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of the original, and the reader who is able to study the New | δ' ουδέν ενην. Θηρία δε παντοία, πλείστοι μεν όνοι άγριοι, πολλοί Testament in Greek will find that he is able thereby to solve δε στρουθοκαι οι μεγάλοι» ενησαν δε και ωτίδες και δορκάδες: ταύτα many difficulties, and throw a clear light upon passages which δε τα θηρία οι ιππείς ενίοτε εδίωκον. Και οι μεν όνοι, επεί τις previously had been quite beyond his comprehension.

διώκοι, προδραμόντες έστησαν άν5 πολύ γάρ τών ίππων έτρεχαν XENOPHON.

θάττον· και πάλιν, έπει πλησιάζομεν οι ίπποι, ταυτόν εποίουν, και Xenophon was a writer who Hourished B.C. 400-359. He ίπποις. Τα δε κρέα των αλισκομένων ήν παραπλήσια τοίς ελαφείοις,

ουκ ήν λαβείν, ει μη διαστάντες οι ιππείς θηρώεν διαδεχόμενοι τους was a pupil of the great philosopher Socrates, who once saved απαλώτερα δέ. Στρουθον δε ουδείς έλαβεν οι δε διώξαντες των his life in battle. He was also celebrated general ; and the εππέων ταχύ έπαύοντο πολύ γαρ απέσπαθ φεύγουσα, τοίς μέν ποσι Anabasis, or expedition up the country (ava, up), is the account of a campaign in which he took a very prominent part. Xeno δρόμο, ταϊς δε πτέρυξιν, αίρουσα, 19 ώσπερ ιστίω χρωμένη. Τα phon lived at a time when the Greek language was at its best, βραχύ, ώσπερ πέρδικες, και ταχύ απαγορεύουσι τα δε κρέα αυτών

δε ωτίδας, άν τις ταχύ ανιστη, έστι λαμβάνειν πέτονται γάρ when dialects were dying out, and Greece was beginning to have

ήδιστα ην. a uniform speech (κοινη διαλεκτος), in which the Attic was the

NOTES. principal element. The “Anabasis” has always had a great

1. 'Opalov (der. from ópoy, together), sven, lovel. charm for all classes of readers, on account of its minuteness of

2. 'Αρώματα, spices : hence our aroma, aromatic. detail, picturesque simplicity of style, and the air of reality

3. Στρουθοι, ostriches, and truth which pervades it. Its plainness and simplicity make

4. Δορκάδες, gazelles (δέρκω, to look), from the brilliancy of their eyes; it the most desirable work for beginners to take up. It is an wriões, bustards, so called from their large ears (ous, motos, an ear). account of an expedition undertaken by Cyrus the Younger to 5. Προδραμόντες έστησαν άν, λασίng run forward, would stop ελοτέ. Αν overthrow his brother, Artaxerxes, King of Persia, and of the re- gives a frequentative sense to the verb. treat of the Greek troops after the death of Cyrus under the

6. Ουκ ήν, it was not ; sc. possible. command of Xenophon himself. Cyrus collected a large army,

7. Διαδεχόμενοι. Δια in composition has a sense of division and altercomposed principally of Greeks, and marched across Asia Minor nation. It means that they stood at different intervals, and thus

caught them, towards Persia. The Greek soldiers, who at first did not know

8. oi de... twinnéw, those of the cavalry who pursued them. Called the object of the expedition, when they suspected that they the partitive genitive. were marching against Artaxerxes, were inclined to be mutinous, 9. 'Απέσπα. Τhe nom. to this is στρουθος. and resolved to ask Cyrus what were his real intentions. It is 10. Αίρονσα, raising them ; so. πτέρυγας. at this point that we take our first extract,

PARSING EXERCISE.
XENOPHON.—“ANABASIS," Book I., Chap. 3.

The student should parse προδραμόντες, έστασαν, θηρώεν, Έδoξε ταύτα, και άνδρας ελόμενοι συν Κλεάρχω πέμπουσιν, οι απέσπα, ανιστη. ηρώτων Κύρον τα δόξαντα τη στρατιά. “Ο δ' άπεκρίνατο ότι ακούοι 'Αβροκόμαν, εχθρόν άνδρα, επί τω Ευφράτη ποταμώ είναι, Babylon, and a battle was fought in which Cyrus was glain by

The army of Cyrus met with Artaxerxes at Cunaxa, near απέχοντα δώδεκα σταθμούς προς τούτον ουν έφη βούλεσθαι ελθείν his brother, after which the chief Greek generals were treacheκάν μέν ή εκεί, την δίκην έφη χρήζειν επιθείναι αυτό, “ήν δεί | rously killed by the Persians. Xenophon was left head of the φεύγη, ημείς εκεί προς ταύτα βουλευσόμεθα.” 'Ακούσαντες δε

helpless host, and he led them back through innumerable diffiταύτα οι αιρετοι 8 αναγγέλλουσι τους στρατιώταις· τοις° δε υποψία | culties to Greece. When they came to the sea-shore, they broke μεν10 ήν, ότι άγει προς βασιλέα, όμως δε έδόκει έπεσθαι. Προσαι- | out into transports of joy:τoυσι δε μισθόν· ο δε Κύρος υπισχνείται ημιόλιον12 πάσι δώσειν ου πρότερον έφερον, αντί δαρεικού13 τρία ημιδαρεικά του μηνός τω XENOPHON.—"ANABASIS," Book IV., Chap. 7. στρατιώτη:14 ότι δε επί βασιλέα άγει, ουδε ενταύθα ήκουσεν ουδείς 'Επειδή δε βοή πλείων τε εγίγνετο και εγγύτερον, και οι άει έν γε το φανερώ.15

επιόντες έθεον δρόμων επί τους αει βοώντας, και πολλά μείζων

εγίγνετο η βοή, όσο δηλ πλείους εγίγνοντο, έδόκει δή μείζον το 1. "Εδοξε ταύτα, these things seemed good, they determined on this course, είναι τώ Ξενοφώντι. Και αναβάς εφ' ίππον και Λύκιον και τους viz., to ask Cyrus the object of the expedition. The neuter plural ιππέας αναλαβών παρεβοήθει- και τάχα δή ακούουσι βοώντων των ταύτα is followed by the singular verb έδοξε, according to the rule | στρατιωτών, « Θάλαττα ! θάλαττα !” και παρεγγυώντων. Ένθα that a neuter plural in Greek takes a verb in the singular.

δή έθεον άπαντες και οι οπισθοφύλακες, και τα υποζύγια ελαύνετο 2. Ελόμενοι, 2 aor. Imid., from αιρέω.

και οι ίπποι. Επει δε αφίκοντο πάντες επί το άκρον, ενταύθα δη 3. Κλεάρχη, a general of the Greek forces,

περιέβαλλον αλλήλους και στρατηγούς και λοχαγούς5 δακρύοντες. 4. Ακούοι, opt. mood, because independent sentence following a prin- και εξαπίνης, ότου δή παρεγγυήσαντος, οι στρατιώται φέρουσε cipal sentence, of which the verb απεκρίνατο is in an historic tense. 5. Käv, contracted for kai tav.

λίθους και ποιούσι κολωνόν μέγαν. 'Ενταύθα ανετίθεσαν δερμάτων 6. Δίκην επιθεϊναι, to lay a penalty upon, to punish. So δίκην δούναι, το πλήθος ώμοβοΐνων και βακτηρίας και τα αιχμάλωτα γέρρα, και ο φαμ α penalty, to be punished. Compare the Latin pαnas summere, pαnas ηγεμών αυτός τε κατέτεμνε τα γέρρα και τους άλλους διεκελεύετα3 dare.

7. "Hv dd. Here the construction changes from the oratio obliqua to the oratio recta, giving Cyrus' own words : "and if" (said he) "he fly"

1. "Ebcov dpóny, were running with (at full) speed. Dative of manner. 8. Aiperoi, chosen by their comrades as spokesmen.

2. "Oow on, by exactly as much as they grew more; exactly as their num. 9. Τοις, the article used for the demonstrative pronoun τούτοις, to

bers increased the shouting increased. them. Note that the article originally was a demonstrative pronoun,

3. Meiçóv Ti, something greater (than usual), something important. and appears as such in Homer, etc. This old use of it is retained to.* Bombeiv is to assist, being literally to run; Béw, to a shout, Bon.

4. Napeßoner, he ran to give aid, or to the noise. lapa means motion in expressions like the present one, and in o uèr—ó de, etc. 10. Mėv, on the one hand, followed by de, on the other.

5. Λοχαγούς, captains (άγω) of a cohort (λόχος). 11. Προσαιτούσι. Προς, when compounded with a verb, has the sense

6. "Οτου δή παρεγγυήσαντος, some one or other ιαείπα prompted ελιετά of addition. They ask additional pay.

(genitive absolute). 12. “Ημιόλιον... ού, half as much αφαίη ας (ήμισυ, half, όλος, whole); ου

7. 'Ωμοβοΐνων, of raw or-hides (ώμος, ταις; βούς, οα).

8. Αιχμάλωτα, taken captive; lit., taken by the spear (αίχμη, 4 8ear; genitive, because it is attracted into the case in which the demonstra

άλωτός, talen). tive would be, if expressed. If put out at length, the sentence would

9. Διεκελεύετο. Δία in composition has a distributive force. Seest run, ημιόλιον εκείνον και πρότερον έφερον. 13. Aape.kov, a dareik, a Persian coin named after King Darius, as we

orders round to the rest. speak of a napoleon, a sovereign, etc. 14. Του μηνός τη στρατιώτη. The article here has a distributive sense.

PNEUMATICS. - VI. To each soldier per month. Myvòs is genitive of time. 15. "Εν γε το φανερή, αι least openly.

COMMON PRESSURE-GAUGE-SAFETY TUBE - ATMOSPHERIC When in the plains near the Euphrates, they came upon some In our last lesson we found that if a gas be kept under a mi

RAILWAY-BLOWING MACHINES-VENTILATION OF MINES, singular animals :

form pressure, and heat applied, it will increase in bulk of XENOPHON.—"ANABASIS,” Book I., Chap. 3. its volume at oo for every degree the temperature is raised. 'Εν τούτω δε τω τόπω ήν μεν η γη πεδίον άπαν ομαλόν,1 | Suppose, now, that the gas be comined so that it cannot increase ώσπερ θάλαττα, αψινθίου δε πλήρες ει δέ τι και άλλο ένην in volume, we shall find that as the temperature increases the "Αης ή καλάμου, άπαντα ήσαν ευώδη, ώσπερ αρώματα2 δένδρον | elastic force will increase too, and in the same proportion as its

NOTES.

NOTES.

volume would were it free to expand. The rule may be stated it, the funnel preventing the escape of the liquid. If, on the thus :

other hand, the pressure inside becomes less than that without, If any gas be confined so that it cannot expand, and its owing to the absorption or condensation of the gas, the pressure temperature be raised from 32° to 212°, the elastic force will of the air will force the liquid into the bulb, and air will then be increased by 0-366 of its original amount.

bubble up through it. In this way the tube prevents the Sometimes the steam in an engine is exposed to a high tem- difference between the pressures from becoming dangerous, and perature after it is first evolved, and is then said to be super at the same time, under ordinary pressure, excludes all air. I heated. Its tension is increased by this, and thus it can accom. We have seen that power may be stored up in compressed air; | plish more work, and at the same time “priming," or the hence it is sometimes employed to drive an engine in place of

condensation of the steam in the cylinders, is to a great extent steam. Of course some power must be first employed to comprevented.

press the air, and therefore in ordinary circumstances no adAfter what we have now seen respecting the change produced vantage will be gained by the substitution, but in many special in the volume of a gas by variations in the temperature or cases it may be and is employed. If steam has to be conveyed pressure, we can very easily tell the specific gravity of a gas to any great distance, there is a considerable loss by condensaif we know the weight of any volume of it, and also its tem- tion in the pipes, and in some places it is inconvenient or perature and pressure. We have merely to ascertain the volume impracticable to have the boiler near the machine. In such it would occupy at the standard temperature and pressure, and cases, therefore, the steam may be employed in the compression then compare its weight with that of the same volume of air. of air, and by this the power may be transmitted to the place In the same way we can calculate the weight of any volume of where it is required. gas, or the volume that a given weight of it would occupy. In mining operations this is especially advantageous. A

We described in our last lesson a manometer for measuring narrow seam of coal, in which there is no room for an engine, high pressures like that produced in the boiler of an engine. has sometimes to be cut out by a machine, and even if the This acted by the elastic force of compressed air: a spring is engine could be placed there, the steam and smoke would premore commonly used, but it is somewhat liable to lose its elas- vent a man being by it; such machines are therefore driven by ticity, or to become injured by the moisture of the steam. compressed air. The same remarks apply to a narrow tunnel, These, however, only record high pressures, and not minor as, for example, that which is now being driven through Mont changes like those produced by alterations of temperature. Cenis; and here, too, compressed air is used instead of steam. We want, therefore, some means of measuring these, and for There is also this further advantage attending the use of air, this purpose we employ a U-shaped tube, open at each end. that the machine can be more easily moved, for a portion of the The bend is filled with water if very low pressures are to pipe may be made flexible, which cannot well be done with be measured, and with mercury if to be used for those rather steam-piping. greater.

The most important application of the pressure of air to If the gas whose tension is to be ascertained is allowed to driving machinery is seen in the atmospheric railway. At press on the liquid in one limb, it will depress it and raise that present this has not come into practical use, but it appears in the other, and the difference in level between the two will probable that the principle will ultimately be adopted in our indicate the pressure. A sliding-scale is usually attached to underground railways, as it will effect å saving in working show this difference. In this way we shall find that the pressure expenses as well as in construction, to say nothing of the much of the gas, as usually supplied to our houses, is seldom equal greater safety which would be ensured by its use, and the to two inches of water, a very small amount indeed when we greater purity of the air in the tunnel. remember that the pressure of the air will sustain a column of The original plan proposed, and actually carried out on a water over thirty feet high. It is, however, found to be quite short piece of line near Paris, was somewhat as follows:-A sufficient to overcome the resistance caused by the friction of large iron tube, having all along the top an opening which was the gas against the pipes, and a greater pressure would only closed tightly by a flexible lid, was laid along the middle of the cause a greatly increased loss by leakage from the mains. line. This tube was made uniform in size, and a pair of pistons, This pressure is produced by weights placed upon the gaso made to fit it, were fixed one to each end of a little carriage

meter, and can in this way be regulated to a which travelled along in the tube. From the middle of this considerable extent. It is found, however, that carriage rose an arm which projected through the slit, and was considerable variations occur, it being greater attached to the carriage on the line. A coulter-shaped piece of just before the majority of people light it in metal was placed on each side of the arm, so as to open the slit their houses, and again in large towns about for it to pass along, and the aperture closed of itself as soon as eleven o'clock, when many burners are turned off. the arm had passed. The pistons were also attached to short These variations in pressure cause a loss in illumi-arms, so that the valve admitted no air in front of them. At nating power, and several regulators have accord- each end of this tube was fixed a powerful double-acting airingly been devised to obviate this. The principle pump, and whenever it was required to start the train, the on which they act is merely that a conical valve is pump at the end to which it was going was set to work. It moved by the pressure so as to close to a greater or soon produced a vacuum in the tube, and the pressure of the less extent the pipe along which the gas passes. air behind the piston was sufficient to drive the train. There

There is a small but useful piece of apparatus, were, however, many practical difficulties in the carrying out known as the safety-pipe (Fig. 17), which may be of this plan. The valves could not be got to close well, and explained here, as it acts in a similar way to the hence there was a considerable leakage of air which greatly pressure-gauge just mentioned. In many chemical diminished the power. All the strain, too, was transmitted experiments in the laboratory, as well as in the through the arm, and thus there was danger of breakage. From manufactory, a large amount of gas is evolved by these and many similar causes the design was not carried out

the changes taking place within some closed elsewhere. Fig. 17.

vessel. If no escape be allowed for this, the More recently, however, an altogether different plan was

pressure may increase to such an extent as to tried with much greater success. In this the tube was built of burst the vessel; while, on the

other hand, it is desirable not to brickwork, and made of such a diameter as to take in it an allow the gas to be lost. A safety-pipe, similar to that shown in ordinary-sized railway carriage. A trial line, of nearly a mile in the annexed figure, is therefore introduced. This allows a portion length, was constructed in the grounds of the Crystal Palace at of the gas to escape

when the pressure reaches a certain limit. It Sydenham. The line was made with steeper gradients and is, in fact, a safety-valve of low pressure. A glass tube has a sharper curves than any line yet worked, so as to give the bulb e blown near the middle, and each end

is then bent back system a full trial. The tunnel was carefully constructed, so as upon itself. The upper end is also shaped into a funnel, which to be of uniform size, and one end of the carriage was made should be rather larger than the bulb. Water or mercury, ac- nearly to fit it, an aperture of a few inches being left all cording to the pressure required, is now ponred into the funnel

, round. A brush fixed round the carriage nearly filled this, 80 28 to fill the bend and part of the bulb. If the

pressure inside and was found

to exclude the air sufficiently. The ends of the the vessel becomes too great, the liquid will be forced into the part tunnel were closed by air-tight doors, and in a building near C of the tube, and any excess of gas will then bubble

up through one end was fixed a large fan, constructed somewhat after the

plan of a centrifugal pump, and so arranged that by causing it | They consist of two ordinary bellows, placed one abore to rotate in one direction it exhausted the tube, 'while on re- the other. When the ander board E (Fig. 18) is lowered, the versing it the air was condensed. The pipe leading from this valve c opens and admits the air ; this is forced, by the rising entered the tunnel at a little distance from the end. The car of the board, through the valve B into the upper bellows. A riage being now placed just in the mouth of the tunnel at the weight A, placed on the top, drives the air with a constant further end, the engine was set to work, and, as soon as a slight pressure out of the nozzle D. The board F is fixed, and a amount of exhaustion was produced, the pressure outside forced is usually worked by a it rapidly along. As the whole area of the carriage was exposed lever. Though these to the pressure, it was found that only a very small degree of bellows are powerful rarefaction was required, a pressure of a few ounces to the inch enough for a blackbeing quite sufficient to impart to it a great velocity. As soon smith's forge, when the as the carriage had passed the portion of the tunnel where metal has only to be the exhaust-pipe entered, it ceased to be carried forward by softened sufficiently to the pressure of the air, but it had acquired an amount of cause it to weld, they momentum sufficient to propel it with considerable violence will not answer for a beyond the end of the tunnel. The doors at the end were, how. furnace for melting iron; ever, closed by powerful springs. The air, therefore, enclosed and a fan, driven by between them and the carriage became more and more com- steam, is usually em

Fig. 19. pressed, until the pressure was sufficient to open the doors, ployed in this case. The and allow the carriage to run slowly out. The air acted, in air enters at the axle, and is thrown off by centrifugal force from fact, as a buffer, and brought the carriage to rest with scarcely the edges, and conducted along large tubes to the furnace. any shock. When the carriage was to be sent back to the other Another purpose for which these machines are employed is end, the engine first exhausted the tube until the carriage passed in the winnowing of corn. In former times, and in some places the opening, the doors were then closed, the engine reversed, and at the present day, the corn, when threshed, is thrown up in the then the air behind was condensed, and drove the carriage to air, and the wind carries away the chaff. The plan now adopted the other end on the same principle as a boy drives a pea is to allow it to fall through a narrow slit, and canse a rapid through his pea-shooter by the pressure of his breath.

current of air, produced by rotating fans, to remove the chaft. The experiment appeared satisfactory, though no practical One great advantage of this plan is that the strength of the use has yet been made of it. The carriage, with passengers in blast may be so regulated as not only to remove the chaff, but it, could be started from one end, driven round the curves, and to separate also the small and shrivelled grains. up and down the steep inclines, and yet stop at the other end, The pneumatic screw is another simple blowing machine, ased nearly a mile off, in the space of about one minute. The system for purposes of ventilation. It acts on exactly the same prinappears to possess many advantages. Much greater inclines ciple as the Archimedian screw, an axle with a spiral flange can be allowed, and all danger of the carriage running off the being made to rotate in a cylinder. This is placed at one end line on sharp curves is avoided. There is also much greator of the tube or shaft, and produces a powerful current, the safety from accidents. Collision is impossible, for two car- direction of which depends upon the direction in which the riages can never be travelling in opposite directions at the same screw revolves. time, nor can one overtake the other. The boiler, too, being This machine is sometimes employed for the ventilation of away from the train, cannot injure the passengers if it explodes, mines, and is fixed above one of the shafts. A second shaft and the only inconvenience then would be that the passengers allows fresh air to pass down it, and replace that removed by would have to walk along the line to the nearest station. the fan, and thus a constant current of air is kept up through Further, as the trains would travel very rapidly, one line would, the mine. The main galleries below are so arranged, by in most cases, be sufficient, and the additional expense incurred means of boarding and doors, that the fresh air must traverse by the careful building of the tunnel would, in many places, be the greater part of the mine before it can find its way to the compensated for by the smaller amount of land required. The "upcast" shaft, as it is termed. As the air will always find tunnel, too, unlike our present ones, would be well ventilated, as the most direct road, great care is required in the arrangements the air would be entirely changed each time a train passed for effecting this. through. There are, of course, many practical difficulties which In most mines in England blowing machines are now dismight occur in the actual working, but the plan seems to pensed with, and in their stead a large furnace is placed at the promise well, and to be worthy of a thorough trial.

base of one of the shafts. This greatly rarefies the air above it, We must now notice the construction of a few common and thus renders it much lighter than that around. It ascends, pneumatic machines, and perhaps the most important are those therefore, and a fresh supply rushes down the second shaft to used for blowing. In furnaces for reducing and melting metals take its place, and in this way good ventilation may nearly it is found impossible to cause a sufficient degree of heat to be always be obtained. The plan, too, is more simple than the produced unless a large additional quantity of air be forced into use of fans, and less liable to get out of order. Sometimes the the fire, so as to quicken combustion. In mines, too, and furnace is placed in a recess, part of the way up the shaft; underground passages ventilation must be carried on by artificial sometimes, too, only one shaft is sunk, and divided by bratticing means, and for these and other purposes blowing machines are into three or more divisions, one for the pumps and working employed.

machinery, the other two for the "upcast” and “downcast." The most simple of these is the common household bellows, This plan is, however, very dangerous, and many of the fearful

so familiar to all. In the accidents we hear of in mines are to be attributed to its
lower of the two boards adoption.
is a circular aperture, Ventilation in our houses and public buildings is carried on
over which a piece of in a similar way. It is much to be regretted, however, that the
board is hinged, so as to principles on which this should be arranged seem to be so little
act as a common clack understood or carried into practice. If we hold a sheet of
valve. When the boards paper near a large fire, we shall soon see by the powerful draught
are separated, the air that there is a strong current of air up the chimney, and cold

opens this valve, and air rushes in at the cracks of the doors and windows to supply enters, but as they are again pressed together this valve closes, its place. A good fire, therefore, adds greatly to the ventilation and air is then forced to escape by the nozzle. With of a room. As, however, the heated air rises, it is an important this kind, however, only an intermittent current can be pro- thing to have some outlet for this, and an opening into the duced, for while the boards are being separated, air is drawn chimney near the top of the room will usually be effectual. in at the nozzle as well as at the valve, though in a less In public buildings the foul air is usually

carried off near the degree. This was often found to be a serious disadvantage, roof, and arrangements ought to be made by which fresh air and therefore two bellows, working alternately, were nsed can enter in a number of small streams at different places, in in many furnaces. Double-acting

bellows are, however, used stead of flowing in a large body through an open door, and thus now in nearly all forges, and these produce a uniform stream. creating a violent draught.

Fig. 18.

THE

LESSONS IN ASTRONOMY.-II.

and expressed their wonder at its manifest disproportions. Still

such is the hold that preconceived notions obtain over the EARLY ASTRONOMICAL INSTRUMENTS HISTORY OF

human mind, especially when those views are supported by SCIENCE (continued) — COPERNICUS AND HIS SYSTEM

priestly authority and made matters of religion, that for TYCHO BRAHE-KEPLER.

centuries no one seems to have referred to the old theory of In judging of the various systems devised by the ancients to Anaxagoras, or proposed any new one to clear up the difficulty. account for the grand mechanism of the heavens, we must bear At length, however, about the year 1472, there was born one, in mind, and make allowance for, the very imperfect nature and Nicholas Copernicus, who, leaving all the speculations of former construction of the instruments they possessed; and when this observers, inquired for himself into the motions of the celestial is done, instead of wondering at the errors they made, we shall bodies. He first examined all the ancient observations he could often be surprised at the accuracy of their observations; some find, and then commenced for himself a system of close and of these which still exist being sufficiently accurate to be at careful study of the heavens. He compared the actual places times of service to astronomers in the present day,

occupied by the sun and planets with those which, according to The simplest and probably the most ancient astronomical former theories, they ought to occupy, and thus obtained a instrument consisted of a vertical pillar set upon an even sur- better knowledge of their irregularities and variations than any face, so that, by observing the shadow, the direction of the sun astronomer before his time. He continued this course for many and its altitude at any period might be measured ; by noticing, years, and at length arrived at the conclusion that Mercury and also, the direction in which the shortest shadow was cast by the Venus revolved around the sun, instead of round the earth. He pillar, they could ascertain the north and south points of the gradually extended his reasoning further, and at last started heavens. It is believed by many that the obelisks and stone his celebrated theory, which regarded the sun as the centre of pillars which were common among Eastern nations were con- the system, with the earth and the other planets all revolving structed for some such purpose, and that they were frequently in regular order around it. By this grand idea all the complisurmounted by a ball, in order that the position of the shadow cated and bewildering schemes which had puzzled so many might be more easily marked. Some of these obelisks were observers were at one stroke swept away. Instead of the afterwards removed to Rome for the same purpose. These cumbrous machinery of crystal spheres revolving one within the instruments were called gnomons.

other, the utmost simplicity is seen to prevail ; order and reguThe telescope, which has made such astounding revelations to larity take the place of almost inextricable confusion; and as men of modern times, and which has so greatly extended their the observer transfers his station of observation from the earth knowledge of the universe, was quite unknown in early ages. to the sun, the planets, which had previously appeared to Instruments for measuring time were also very imperfect, wander on in ever-varying directions among the stars—now although, as will be seen further on, the importance of noticing retracing their steps, and then, after an interval of rest, starting the exact moment of the occurrence of any of the celestial afresh-are seen to be steadily moving on in elliptic orbits phenomena is very great. Various forms of the sun-dial were around the central luminary of the system. The movements of in ise, but these could only be of service when the sun was the inferior planets Mercury and Venus, the reason why they shining, and even then could not give very accurate indications. were never seen very far removed from the sun, the retrograde Other instruments were therefore planned, and the one most motions of the planets, and their irregular movements, were commonly employed was the clepsydra, or water-clock, in which all clearly explained by this grand yet simple theory. the hour was shown by the amount of water that had passed We can with difficulty form an idea of the prejudice with through an aperture. Sand was afterwards used in the place of which this scheme would be received; the earth was by it water, as its flow was found to be more regular and even. degraded from its central place, and reduced to the rank of one

Rather strangely, we have come back very recently to a of the planets; and that which men had always been wont to method of measuring minute intervals of time similar to this old regard as fixed and immovable, was now declared to be in rapid plan. A vessel is provided with a small aperture from which a flight around the sun, and, at the same time, to be ever whirling fine stream of mercury is issuing, and when it is required to note round on its own axis. He himself foresaw the effects of this any brief interval-as, for instance, that occupied in the passage prejudice, and hence he seems to have been long before he fully of a planet between two lines situated in the field of view of a accepted the theory, and then to have waited still longer before telescope-the mercury is diverted into a separate vessel at the he ventured to make it public. His work on the subject, entitled moment of the disc of the planet coming into contact with the “On the Revolution of the Heavenly Bodies," was finished in first line, and allowed to flow on until it has passed the second, the year 1530, but he delayed publishing it for several years, when the stream is allowed to flow as at first. The amount of although a few friends, to whom he had communicated and mercury in the vessel is then accurately weighed, and by com- explained his views, at once adopted them and urged him to do paring it with the amount which is known to flow out in a given so. At last, however, he gave his consent to its being printed, interval-say, for instance, five seconds—the exact duration of but his dedication almost takes the form of an apology for the passage can be noted.

venturing to suggest such views, and his ideas were put forward A few other rade instruments were also occasionally employed, rather in the shape of an hypothesis than of a definite system. but their construction was very imperfect, and we are not there We must not, however, suppose that Copernicus formed a fore surprised at the slow progress of the science. Among the complete system to account for all the motions of the planets ; Romans, too, science never found a congenial home; glory in his life was too short for this task. His work was rather to war being the object of their ambition, rather than the peaceful indicate the true theory of the universe, leaving it for others to yet glorious triumphs achieved by intellect. After the age of trace out more accurately the exact curves in which the planets Ptolemy little progress appears to have been made, and even moved, and to ascertain their various distances, sizes, and rates known truths were to a great extent forgotten. His system was of motion. This work was taken up by Kepler, who has someindeed universally received for many centuries, more especially times been called the “ Legislator of the Heavens," as it was he as it was supported by the authority of Aristotle ; and fresh who first laid down the laws and rules which govern the moveadditions to it, in the shape of eccentrics and epicycles, were ments of the heavenly bodies. We shall notice more about this made ; but few, if any, new discoveries appear to have been celebrated astronomer shortly, but must first look at the labours effected, and no noteworthy name appears on the

pages of of another distinguished man who preceded him-Tycho Brahe.

He was of Danish extraction, and was born very shortly after After the fall of the Roman empire the science found a home the death of Copernicus. It is said that his attention was first among the Arabians, who, in the eighth century, seem to have directed to the science of astronomy by an eclipse which dovoted much attention to its study, and to have made con happened at the time predicted, in the year 1560, and incited siderable advances in it. By them the length of the solar year him to learn something of the wonderful science which enabled was calculated to within a very little of its true amount; the such predictions to be made. When at the University of Leipsic, obliquity of the

ecliptic was also measured ; and at a place in much of his night was often devoted to observation of the stars, the desert, near Palmyra, the length of a degree was ascertained and thus he soon attained considerable proficiency; but there with very creditable accuracy. The Ptolemaic system was, how is one thing which tends rather to lower him in our estimation, ever, firmly received, though many of the more thoughtful and and that is his partial rejection of the Copernican system, and careful observers seem to have been far from satisfied with it, the proposal of a new one, in which the earth occupied the

84

history,

VOL. IV.

central place, with the moon and sun revolving round it, while bination of several circular movements; and again he diligently all the planets revolved round the sun.

calculated its position, till, just as he seemed to be on the verge The Copernican system, however, was, we must remember, at of success, the planet once more wandered away from the path this time a mere theory unsupported by proof, and the main which he had assigned to it; and once more he had to commence reason of Brahe's rejection of it was that, if the earth revolved his observations from the beginning. In this way he continued in a large orbit, he thought the fixed stars ought to appear in a to try one hypothesis after another, submitting each to the test different position when seen from one extremity of the orbit to of most careful observation, till at length no fewer than nineteen that which they occupied when seen from the other extremity; different theories had been proposed, and the movements of the and not being able to observe this change, he concluded that the planets compared with those which were calculated by these earth must be at rest. The principle of this argument was theories; and yet the true solution of the problem was still onright, and in reality there is a minute difference in the appearance found. His perseverance, however, never failed, and he toiled of the stars; it is, however, too minute to be observed, except on, though eight long years had been occupied in the task. One by the most delicate instruments. The reason why it is not important negative result he had, however, arrived at, and this more clearly seen is that, great as is the diameter of the earth's was that, whatever was the nature of the curve the planets orbit, the distance of even the nearest fixed star is so immensely described, it was not a circle, nor a combination of circles. This greater that the change produced is scarcely visible. We may was one great step towards the solution of the task. From the notice this same effect as we are carried rapidly along in a train; very earliest ages it had been assumed that, as the cirele the objects situated near to the line of railway seem to move seemed the perfection of form, all the heavenly bodies must move past us very rapidly, those further off have a less apparent in circles; but Kepler now cast off this trammel, and then speed, while lofty objects in the distance scarcely seem to move applied himself afresh to his task. at all. Every minute changes the apparent position of those In looking at the greatness of his work we must remember which are near, while it is only after the lapse of some little time that the difficulty is much increased by the fact that our station that we perceive the motion of those at a distance; and, sup- of observation is itself in rapid motion. Could we view the posing the line of rails were perfectly straight, we might travel planets from the sun, we should easily see their courses ; but on for hours, and not be able to detect the slightest alteration in as we cannot do this, allowance has to be made in every calcuthe apparent position of the sun. We see thus that the con- lation for the movement of our standpoint, and this motion was clusion which Brahe arrived at was wrong, though his premises not then clearly understood. were right; and we shall find further on the great importance Having discarded the theory of motion in circles, Kepler now which is attached to this change of position, or “parallax," as proceeded to try other forms, testing them as before, and the it is called, all the distances of the heavenly bodies being deter- first that occurred to him was the ellipse. The same series of mined by means of it.

calculations was accordingly gone through again, and this time His fame, however, as an astronomer rests upon the care and the motion of the planet was found to agree with that assigned accuracy of his observations. A new star which appeared in the to it by the theory. The great problem of the heavens was now year 1572, and continued visible for about a year and a half, solved, and the joy with which Kepler enunciated the first of was specially observed by him, and he recorded a large number the laws which bear his name can scarcely be imagined. This of very careful observations on the planets and stars, some of law may be stated as follows:

The planets revolve around which are of great use for reference at the present time. To him, the sun in elliptical orbits, the sun being situated in one of the too, we are indebted for a catalogue of many of the fixed stars, foci. which, though it contained a much smaller number than that of As this is one of the fundamental laws of astronomy, we must Hipparchus, was greatly superior to it in accuracy.

explain it rather more fully. In every circle there is a point A table showing the allowance to be made in the apparent called the centre, such that all straight lines drawn from it to position of the heavenly bodies, on account of the effect the circumference are equal. No such point is to be found in an produced by the refraction of the air, was also calculated by ellipse; but in the longest diameter two points can be found to him. The nature of this refraction will be fully explained further situated that, if straight lines be drawn from one to any point in on. We may mention, however, that it causes all bodies near the circumference, and thence to the other, the sum of these the horizon to appear at a greater altitude than they really have lines will always be equal. These points are called the foci. attained ; and hence, in important observations, allowance must Explanations of the practical methods by which the curve of be made for its action.

an ellipse may be traced from any two points as foci, hare About the year 1575 Tycho Brahe attracted the attention of already been given in Problem LVIII. of “Lessons in Geometry" Frederick II. of Denmark, who gave him a small island on the -"How to trace the curve of an ellipse by mechanical contriBaltic, and an annual allowance. Here he built himself a large vances” (see Vol. II., page 252); it is therefore unnecessary to house and observatory, which he called Uraniborg, the “ Castle repeat them here in detail

. It will be needful, however, to call of the Heavens," and in this he lived for years, occupied with the reader's attention to what is termed the “eccentricity" of his favourite science, and assisted by the best instruments which an ellipse, as it is a term that is constantly used in speaking of could be procured. After the king's death, some of those who the orbits of the heavenly bodies. In Fig. 84 (Vol. II, page were envious of his honours succeeded in depriving him of his 252), G is the centre, and the fraction of which the namerator is allowance and his observatory. He did not, however, despair, GA and the denominator is GC-or, in other words, the proparfor soon after he was received at Prague by the emperor, tion between G A and G c, which is the half of the major axis and an observatory erected for him and his pupils. Here he is called the eccentricity. In the figure, however, this is repreremained until his death, which happened a few years later. sented very much greater than it is in orbits of any of the pla

Among his pupils was Kepler, to whom we have already nets, and their paths therefore differ less from a circle. referred. He acquired from Brahe the habit of accurate observa The consideration of the remaining two laws of Kepler must tion, and was far more successful than his master in the theories be deferred till our next lesson. which he formed. Naturally he was possessed of a quick and kvely imagination. He commenced with careful observation, and then formed his theories in accordance with the facts; and pro

READINGS IN LATIN.-II. ceeding in this way, he soon made several important discoveries. The task to which he now devoted his time and energies was

VIRGIL. to discover the nature of the paths described by the planets. VIRGIL was a Roman poet who was born in the year 70 s.c. and Starting with the hypothesis of the sun being in the centre of died 19 B.C. He flourished in the

period which is known as the the system, he began to watch attentively their

places, and, to "golden
ege” of Latin poetry, of which

he was one

of the most simplify matters, he confined himself at first to the motions of brilliant ornaments. The works by which he is best known are the planet Mars.

(1) the Bucolics, a book of pastoral poetry, consisting of ten He calculated the place it onght to occupy according to the eclogues, as they are called ; (2) the Georgies, four books of theory of its revolving in a circular orbit, and soon found that what is known as “ didactic" poetry, containing instructions in the place it really occupied in the sky differed considerably from the art of agriculture and similar occupations; and (3) the that assigned to it. This theory was thus at once shown to be Æneid, an epic poem in twelve books, each of considerable incorrect, and he had therefore to form

a fresh one by the com- length, the subject of which is the wanderings of the Trojan

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