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In this earthquake a remarkable proof was offered of the fact that the land must have sunk, and the present floor have been above alluded to, that the wave is more readily propagated in raised above the level of the water. In the early part of the some strata than in others. The destructive effect was confined third century, the Emperor Alexander Severus beautified the to those houses which were built on the Tertiary strata. The temple, of which these are the pillars. lower part of the city, which rests on blue clay, was most At what time the temple was deserted we cannot conjecture; severely shattered; whereas that part of the city which was but in 1749 the following facts were brought to light by excabuilt on the limestone or basalt escaped. The undulatory vating: -That when the sea broke in, the salt-water caused a hotmovement passed along the earth's surface at the rate of spring which existed to throw down a dark calcareous deposit, twenty miles an hour. The sea-wave rolled about four miles in two feet thick; above this a layer of volcanic tufa reposed, which that time. This wave is generally the cause of as much loss of must have been ejected from a neighbouring volcano; this deposit life as the actual violence of the shock. This may well be sup- is not regular, varying from five to nine feet in thickness. The posed from the fact that at Cadiz the wave was sixty feet high. eruption seems to have formed a barrier which kept out the This wave is largest when the point of disturbance is under the waters of the sea, so that the hot-spring continued to deposit its ses; then the sea-bound towns are subject to a double inunda- carbonate of lime, but without any marine admixture ; thus tion. The undulatory movement, when it reaches the shore, about two feet more were added to the matter which embedded canses a great commotion, as, when a basin of water is moved, the bottom of the columns. More volcanic tufa was now placed the water does not at once participate in the motion, and there- upon the lime deposits, either by a storm or another eruption, fore washes up the sides of the vessel. This disturbance no making a total deposit of eleven feet. All this time the land had sooner subsides than the sea-wave, which has für wed the been sinking. The sea now surrounded the pillars, which finally "ground" wave at a slower pace, rushes in upon thu shore, its sank nine feet more; thus half their height was above the water, waters blaok with the sediment of the ocean-bed.
and of that which was beneath the surface eleven feet was South America has for centuries been the scene of repeated embedded and nine exposed to the water; in this space the earthquakes. A few years after Lima was first built, in 1582, the pillars were perforated by a bivalve, Lithodomus, which is indi. city was ruined, and since then the catastrophe has been re- cated in the figure by the dotted parts. Thus, if we include the peated some twenty times. In all the cities of that neighbourhood lower pavement, the land must have sunk twenty-five feet from the ecclesiastical year is full of anniversaries commemorating the commencement of the Christian era. When the upheaval terrible overthrows or marvellous escapos. But none of these began we cannot say, but we know it was in progress in 1530, calaznities seem comparable to that which has just paralysed and in 1838 the pavement was again above the sea-level. The the country. Two shocks, on the 13th and 16th of August, downward movement has again commenced at the rate of about 1868, passed over Peru and Ecuador, ruining every town and one inch annually. city, and leaving between two and three hundred thousand dead Here, then, we have an evidence of a structure which has to putrify in the tropical sun. Arica, a seaport town, was com- undergone an upheaval and subsidence of at least twenty feet, pletely covered by the wave. The writer of these pages hears and still stands to attest the quietness and regularity, of the from one who survived that, upon the first shock, at 5.15 in the movement. afternoon, he, with some others, jumped upon a barge, when the From the cases cited, seeing the difficulty of proof on account great wave carried them on its crest completely over the town of the peculiar circumstances of position requisite for such proof, above the spire of the church, and left them unharmed nearly a we may consider that this motion of the earth's crust is far mile inland.
more general than we suppose, and may fairly be required to The chief geological effect of earthquakes is shown in the account for the successive upheaval and depression necessary permanent alteration of the level of the land. In 1822 the coast for bringing the aqueous rocks to form the surface of continents. of Chili was raised some two feet, while farther inland the eleration was more than double this quantity. In 1855 the coast of Now Zealand for ninety miles evidenced a rise of nine feet.
READINGS IN FRENCH.-IX. (For many other facts illustrative of the alteration of level-a
FEDORA. result of an earthquake--in all parts of the world, chap. xxvii. of Vol. II. of Lyell's “Principles " may be consulted.)
SECTION I. But that gradual alteration of level which is not accompanied C'ÉTAIT en mil huit cent douze ;' Napoléon, à la tête des ses by convulsive movements is more important than these local troupes victorieuses dans les plaines de la Moskowa, était entré variations. It is difficult to establish these facts, because we dans l'antique capitale de l'empire des czars, et de là menaçait have no standard which is not itself subject to alteration. Care- la nouvelle ville fondée par Pierre-le-Grand.Poussé par un ful investigation of the coast of Sweden has shown that most of patriotisme fanatique, le gouverneur de Moscou, Rostopchin, the Scandinavian peninsula is rising at the rate of four feet a prit alors cette résolution qui a porté un coup si funeste au century. The coast is favourable for the observation. There succès de nos armes, celle d'incendior" la ville, dont l'empereur are no tides in the Baltic, and the cliffs descend perpendicularly Alexandre lui avait confié la garde. Nous ne raconterons pas into the sea ; the water-level has been repeatedly marked, and toutes les circonstances de cet épouvantable drame. Chassés de the rise judged by its change. In few other places are the leurs demeuresó en feu, croulant sous les efforts des flammes, same advantages. Mr. Darwin has suggested an ingenious c'était un spectacle affreux que de voir tous les habitants mêlés proof of the sinking of the ocean-bed in the Pacific. It is known à nos soldats, forcés de fuir en emportant ce qu'ils pouvaient that the coral insect cannot live below twenty fathoms, the dérober à la violence de l'incendie. pressure of the water beyond that depth being too great for its La petite fille d'un négociant, à peine âgée de six ans, se existence. How, then, can the fact be accounted for that many trouva perdue dans le tumulte.7 Abandonnée, transie de froid, of the coral structures have their foundations resting on the elle errait çà et (a) lds à travers les rues que le feu épargnait ocean-bed at profound depths ? There is only one reasonable encore. Son père et sa mère avaient disparu, et personne ne solution of the difficulty, that they build upon a sinking founda- semblait vouloir la recueillir. La nuit se passa ainsi toute tion, and this very fact impels their labour and increases the entière ; et quand le jour commença à poindre, Fædora, exténuée demains they conquer frem the sea.
de fatigue et de faim, s'affaissa devant la porte d'une église et We have reserved one well-known proof of this repeated se prit (b) à dormir. oscillation of the earth's crust, that of the Temple of Serapis, Sans doute elle ne se serait plus réveillée," la mort serait near Puzzuoli, in the Bay of Naples.
venue la surprendre, si une vivandière, qui par hasard vint (c) The ruins of this temple consist of three pillars of marble établir son petit marché de vivreslprès de cette église, ne l'eut hewn out of solid blocks. They are rather more than forty feet aperçue et ne se fut sentie touchéo de compassion's pour la high.
malheureuse enfant. Elle aussi avait des enfants ! C'est The history of this remarkable temple seems to be this : pourquoi elle s'empressa de prodiguer ses soins à la petite From certain inscriptions discovered in the neighbourhood we orpheline 15 Fodora ne savait comment lui témoigner sa learn that, in 105 B.C., a temple dedicated to Serapis existed reconnaissance.16 Elle devint bientôt pour sa seconde mère une on the sea-shore. In 1828 the handsome mosaico pavement aide fort intelligente. Peu à peu, elle apprit (d) à comprendre of this temple was discovered five feet beneath that from sa bienfaitrice et pat (e) lui exprimer tout ce que son cæur which the pillars rise. The existence of this parement indicates renfermait de reconnaissance et d'amour.
Cependant l'armée de Napoléon commença sa retraite, 18 et la flocons et obscurcissait le ciel de manière à ce qu'on ne pût rien vivandière dut (f) quitter Moscou. Les parents de Fodora voir à trois pas devant soi. existaient-ils encore ? C'est ce que rien n'était venu révéler. 19 "C'est quelque voyageur égaré qui demande du secours ou qui Fodora partit donc avec l'armée française. 20 Qu'on juge de ce est attaqué par les bêtes féroces, 23 car il est impossible de se qu'un enfant de cet âge eut à endurer pendant une pareille livrer au plaisir de la chasse par un temps semblable," s'écria retraite! Au passage de la Bérézina, Fodora eut encore le Polowski, et il donna l'ordre à ses gens d'aller à sa recherche. malheur de se trouver séparée de sa bienfaitrice, 21 soit (g) que Lui-même se mit (i) à la tête du cortège, 24 qui se dirigea vers la celle-ci eat péri dans les flots, soit qu'elle crût (h) la jeune enfant forêt. Quelque temps après, il reparut. Les domestiques por. égarée! Quoiqu'il en soit, l'orpheline ne la trouva plus,-2 et elle taient sur un brancard le corps d'un Russe ensanglanté.25 se vit de nouveau délaissée.
Fodora se précipite au devant son compatriote ; elle-même COLLOQUIAL EXERCISE.
veut panser sa blessure. Bientôt celui-ci put témoigner
reconnaissance aux hôtes du château et leur raconter son 1. En quelle année Napoléon 12. Que vint faire la vivandière histoire. entra-t-il dans Moscou ?
près de l'église ? 2. Quelle ville l'Empereur mena- | 13. La vivandière ent-elle pitió de
COLLOQUIAL EXERCISE. çait-il de là ? la petite malheureuse ?
1. Où Fedora fut-elle conduite ? 14. Que raconta-t-elle à ses bien3. Par quoi Rostopchin fut-il 14. Pourquoi eut-elle pitié de
2. Qu'étaient devenus ses com faiteurs ? poussé ? Fedora ?
15. Comment Polowski et 4. Quelle résolution prit alors le 15. Que fit la vivandière ?
3. Où se trouva la petite Mos femme traitèrent-ils notre hégouverneur ? 16. Fedora parut-elle reconnais
roine ? 5. Où étaient les habitants ?
4. Que ''elle soudainement au 16. Apprit-on bientôt des nou6. Que s'efforçaient-ils d'em- 17. Qu'apprit-elle peu à pen ?
milieu de la forêt ?
velles de ses parents ? porter ? 18. Que fit l'armée quelque temps
5. Que fit Fædora à l'approche de 17. Prit-on soin de son éduca7. Qu'arriva-t-il à la petite fille après ?
tion? d'un négociant ?
19. Avait-on découvert les parents 6. Comment échappa-t-elle à un 18. Quel âge avait-elle ? 8. Que faisait la petite ? de Fedora ?
si grand danger?
19. Que faisait-on chaque année ? 9. Où étaient son père et sa 20. Que fit-elle alors ?
7. Que vit alors Fodora ?
20. Que faisait Fædora dans une mère ? 21. Qu'arriva-t-il au passage de
8. De quelle manière l'étranger de ces réunions ? 10. Dans quelle situation
regarda-t-il l'orpheline ?
21. Qu'entendit-on alors ? trouva-t-elle à la pointe du 22. L'orpheline retrouva-t-elle sa
9. Qui était l'étranger ?
22. Quel temps faisait-il dans ce jour ?
10. Que fit alors le gentilhomme moment? 11. Se serait-elle réveillée ?
23, Que dit Polowski on entendant 11. Que fit-il pour l'enfant ?
de feu ?
12. Par qui Fedora fut-elle ac- 24. Que fit-il alors? (a) çà et là, here and there. (c) From pouvoir.
25. Que portaient les domes(b) še prit à dormir, fell asleep; (1) Dut, was compelled to; from 13. Fut-elle longtemps à se ré tiques ? from prendre. devoir.
tablir ? (c) From venir. (9) Soit, be it; from étre.
NOTES. (d) From apprendre. (h) From croire.
(a) Parvint, reached; from par-; (e) From pouvoir. SECTION II.
(f) Coup de fen, shot,
(9) From accueillir. Cependant Fedora parvint (a) jusqu'en Pologne avec un dé- De sorte que, so that.
(c) From recueillir.
(h) From apprendro. tachement de troupes ;' plusieurs de ses compagnons de voyage id) Ce qui lui restait de forces, (i) From mettre., avaient succombé, moissonnés par le froid ou par la faim, et les her remaining strength. autres se dispersèrent tout à coup, de sorte (6) que la petite Moscovite se trouva seule, abandonnée au milieu d'une forêt.3 Mourante de froid, ayant de la neige jusqu'aux genoux, elle vit
PNEUMATICS.-I. soudain un ours se diriger vers elle ;* alors elle recueillit (C) ce
OBJECTS OF THE SCIENCE-PROPERTIES OF THE AIR-ITS qui lui restait de forces (d) et voulut s'enfuir. Mais, hélas !
WEIGHT-DIVING-BELL--AIR-PUMP-FIRE BALLOON. comment une enfant si faible, et dont tous les membres sont In our first lesson on Hydrostatics we saw that all bodies were presque engourdis, pourra(e)-t-elle échapper à ce danger ? Déjà divided into three great classes-solids, liquids, and gases l'ours est sur le point de l'atteindre, Fedora pousse un cri, according to the relations subsisting between their ultimate parappelant au secours. Par une faveur inespérée de la provi- ticles and the relative distances at which they are placed from dence, au moment où la bête féroce se précipite sur elle, un one another. Of the properties of the first and second of these coup de feu (f) part, et l'ours tombe. Bientôt un étranger classes we have treated in our lessons on Mechanics and Hydroarrive à la place où Fodora s'était arrêtée, à peine revenue de statics ; and the science which is now to engage our attention son effroi.? ' Il regarde avec bonté et d'un oeil de compassion is concerned with the motions, pressure, weight, etc., of the cette enfants dont le ciel venait de lui confier le salut.
third. The term " Pneumatics” is derived from the Greek word C'était un gentilhomme polonais appelé Polowski ;' il tira de pneuma, which signifies “ breath” or “air,” and it therefore sa gibecière de la viande froide, du pain, du vin, et en offrit à means the science which treats of air; not that it is occupied Fodora, 10 ce qui la ranima bientôt. Puis il prit l'enfant par la exclusively with air, but as in Hydrostatics water is taken as a main et l'emmena dans son château," éloigné d'environ deux type of all liquids, so here air is taken as a type of all gases, lieues.
being the most familiar of them all. There are many different Là, Fædora accueillie (9) avec bienveillance par la femme du gases, but in their physical properties they, for the most part
, noble Polonais, 12 ne tarda pas à se rétablir dels toutes ses very closely resemble common air ; and the points of difference souffrances. Elle put alors leur raconter tout ce qu'elle savait in their composition and chemical properties it is not our prode son histoire.14 Emus jusqu'aux larmes par le récit de vince to treat of here. When, therefore, in these lessons we l'enfant, Polowski et sa femme la comblèrent des plus touch-speak of air, it should be remembered that the results obtained antes caresses, 15 et Fodora n'eut bientôt plus que le souvenir de are, with necessary modifications, true of other gases. ses maux.
In many of their properties gases are very closely allied to Plusieurs années s'écoulèrent ainsi sans qu'on apprit (h) rien liquids, hence many of the principles we arrived at in Hydrodes parents de Foedora. Cependant, elle avait grandi en sagesse statics, as relating to liquids, apply equally to gases-their et en beauté ; rien n'avait été négligél7 pour former au bien son particles move over one another with scarcely any friction, and cæur et son esprit. Elle avait alors quinze ans.18 Chaque they transmit pressure equally in all directions. année, le jour de sa délivrance était un jour de fête.19 Durant There is, however, this great difference, that the ultimate l'une de ces réunions, tandis que Fedora racontait de nouveau particles of any liquid have a certain amount of attraction for les accidents de son enfance2o si agités, et passait en revue tous each other, while those of a gas repel one another; and thus, if les bienfaits dont la comblaient tous les jours ses parents d'adop- the space in which it is enclosed be enlarged, it will at once extion, on entendit l'explosion d'un coup de feu”l parti à quelque pand and completely fill it. If a liquid is contained in n vessel, distance du château,
the pressure it exerts upon the sides results solely from its Le vent soufflait avec violence, 22 la neige tombait à gros weight; but when a gas is thus confined there is, in addition to
this, a pressure on all parts of the containing surface, arising from maintain a supply of pure air, pipes are brought down from the elastic pressure of the gas itself. A gas, too, is highly-in some powerful force-pumps, and by means of these the bell is fact, almost indefinitely-compressible, while, as we have seen, kept full
, and supplied with fresh air. Thick glass windows are for all practical purposes, a liquid is absolutely incompressible. placed in the top to give light to those within. The condensa
Now as air is by far the most important of all gases, we tion of the air by the pressure of the water produces a sense of shall inquire a little into its properties before considering gases oppression, and frequently a pain in the eyes or ears: this, howgenerally. The atmosphere, then, is a layer of air completely sur. ever, gradually passes away. The men are sometimes provided rounding the earth on all sides, and extending upwards to a height with a waterproof dress and helmet, clothed in which they can usually computed at about forty-five or fifty miles. It fills every get out of the bell, and walk about at the bottom, air being conspace on the earth's surface, and presses, as we shall see, on all veyed to them by pipes. Frequently, indeed, the bell is dispensed bodies with an immense force. We are completely surrounded with altogether, and these dresses only used; the air-pipe opening by it; we live, in fact, at the bottom of an immense ocean of it; into the helmet, and the excess and waste air escaping by a suitable and yet, except when it is put in motion, we scarcely notice its valve; heavy weights are then fastened to the feet to keep the presence. Though thus unnoticed, however, it is of the utmost man at the bottom. Sounds made under water are conducted importance to us. Without it all life, animal or vegetable, by it to a considerable distance, and hence by taps on the sides would droop and die ; our fires and lamps would refuse to burn; of the bell messages are transmitted to the surface. One of and when the sun shone, instead of even gradations of light and these plans is frequently used for the recovery of property from shade, we should have either almost intolerable brightness or the sunken vessels, and for fixing tackle, so as to endeavour to raiso blackest darkness. No clouds would shade the sun, nor any rain them to the surface, and large amounts of treasure have frefall to water the earth ; all would be a barren, lifeless blank. We quently been thus recovered. The action of the condensingsee, then, something of the benefits we derive from it, and these pump, for forcing down the air, will be explained in a future surely render it desirable for us to study some of its phenomena. lesson. Its chemical properties have already been explained in our lessons Since air is a material substance it has weight, and we must on Chemistry; we need, therefore, say little about them. It is now see how to prove this, and also to ascertain what its weight not a simple gas, but a mixture consisting almost entirely of really is. oxygen and nitrogen, in the proportion of nearly 21 parts by To weigh an ordinary substance, we have merely to place it volume to 79 of the latter. Small quantities of carbonic acid in one scale of a balance, and place our weights in the other. and watery vapours are also present. The former of these is a This plan, however, will not answer here, since the air presses poisonous gas given off in the breath, and by fires, and burning on both; we have, therefore, to proceed in a different way. A bodies, and would speedily accumulate, so as to destroy life, had large glass globe, having an opening which can be closed by a not the Creator mercifully caused that trees should feed upon it, stop-cock, is procured, and by means of an air-pump it is comremoving the carbon it contains, and building that into their pletely emptied of air, and very accurately weighed; the air is own structures, while they set free again the oxygen which was then admitted to it, and the difference thus produced in the united with it in the gas. Winds mix the different portions of weight accurately noted. By now filling the globe with water the air, and thus remove this gas from crowded cities and bring its cubic contents may easily be ascertained, and from this we in its place purer air.
can calculate the weight of a cubic foot or any other volume of The watery vapour in the air varies very greatly in amount, air. and, as will appear, performs a very important office, being the In this way it is found that 100 cubic inches of air weigh, cause of rain and dew.
at the ordinary temperature and pressure of the air, a little over Though we notice the presence of the air so little, it is a 31 grains, and hence a cubic foot weighs about 11 oz. Though material substance; that is, it occupies space to the exclusion this weight appears small, and actually is so, when compared of other bodies.
with the weight of solids or liquids, yet if we calculate the A simple experiment will furnish conclusive proof of this. weight of air in any building we shall find it much more than Float a cork on a vessel of water, and invert over it a glass we expected. Suppose, for instance, we have a room 20 feet by jar. On pressing the jar down, the position of the cork will 15 and 10 feet high, it contains 3,000 cubic feet. The air in it show that the level of the water inside is below that outside; therefore weighs about 3,750 oz., or rather more than 2 cwt. something, then, must be there to press it down, and that The weight of other gases may be ascertained in a similar way, something is the air contained in the jar. If we have a and by comparing the weights of equal bulks of them with that stop-cock inserted in the top of the jar, or use a bottle with the of air, at the same temperature and pressure, we can ascertain bottom cut off, on opening the mouth the air will rush out, and, their specific gravities. the pressure being removed, the water inside will rise to its In making this experiment an air-pump was required; and as former level. We see, then, that though the jar would have this piece of apparatus is necessary in nearly all pneumatic been stated to be empty, it was in reality full of air. In the experiments, it will be as well to explain its construction at same way a bladder or air-cushion may be filled with air, and once, before passing on to notice the effects produced by the will sustain pressure almost as if it were solid.
weight of the air. If we take a large sheet of thick cardboard, or an open The simplest instrument for removing the air from any vessel umbrella, and run, holding it so that the air meets its flat sur. is that known as the exhausting syringe, and is represented in face, the resistance we shall experience will afford an additional Fig. 1 on the next page. A is the globe from which the air proof that the air which thus opposes the motion is really a is to be removed; this is furnished with a stop-cock, B, and material substance.
screws on to the end of the syringe ; C D is the cylinder, which The experiment we mentioned above-namely, immersing a is accurately turned inside, and in which the piston E works airglass jar in water—though so simple, is an important one, as it tight. In order to prevent leakage past the sides of this, a illustrates to us the principle of the diving-bell
. In laying the groove is turned in it
, as shown, and cotton or some similar foundations of bridges, piers, or other structures rising out of packing is wound tightly round, and then saturated with oil. In water, it is very desirable, and, in fact, absolutely necessary at this way it is made to fit much more tightly, and the wear is times for some person to be down at the place where the work likewise greatly diminished. A pipe, closed by a stop-cock F, is going on. Now if a coffer-dam had to be constructed to opens into the cylinder near its lower end. keep out the water it would add greatly to the expense of the Let the piston be at the lower end of the cylinder; the valve work, and also to the time occupied, but this can be dispensed F must now be closed and B opened; the piston is then raised to with by the use of a diving-bell. This consists merely of a large the top, and the air contained in A will expand and fill the iron vessel, made strong enough to resist the pressure of the cylinder. B is now closed, and F opened, and as the piston water. It is open at the bottom, and has a ledge round it, on descends it will force the air contained in the cylinder through Which people may sit. The bell is raised a little above the F. The taps are again reversed, and a similar process repeated level of the water, so that the workpeople may enter it, and till nearly all the air is removed. then it is gradually lowered into the water by means of chains; If the area of the cylinder be just equal to that of the globe, the air inside keeps out the water, so that those within remain one-half the air in the latter will be removed by the first stroke, dry. As the bell descends, however, the air becomes compressed, and the density of that within will be of what it was; similarly, and the water rises a little way. To remedy this, and also to after the second stroke it will be }, after the third ], and so on.
It would be found very inconvenient, however, to open and ( valves at the top and bottom are fixed to opposite ends of a rod close the taps continually; in practics, therefore, they are re- which passes through a hole in the piston. As the piston rises placed by valves, which are constructed of small piece of oiled it closes that at the top, and forces the air above it out by : silk covering a hole bored in a metal plate. These only allow of separate valve, while at the same time it opens the lower exhaust the passage of the air in one direction, and that at r is made to valve, and thus draws a further supply of air from the receiver. open outwards, while B opens upwards.
Similarly, in descending it forces out the air below it, and exThis syringe will answer well for exhausting a globe, but the hausts above. It thus has the advantage of the double-barrel mouth of this must necessarily be small, and so but few things pump without its complications. It is also constructed so that could be introduced and operated upon, The pipe
the cylinder oscillates, and the piston is worked by a from the bottom of the syringe is therefore pro
crank; thus, instead of the usual alternate motion, longed, and made to pass through the centre of a fixed
c which is very awkward, a fly-wheel turned by a winch metal disc, the surface of which is ground so as to be
gives motion to the pump. perfectly true. Glass receivers are also ground true on
We can now pass on to give other illustrations of their open ends, so that when a little tallow is smeared
the weight of the air. If we take a ball of cork, and on to fill the small irregularities of the surfaces no air
suspend it from one end of a scale-beam, placing a can pass between them and the pump-plate. Any sub
piece of metal at the other end so as just to balance stance can then be placed on the plate, and have the
it, and then transfer the whole to the receiver of the air removed from it.
pump, and remove the air, we shall find that the cork In the better class of air-pumps two syringes are
will overbalance the weight, and descend. The reason placed side by side, so as to work alternately; the
of this is that the cork displaces a larger bulk of air construction of the whole arrangement is then similar
than the piece of metal, and therefore is supported to to that shown in Fig. 2. AA is a pipe which opens
that extent by the air; but as soon as this support is through the pump-plate, and also communicates with
removed it sinks. If, therefore, we would know the each of the barrels, BB', valves, C, C, opening upwards,
true weight of any substance, we must weigh it in being placed at the bottom of the barrels. The
A vacuo. piston-rods are cut into notches which work in the
The ascent of a balloon furnishes another illus. teeth of the wheel D, so that as the handle e is
tration of this. A body floats in water because it worked alternately backwards and forwards they rise Fig. 1.
has less weight than in equal bulk of water, and and fall, and while the piston in B is rising that
in the same way a body will float in air if it has less in B' is being depressed. The great advantage of this is that weight than an equal bulk of air. Now a balloon is so con. the pressure on the handle remains almost constant. When the structed as thus to be lighter, and therefore the weight of the receiver becomes nearly exhausted, the external air presses very surrounding air buoys it up. If it were possible to construct & heavily on the top of the piston, which therefore requires a con. hollow vessel strong enough to bear the pressure of the air, and siderable force to raise it; but when there are two arranged yet weighing less than the air it displaces, it would ascend ; thus, the pressure on the one nearly balances that on the other, this, however, has not been accomplished, nor does it seem at and thus much less labour is required. As the air in the receiver all likely to be done. The simplest balloon is a common soap F becomes rarefied, the external air presses on it with consider. bubble, the breath used in blowing it is warm, and sufficiently able force, fixing it firmly on the plate, so firmly, indeed, as to lighter than the air to carry up with it the delicate film of tender it almost impossible to stir it. An opening, closed by a soapy water which envelopes it. If bubbles be blown with screw, is therefore provided, by which the air can be allowed to hydrogen gas instead of air, they will ascend more rapidly. A enter the receiver when it is desired to remove it from the plate. peculiar soap solution is now to be obtained, bubbles blown A gange to indicate the degree of rarefaction produced is affixed with which may be attached to a small paper disc, and made to the machine at G. Since at each stroke the air in the receiver to take up with them a miniature car. expands and fills the cylinder, it is obvious that the larger the The Montgolfier, or fire-balloon, was the first used, being incylinder is in comparison with the re
vented towards the end of the last cenceiver, the less the number of strokes re
tury. It consists of a large, hollow vessel, quired to produce any given degree of
made of varnished silk or thin canvas, and exhaustion.
surrounded by a network of ropes suffiIf the capacity of the cylinder be that
ciently strong to support a furnace just of the receiver, of the air will be re
under the open mouth of the balloon, and moved by the first stroke, and conse
also the car, in which the aëronaut can sit. quently only will remain; of this will
A large fire being kindled in the furnace be removed by the next stroke, leaving in
the air inside becomes highly rarefied, and the receiver 1-7, or of the original
thus weighs so much less than an equal quantity. In this way we see that after
bulk of the external air that it will raise five strokes, upwards of j of the air will
the balloon with its furnace and car. Fresh have been removed, while if the cylinder
B supplies of the fuel, which is of a highly have only lo of the capacity of the re
combustible nature, are laid on when it is ceiver twelve strokes will be required to
desired that the balloon should rise to a produce the same degree of exhaustion.
greater height, while damping the fire soon It is important, then, to have the cylinder
causes it to fall. The use of these balas large as practicable; still, since the
loons is, however, attended with great pressure of the air on it may become
danger, as the flames may catch the car thirteen or fourteen pounds to the square
or the balloon itself. Several accidents of inch, there is a limit soon imposed by the
this kind have occurred, and only recently power which would be required to work the machine. The | some attempts made at the Crystal Palace to send up a firevacuum produced by an air-pump constructed in this way balloon ended in this way. is not absolutely perfect, for the pressure of the air in the re. Another objection to their use is that a large supply of fuel ceiver has to open the valves, and
when a certain amount has must be taken up with the balloon, and this adds considerably been removed, that which is left ceases to have sufficient expan- to the weight to be raised. Hence these are rarely used, except sive force to do this, and then no further exhaustion can be pro- for curiosity ; the gas balloon—which we shall describe in our duced. To obviate this the valves in the best machines, instead next lesson, and which is much more manageable — having of being made of oiled silk, are conical plugs fitting into settings, almost
superseded them. Frequent attempts have been made and are worked by the piston instead of opening by the force of to produce a machine acting on the principle of the screw, which the air. In this way a much more perfect vacuum may be shall be able to rise in the air, but at present they have been obtained. A still further improvement
has been effected
. Bianchi. His machine has only one barrel
, but this is so con- Aëronauts are, however, still at work on the subject, though the structed as to be double-acting. The conical plugs closing the solution of the problem seems far distant.
LESSONS IN GEOGRAPHY.-XXXIII. we have yet all to learn-by the indefatigable and adventurous
The surface of the whole continent is reckoned to contain AFRICA, the mystery of ancient and the problem of modern just about 12,000,000 square miles, and about 130,000,000 in. times, is the south-western part of the Old World. This con. habitants ; which gives on an average nearly eleven to every tinent is situated chiefly in the torrid zone, the exceptions square mile. being Egypt, Barbary, and the British colonies at the Cape of Boundaries.-Africa is bounded on the north by the MediGood Hope. The central regions and the coast on the eastern terranean Sea ; on the south by the great Southern Ocean, or and western sides were considered as almost wholly uninhabit- rather the waste of waters in which the South Atlantic Ocean able by the ancients, as far as they chanced to be acquainted and Indian Ocean meet, lying to the north of the Antarctio with them, and with regard to Europeans these districts have Ocean ; on the east by the South Atlantic Ocean; and on the not greatly improved in character in this respect at the present west by the Indian Ocean. day; although, judging by the accounts of all the recent explor Oceans, Seas, Gulfs, etc.--The Red Sea and the Gulf of Aden ing expeditions into its interior, there is a bright future yet in lie to the north-east of this continent; and the Mozambique store for the negro races of Africa, who are fitted to endure the Channel, 250 miles in width at its narrowest part, to the east, intense heat, the heavy periodical rains, and the fever-fraught between the south-east coast and Madagascar. The South exhalations of the marshy
Atlantic and the Indian districts of the land in
Oceans commingle their which an all-wise Creator
waters south of the Cape has placed them, when
of Good Hope; the Atintercourse with English Nose
lantic washes the shores of traders shall have awa tandig
Guinea and Lower Guinea, kened them to the bene
in the Gulf of Guinea, and fits that civilisation and
the Bights or Bays of the arts of peace, agricul.
Benin and Biafra. The ture, and commerce bring
Strait of Gibraltar sepato every land that uses
rates the rooky coast of them aright. Home of Memphiş Helouan
Marocco and Spain. In Extent.--Africa extends Atfield
the Mediterranean are the
Almbah from north to south about redine
Galfs of Sidra and Cabos, 5,000 miles; the most nor
on the north of Tripoli thern point being a headPeshury Bibaba
and Tunis ; and in the land of Tunis, called Ras. Abu Girgah
south, on the shoros of al-Kran, in lat. 37° 20' N., Minich"
Cape Colony, are St. Helena and long. 9° 48 E.; and ENTRAL
Bay, Table Bay, False Bay, the most southern point,
Algoa Bay, etc. ; while on Cape Agulhas, in lat. 340 EGYPT
the east coast are Delagoa 50 S. and long. 19° 57' E.
Bay, Sofala Bay, and many It extends also from west
others of less extent and to east about 4,600 miles ; b'Boomlich Saateh
importance. the most western point
Islands.-The principal being Cape Verd, in lat. Girgch Saiad
of the African islands, Ma14° 45' N. and long. 17°
dagascar of course being ex32 W.; and the most
cepted, lie in small groups eastern point, Cape Guar
and clusters. The Azores, dafui, in lat. 11° 41' N.,
which are usually reckoned
Canch and long. 51° 22' E. The UPPER EGYPT
as African islands, though equator, passing over the
they lie above the latitude Gulf of Guinea, crosses
of the most northerly point
El Arenich, this continent over Lower
of Africa, and in the lati
Siassouan Guinea on the west, near
tude of Portugal, to which the island of St. Thomas
Cataracts Dard C.Benass,
they belong, are about 800 and mouth of the Gaboon
miles to the west of the River, and over Zangue
last-named part of the bar on the east, near the
mainland of Europe. Mamouth of the river Juba ; Lon. E
deira, which also belongs
Greenwich. it thus cuts off about one
to Portugal, and Porto third of this continent to
Santo, lie off the west coast the south in the form of a peninsula ; the Tropic of Capricorn of Marocco. · The Canaries, belonging to Spain, of which the cuts off from this peninsula, in like manner, a smaller one, con chief are Teneriffe and Grand Canary—the former remarkable taining Cape Colony and Kaffraria. The Tropic of Cancer for its high mountain, called the Peak of Teneriffe, which cuts off Northern Africa, crossing Sahara, or the Great Desert, rises to the altitude of 12,180 feet above the level of the and the Libyan Desert, and dividing Egypt from Nubia. Be- sea, or rather more than 2 miles of perpendicular heighttween the two tropics, and between the meridians of 10° and lie off the north-western corner of that part of the Sahara that 30° E.—that is, nearly those of Tunis and Alexandria—there abuts on the Atlantic. Of the Cape Verd Islands, the chief is & vast tract of unexplored country, especially south of are Santiago and St. Vincent; the chief town of the former the equator, which is little less than one-half of the con- being Porto Praya, formerly the seat of government, which has tinent; the only parts of this vast region which have been been transferred to Porto Grande, the chief town in the latter. partially explored being, firstly, the countries that lie around Among the tropical islands should be reckoned Fernando Po, Lake Tohad; secondly, the regions in which lie the great equa- Prince Island, St. Thomas Island, and Annabon, all in the Gulf torial lakes of Africa -- namely, Lake Victoria Nyanza, dis- of Guinea. Ascension Island and St. Helena--the spot on which covered by Captain Speke ; Lake Albert Nyanza, discovered the Emperor Napoleon fretted through the last few years of his by Sir Samuel Baker; and Lake Tanganyika, discovered by existence-lie out in the midst of the South Atlantic Ocean. Captains Burton and Speke; and thirdly, the countries between Madagascar, in the Indian Ocean, is about 1,050 miles long, and the parallels of 59 and 25° S. latitude, which have been about 300 miles broad in its widest part; it is reckoned to contraversed in their breadth from the Indian Ocean to the tain about 200,000 square miles. The population is reckoned to Atlantic Ocean, and in their length from the Kalahari Desert be between 4,000,000 and 5,000,000. The Isle of Bourbon or to the equatorial districts west of the great lakes--about which Réunion, belonging to France, lies about 400 miles eastward of VOL. III.