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any extent, the elastic force, however, increasing greatly. If we
now remove the pressure the air will expand again, so as to fill HEIGHT OF ATMOSPHERE—ELASTICITY OF AIR—BOYLE AND exactly the same space as it did before, and the piston will stand MARIOTTE'S LAW-MANOMETER — AIR-GUN-AIR-THERMO
just where it did at first. METER--EXPANSION OF GASES BY HEAT-CO-EFFICIENT OF
A familiar example of this compressibility of the air is seen if
we invert a tumbler over a piece of cork floating on the surface We have now seen the most important effects of the weight and of water. The water will rise a little way inside the tumbler, consequent pressure of the air, and also the means of measuring just as it does in a diving-bell, unless a fresh supply of air is this pressure and its variations. The question, what is the true introduced from above by means of the force-pamp. height of the atmosphere, is one which we cannot fully solve. Though the apparatus just described shows very clearly the Each sncceeding layer of air which we meet in ascending is less fact of the compressibility of the air, and the consequent increase dense than the one below it, and thus, theoretically, the limit to of its elastic pressure, yet it does not afford a very ready way of the height of the air is when the repulsive force exerted by its measuring exactly the alteration of volume, and a different plan particles on each other is exactly balanced by the earth's attrac-has therefore been adopted by experimentalists on this point. tion for those particles. We cannot calculate exactly at what the laws of this compression were studied at the same time in height this would occur, but the question is of no great practical France by M. Mariotte, and in England by Mr. Boyle,
and they importance, and we may safely say that the limit of the atmo- both arrived at the same result, which is known as Boyle and sphere is about fifty miles above the sur
Mariotte's law, and may be stated as folface of the earth. A few scientific men
lows:have, as a result of observations on the
The temperature remaining the same, refractive power of the air as shown in its
the volume of a given quantity of gas cansing twilight, stated the limit to be
varies inversely as the pressure which it much above this; but if any air does exist
bears. at a greater height, it is in a state of such
Fig. 15 represents the apparatus which rarity that it would scarcely be possible
is usually employed in the proof of this, to prove its presence even by the most
and which is known as Mariotte's tube. A delicate tests.
long glass tube is sealed at one end, and We now pass on to notice the compres
bent round not far from this end, as sibility and elasticity of the air. As we
shown in the figure. It is then fixed to al saw in our first lesson, if any gas be con
board, and a scale divided into inches and fined in a vessel, it exerts a pressure
decimals of an inchis placed against each against the sides altogether apart from
limb. Both these scales commence at its weight, and this pressure is exerted
exactly the same level, and a little meragainst the upper part of the vessel as
cury is first poured into the tube, so as to well as upon the lower side. Now, this
Fig. 19. fill the bend and stand in each tube even pressure arises from the elastic force of
with the lower end of the scale. This may the gas, and depends alone upon its com
with a little practice be easily accompression and temperature. If the volume
plished. The air in the shorter leg is now occupied by it be in any way diminished,
exposed to the pressure of the atmosphere, that is, if the same quantity of gas be
which, for simplicity, we will suppose to made to occupy a smaller space, the pres
be just equal to that of a column of mersure will be increased; or if the space oc
cury 30 inches high. Now pour mercury cupied remain the same, and the tempera
into the longer limb till it stands 30 inches ture be raised, the pressure will also be
higher than in the other limb; the air in increased.
this will then be compressed with double We have, then, to investigate the propor
the force that it was before, and, by notion which this increase of pressure bears
ticing the height on the scale, we shall to the diminution of volume. That air is
find that it now occupies 10 divisions compressible to a very great extent, and
instead of 20-that is, under double the that the pressure it exerts increases with
pressure it occupies half the space. Let the compression, is readily seen. Procure
a further quantity of mercury be now a stout glass or metal tube A, (Fig. 14),
poured into the tube, till it stands in with a stopcock, B, let into it near the lower
the long limb 60 inches above the height end, and a piston, c, fitting it air-tight.
in the other, and again notice the space When the tap, B, is open, we can place
occupied by the confined air; we shall the piston at any part of the tube, and
now find it to be 6j divisions, showing the pressure on each side of it will be the same. If now we that with three times the pressure it occupies only one-third close the stopcock, the air within will be cut off from all com- the space. Experiments have been carried on in this way munication with the external air, and therefore
no pressure will till a pressure of nearly 30 atmospheres has been obtained, and be communicated to the under side of the piston from without; this law is found to hold true with most gases. With some, and yet it does not
fall, though pressed upon by the air with a however-as, for instance, carbonic acid—it is found that when force of 15lbs. per square inch. The reason is that the elastic very high pressures are attained they suffer rather more
diminuforce of the confined air is sufficient to balance this pressure, and tion in volume than this law points ont, but the cause of this is therefore the piston remains at rest. Were we to place the whole believed to be that they are then approaching the degree of conunder the receiver of an air-pump, and thus diminish the in- densation at which
they assume the liquid state. In fact, ternal pressure, we should find that the elastic force of the carbonic acid has been liquefied at a pressure of about 50 atmoair within would overcome the diminished pressure, and cause spheres. In conducting these experiments, allowance must be the piston to rise. If now we increase the pressure, either by made for the increase of temperature in the gas caused by the adding weights to the top of the piston or by pressing with the condensation. It should, therefore, be allowed to cool to its hand, we shall find an increasing resistance to our efforts. This original temperature before the measurement is taken. We may arises from the increased tension of the air, and, if a weight
has state, then, generally that the
volume of a gas varies inversely as been placed on, we shall find that after sinking a little way the the pressure. The importance of knowing this is great, for, as piston will come to rest, showing that the elastic force of the en- the pressure of the air is continually changing, a given weight of closed air is then equal to the pressure of 15lbs. per inch, and to any gas will occupy more space at one time than at another. the added weight in addition. Let another similar weight be now Hence, in all experiments with gases, the pressure as shown by a added to the piston, and it will sink still lower, though it will barometer has to be noted. As we have already stated, 30 not move as much as it did before. In this way we shall find inches is taken as the standard height; if, then, we have an
more weights we can compress the air to almost amount of any gas which occupies, say, 230 cubic inches when VOL. IV.
that by adding
the ḥarometer stands at 28-90, we must find how much space it in the construction of a fountain. A strong metal vessel is would occupy when the pressure is increased to 30 inches, and constructed, and a tube, dipping nearly to the bottom, is fitted this we can easily do by the following equation :
tightly to its mouth. A stopcock is inserted in this tube, and As 30 : 28-90 :: 230 : 221-56 cubic inches.
a screw is also cut in the upper end of it. The vessel is then
filled to about three-fourths of its height with water, and, by This, then, is the space which the gas would occupy when the means of a condensing syringe screwed on to the pipe, the air barometer stands at 30.
within is powerfully compressed, the fresh air bubbling up When we speak of a pressure of so many atmospheres, it through the water. The tap is then closed and the syringo remust be remembered that by an atmosphere is meant a pressure moved, and when it is desired to start the fountain we have only of 30 inches of mercury; and as a cubic inch of mercury weighs to screw a jet on the tube, and on turning the tap the tension of 0:491lb., the pressure on a square inch is 30 x 0-4911b. the air will be such as to force the water through the jet with 14.73lbs. We can thus easily solve questions like the following: sufficient velocity to raise it to a considerable height.
What is the pressure on a portion of the surface of a boiler The air, it must be remembered, does not create any force; it measuring 3 inches each way, when the steam has a tension of merely stores up the force exerted by the hand in working the 4, atmospheres ?
syringe. It is, in fact, a reservoir of power, and in many inThe pressure on each square inch is equal to the weight of stances it becomes of great service by its action in this way. 41 x 30, or 135 inches of mercury, and the surface has an area In this case the air was condensed, and its elastic force of 9 square inches; the total pressure, therefore, amounts to thereby greatly increased; at ordinary pressures, however, it has 135 X 9 X 0:4911b. = 596.565lbs.
quite enough elastic force to act in the same way if we allow In many operations it is important to have some means of the jet to play into a vacuum. There are two modes of showing measuring the prossure exerted by a gas or vapour, and the this experiment. instruments employed for this eni, are called manometers. In If a small vessel, similar to that described above, be placed nearly all of these the pressure of the atmosphere is taken as under a tall receiver, and the air rapidly removed, the effect will the unit. The pressure-gauge of a steam-engine is merely one be seen. The simplest plan of making the vessel is to take form of this instrument.
a small flask with a tightly-fitting cork, through which is passed The most common manometer is that which acts by means of a glass tube, drawn to a jet at the upper end, and reaching compressed air. It consists of a small vessel of mercury with a nearly to the bottom of the flask. To show the experiment tube closed at the upper end dipping down into it. This vessel well, the receiver must be very rapidly exhausted, otherwise the is so placed that the surface of the mercury is exposed to the air slowly expands, and merely causes the water to run slowly pressure which we want to measure, and as this increases the out of the jet. The plan usually adopted is to exhaust a second mercury is forced up into the tube, compressing the air above it, large receiver on another pump-plate, and so arrange the two and indicating the pressure by a scale marked
at the side of the that a connection may be made between them by opening a tap. tube. The graduations on the scale are not equidistant, that the air is thus almost instantaneously rarefied to a considerable marked 2 being at the middle, that marked 3 one-third of the extent, and the experiment answers. The second plate in this height from the top, and so on. Sometimes, instead of a vessel description of pump is known as the transfer-plate, and is freof mercury there is merely a U shaped tube, with the bend filled quently found very convenient. with mercury and the open end connected with the boiler. It The other mode of exhibiting the experiment is rather simpler. acts, however, in the same way.
A jet is screwed into the aperture of the pump-plate, and the pump If, instead of the pressure on any gas being greater than that is so constructed that the plate, together with a portion of the of the air, it is less, the gas still expands in the same proportion. exhaust-pipe closed by a stopcock, may be removed without the Thus, if one-half the pressure be removed it will fill twice the admission of any air. The end of this tube is then plonged space. To prove this, a graduated tube is nearly filled with beneath the surface of water, and on opening the tap, the water mercury, and inverted into a tubular vessel filled with the same will be forced up through the jet by the pressure of the air, and liquid. It is first sunk so deep that the level of the mercury is thus produce a very pretty fountain in vacuo. the same inside as outside, and the volume of the contained air Having seen the mode of ascertaining the alteration which is is carefully noted. The tube is now raised till the air has ex- caused in the volume of any quantity of a gas by variation in panded to exactly double this volume, and the mercury in the the pressure, we have now to examine the effects produced by tube will then be found to stand at just half the height of the variations in the temperature. These variations are considerable ; barometer above that outside.
it is therefore necessary, in order to measure the exact quantity A simple experiment shows that if external pressure be of a gas, to bring it to a standard temperature, and, as already removed air will expand forcibly. Procure a very shrivelled stated, 60° has been fixed on as the most convenient. There is apple, and having placed it under the receiver of the pump, however, often difficulty and loss of time in bringing & 523 remove the air. The apple will expand, and look quite plump exactly to any temperature ; we want, therefore, when we know and fresh. If, however,
you admit the air in order to remove and the volume at any other temperature, to be able to calculate enjoy the apple, the pressure at once shrivels it up as before. what it would be at 60o. In the same way, if one of the thin india-rubber balls frequently If we dip the neck of a retort beneath the surface of water. sold in the streets be nearly emptied of air and tied at the and apply heat to the bulb, we shall find a number of bubbles of mouth, the little air left in will, when it is placed under the air passing off through the water; and when the source of beat receiver, expand with sufficient force to distend the ball. So, is removed and the air cools again, the water will rise in the too, if a hole be pricked in the large end of an egg, the bubble neck of the retort to take the place of the displaced air. So, of air at the other end will expand sufficiently to drive out all the likewise, if we nearly fill a bladder with air, and, having tied the contents.
neck tightly, place it before the fire, the air in it will expand Several experiments may also be easily performed showing 50 as completely to distend, and perhaps burst the bladder
. the large amount of elastic force which may be stored up in We see, then, that
the air alters in its volume by a change of compressed air. The air-gun is, perhaps, the simplest illustra- temperature. tion of this. In it the elastic force of the air takes the place of This property of air is sometimes employed in the construc gunpowder, and propels the bullet with great velocity. A strong tion of a thermometer. Two forms of air-thermometer are recopper ball is made to screw on just below the lock of the presented in Fig. 16. In one, a straight glass tube, B, with a gun. By means of a condensing syringe, air is powerfully com- bulb blown at one end, is placed with its open end downwards pressed into this, and when the trigger falls it presses a pin, in a vessel of coloured water, A. Heat is first applied to the bulb
, and thus opens a valve in the ball and allows a portion of the c, greater than that which
it is required to indicate; a portion of air to escape. This strikes the bullet, and imparts such velocity the air is thus driven out of the tube, B, and the water rises to it as to make it a very deadly missile. If the ball be weil to replace it. The height at which this water stands depends charged, the gun may be discharged
from twelve to twenty times upon the pressure of the air in the bulb, c, and as this varies successively without condensing the air in it afresh;
the with the temperature, the column serves as'a
thermometer. In power, however, diminishes slightly each time that it is fired, as the other form represented, the tube is turned up so that a small the air becomes less dense.
quantity of air may be included
in the bulb, c, above the water, anı! This expansive force of compressed air is sometimes employed this, as it expands or contracts by the heat, causes the water to
rise or fall in the limb, A B. Beth are graduated by comparison Latin Nouns. Derived Adjectives. Saxon Nouns. with a standard thermometer. These instruments are highly vacca
vaccine sensitive to slight changes of temperature; they are, however, morbus, morbi
discase. affected by the height of the barometric column, and therefore a canis
dog. certain amount of uncertainty is introduced into their indica- auris
hostile The law showing the relation between the temperature ard
ocular the volume of any gas was discovered by Dalton, and has been
field. checked by many philosophers since that date. It may be stated
Mock. as follows:
guardianship. If any gas be allowed to expand freely under a constant pres- manus
hand. sure, its increase of volume when raised from 32° to 2120 will caput, capitis
Tread, be equal to 0-366 of its original volume, and this law of increase merces
horse. holds true in the same proportion for intermediate temperatures. equus, or eques
home. Now, there are 180° between these two temperatures; the ex
island. pansion for each degree is, therefore, Tło of 0-366, or about a, culina
kitchen. and this fraction is called the co-efficient of expansion. A gas, lax, lucis
light. then, expands ia of its volume at 32° for each degree that it is pulmo
lungs. raised above that point. This rule enables us to make the cal- mens, mentis
mind. culations we required, for 492 cubic inches at 32° will occupy pecunia
money. 493 at 33°, 510 at 500, 520 at 60°, and so on. Suppose, then,
lunar we have the following question :- A quantity of gas is measured os (oris)
mouth, at a temperature of 76°, and is found to occupy 427 cubio
local inches; what is its volume at 600 ? We first find the proportion
place. vulgus vulgar
Tabble. between the space a gas occupies at 60° and at 76°, and, as we annulus (annus) annular
a ring. have seen, 492 cubic inches at 32° will occupy 520 at 60°, emulus
a rival. and 536 at 76o. The volumes are therefore in the proportion radix, radicis
root. of 520 to 536, and the following rule of three sum will there- regula
rule, fere give us the required volume :
maritime pastor pastoral
shepherd. As 536 : 520 :: 427 : 4144.
shoulder. Very often in chemical experiments corrections have to be latus, latèris
sido. made for pressure as well as for temperature. The process is,
sight. howerer, the same, and each correction may be made separately.
vernal The following examples will give the student good practice in
solar the application of these rules :
thigh. 1. If the barometer stands at 29-04, what is the pressure of the air dens, dentis
tooth. on a surface measuring 5in, x 6}in.?
treaty. 2. In Fig 15, if the length of the graduated portion of the shorter The similarity which exists between the Latin and the correlimb be 10 inches, and the mercury rise in it to a height of 5} inches, sponding English affords the student aid either to learn the at what height does the mercury stand in the other limb, the barometer words which are of Latin extraction found in English, or to being at 29in. ? 3. A volume of gas measures 219 cubic inches when the barometer for instance, that you meet with the word lateral, and know, or,
become acquainted with the Latin vocabulary itself. Suppose, stands at 28-7. How much will it measure at the standard pressure ?
4. Some gas at a temperature of 155o measures 1 cubic foot ; how not knowing, ascertain, that it is a word of Latin origin which much space will it occupy when cooled to 600 ?
signifies that which pertains to the side. Having this informa5. 140 cubic inches of air at 609 is heated till it occupies 215 tion, you are enabled to remember that latus, the noun cubic inches; what temperature has it attained ?
whence lateral comes, denotes the side. Or if you know that 6. When the barometer was standing at 28-78, and the thermometer latus means the side, then you readily infer that lateral means at 71°, a quantity of gas was found to measure 158 cubic inches. that which pertains to the side. In this way, you may make the How much would it occupy at the standard pressure and tempera- Latin roots with which you have become acquainted teach you ture!
the import of scores, nay, hundreds, of derivatives.
And observe, too, the specific service which the Latin element LESSONS IN ENGLISH.-XXXVI. renders. We have the noun side, but we have no corresponding LATIN STEMS (concluded).
Saxon adjective. The want is supplied by the Latin. SOME Latin stems supply us only in part with derivatives, giving, correspond. Thus ager, field, and agrarian do not strictly corre
In meaning, these nouns and adjectives do not always strictly for instance, the noun, and leaving the Saxon to furnish the spond; I mean, you cannot infer the exact meaning of agrarian, adjective; or giving the adjective, and leaving the Saxon to for instance, from the meaning of ager. You are thus taught that furnish the noun. Such a fact illustrates the composite cha- it is an intelligent, not a slavish, study in which you are engaged. racter of our present English tongue. If it be a token of perfec- Rules are not chains, but guiding-posts. tion in a language that it is produced and evolved out of its own Some of the words in the last lists, and in previous lists, elements like a tree, with its stem, branches, and leaves, the which appear as Latin or Saxon, are not exclusively of Latin or English has little claim to perfection. But a perfection of this Saxon origin. To wade, given as a derivative of vado, is a Saxon kind is only theoretical. That is the best language which most root, being common to both the Latin (Celtic) and the Saxon
answers the purpose of speech. Thus viewed, the tongues., Waddle, a diminutive of wade, is also Saxon. Rule English possesses very high qualities. In virtue of the facts and regula may be considered as the same word in different just mentioned, examples of which I am about to append, the forms; also oculus and eye; so insula and island ; leo and lion ; English possesses a most desirable variety, which adds not only mens and mind. Similar facts abound in our language, and to the colouring and polish of our style, but also to its capability show that in order to know one language well you must study
several, and that the proper way to study languages is to study LATIN NOUNS WITH THEIR DERIVED ADJECTIVES, AND COR- them in their mother tongues--in the primitive groups or classes RESPONDING SAXON NOUNS.
where they are found, and whence they shoot and branch. Derived. Adjectives. Saron Nouns.
I subjoin a list in which the richness of our language is still
more exemplified :-
Latin Nouns. Latin Adjectives. Saxon Adjectives. Saxon Nouns. cadaver cadaverous carcass, corpus, corporis corporeal bodily
Latin Nouns. initiam pectus, pectoris
Latin Nouns. Latin Adjectives. Saron Adjectives. Saxon Nouns,
THE LOVE OF KNOWLEDGE. frater fraternal brotherly brother.
But while I am descanting so minutely upon the conduct of the onus, oneris onorous burdensome burden.
understanding, and the best modes of acquiring knowledge, some men dies diurnal daily day.
may be disposed to ask, "Why conduct my understanding with such mors, mortis mortal
endless care l-and what is the use of so much knowledge ?" What terra terrestrial earthly
is the use of so much knowledge ?- what is the use of so much pator paternal fatherly father.
life --what are we to do with the seventy years of existence culpa culpable faulty
allotted to us ?--and how are we to live them ont to the last? I ignis igneous fiery
solemnly declare that, but for the love of knowledge, I should concaro, carnis carnal
sider the life of the meanest hedger and ditcher as preferable to that capillus capillary hairy
of the greatest and richest man here present. For the fire of our minds edium odious hateful hate.
is like the fire which the Persians burn in the mountains-it flames salus, salātis salutary
night and day, and is immortal and not to be quenched! Upon somecor, cordis cordial
thing it must act and feed-upon the pure spirit of knowledge, or upon cælum celestial heavenly heaven.
the foul dregs of polluting passions. Therefore, when I say, in conauxilium auxiliary helpful
ducting your understanding, Love knowledge with a great love, with a glacies glacial
vehement love, with a love coeval with life, what do I say bat love rex, regis regal
innocence-love virtue-love purity of conduct-love that which, if you lex, legis legal lawful law.
are rich and great, will sanctify the blind fortune which has made you vita vital lively life.
so, and make men call it justice; love that which, if you are poor, amor amorous lovely
will render your poverty respectable, and make the proudest feel it mater maternal motherly mother.
unjust to laugh at the mennness of your fortunes ; love that which will noz, noctis nocturnal nightly
comfort you, adorn you, and never quit you-which will open to you robur robust strong strength.
the kingdom of thought, and all the boundless regions of conception, mars, martis martial warlike
as an asylum against the cruelty, the injustice, and the pain that may aqua aqueous watery
be your lot in the outer world; that which will make your motives voluntas voluntary willing
habitually great and honourable, and light up in an instant a thousand femina feminine womanly woman.
noble disdains at the very thought of meanness and fraud! Therefore, sylva sylvan woody wood.
if any young man here have embarked his life in the pursuit of knowmundus mundane worldly
ledge, let him go on without doubting or fearing the event; let him annus annual yearly year.
not be intimidated by the cheerless beginnings of knowledge, by the The diverse meanings of capillary and hairy suffice to prevent darkness from which she springs, by the difficulties which hover around you from thinking that these pairs of adjectives one from the her, by the wretched habitations in which she dwells, by the want and Latin, one from the Saxon-are in each case identical in mean- her as the angel that guards him, and as the genius of his life. She
sorrow which sometimes journey in her train; but let him ever follow ing. Frequently, however, that which is indicated by the one is will bring him out at last into the light of day, and exhibit him to the that which the other signifies. When the two are of the same world comprehensive in acquirements, fertile in resources, rich in import, the one may be used for the other. To which of the imagination, strong in reasoning, prudent and powerful above his two you should give the preference depends on circumstances. fellows, in all the relations and in all the offices of life.-Sydney Smitk. If you are addressing the people, you will do well to employ words of Saxon origin. Nor fancy that by so doing you lower your style. Simplicity in diction, like simplicity in dress, betokens
LESSONS IN GEOGRAPHY.-XXXV. real respectability. Write, because you have something to say ;
CONSTRUCTION OF MAP OF AFRICA, ETC. and if you have nothing to say, do not write ; and if you write, The projection for a map of Africa is constructed on a prinwrite so as to be understood by those for whom you write; the ciple entirely different to that of the conical form of projecbest style is that which is most readily understood.
tion used for Europe and Asia. It will be seen, on reference COMPOSITION AND PARSING.
to our map of the continent of Africa (Vol. III., p. 357), that Make short sentences out of the list which I now give of
this division of the world is pretty nearly bisected, as far as
length is concerned, by the equator, the most northern point Words with their Proper Prepositions.
of the mainland being rather more than 37° north of the equator, Words.
while its most southern point is nearly 35° to the south of that Deviate from,
via, a way.
line. Considering the equator, then, as the centre parallel of Devolve on, upon,
volvo, I roll.
the parallels of latitude that traverse Africa, it is plain that Devote to,
votum, a vou. Dictate to,
dico, I say.
a straight line supposed to pierce the sphere at 20° or 250 Die of (a disease), by (the sword or
north and south of the equator, would be parallel to the aris famine), for (another)
of the sphere, and not inclined to Differ with a person in opinion),
it, as in the case of straight lines from a person or thing in fero, I bear.
piercing the sphere in two points, some quality)
both of which are on the same side Different from,
fero, I bear.
of the equator; and it is equally Difficulty in,
clear that a line entering the Diminution of,
sphere and coming out of it again Disabled from, Disagree with,
in such a manner as to be paralDisagreeable to,
lel to the axis of the sphere, would Disappointed of (a thing not ob
lie in the surface of a cylinder tained), in (a thing obtained),
as in the annexed figure, and not Disapprove of,
in a cone. It is true that the Discourage from,
projection of a map of Africa Discouragement to,
might be developed on the surface Disengaged from,
gage, a pledge.
of a cylinder supposed to circumDisgusted at, with,
gustus, taste, Dislike to,
scribe the sphere after the manner of the kind of projection Dismissal from,
called “Mercator's Projection," in which all the meridians Disparagement to,
and parallels are represented by straight lines at right Dispense with,
dispenser, to set free. angles to each other, and which peculiar mode of conDispose of, to, for,
struction will be explained in a future lesson. This style Dispossess of,
possideo, I possess. of projection, however, which is used in charts and nautical Dispute with,
puto, I think
maps, is not so well suited for representations of very large Study and parse carefully the following admirable remarks. areas of land, as the parts at the top and bottom--or, in other Having done so, write, as well as you can, on the same subject; words, north and south of the map-are distorted, and larger in and if you have kept your earlier attempts, compare them proportion than the central parts; and the mode of projection with the essay you produce on the love of knowledge. The most generally adopted for a map of Africa is, in consequence, momparison will give you both instruction and encouragement. that which we are now going to describe.
The main features of this projection consist in tracing the To do this, a diagonal scale must be constructed (as in the case of parallels in parallel straight lines, instead of representing projections for the maps of Europe and Asia) on the line assumed them by arcs of concentric circles as in the conical projec- at first as being equal to a space of five degrees of latitude. The tion, and by using curved lines for the meridians, instead of method of constructing this diagonal scale has been explained straight lines converging to a certain fixed point, as in the in Vol. II., page 356. We must now turn to the table of geoprojections for maps of Europe and Asia.
graphical miles in a degree of longitude under each parallel of On examination of our map of Africa, to which reference has latitude (Vol. II., page 357), and from this we find that the been made above, it will be seen that the meridian of 15° length of a degree of longitude on the fifth parallel north or E. has been selected as the central meridian of the map, south of the equator is 59:77 geographical miles. Opening the which crosses the equator at right angles, but which does compasses to this extent, as represented on our diagonal scale, not appear in the map itself. For this central meridian line set off distances along the fifth parallel of latitude north and our readers may select the meridian of 15° or 20°, as may south of the equator, on either side of the central meridian, as appear most desirable. We shall, however, in the following far as the border-line of the map will permit, and proceed in the description, take the meridian of 150 as the central meridian in same manner along each pair of parallels of latitude north and our map, and—supposing that the majority of our students who south of the equator, ascertaining the distance equivalent to five are following these lessons in Geography, and constructing maps degrees of longitude under each parallel in question from the from our instructions, are working on a large instead of a table already mentioned, and opening the compasses to the small scale-imagine meridians and parallels to be drawn inter- proper extent in each case by aid of the diagonal scale. The mediate to those which appear in our map, so that these lines points thus found on each parallel will be those through which would be but five degrees apart in our learner's projections, the meridians must be traced. This may be effected by drawing instead of ten degrees as in the map; that is to say, a parallel short straight lines from point to point in each successive paralwould be drawn at every fifth degree north and south of the lel to the north and south, or by means of a thin band of steel, equator, instead of every tenth degree, as in the map, and a so bent that its edge may pass through every point marked for meridian at every fifth degree east and west from the meridian the passage of each meridian across the parallels. The border of 150 east from Greenwich, which we have assumed as the must now be completed, the degrees numbered, and the title of meridian in our projection that crosses the equator at right the map and scales of geographical and English miles inserted, angles, instead of marking in a meridian five degrees east and after which the outline and different places may be fixed in west of this central meridian, as in our map, and then drawing position as before. meridians ten degrees apart east and west, proceeding in each The following table will afford sufficient names for the condirection from the meridians that have been traced five degrees struction of a map of Africa on a small scale. If a large scale each way from the central meridian.
be adopted, as we have advised, the latitudes and longitudes Having drawn two straight lines at right angles to each other, may be obtained from the index of places appended to any ordione to represent the equator and the other the meridian of 150 nary atlas. Our readers will often find that the latitude and east from Greenwich, we must, in order to draw the parallels, longitude of a place according to one index will differ from the first assume a space equal to five degrees of latitude, and set off latitude and longitude assigned to the same place in another eight of these spaces north and south of the equator along the index. This arises in most cases from a difference in the results central meridian. Through the points thus marked draw straight obtained at different times by independent observers, or some lines parallel to the equator on either side of it. Those on the different point being selected by each for making the obser. north of it will represent the parallels of 5°, 10°, 15°, 20°, 25°, vation. 30°, 35, 40° north latitude; while those on the south of it will
TABLE OF LATITUDES AND LONGITUDES OF PLACES IN represent the parallels of 50, 100, 150, 20°, 25°, 30°, 350, 40°
AFRICA. south latitude. If the learner wish to do so, he may delineate more of the southern part of Europe, as in our map, and more of the ocean to the south of Cape Colony, by setting off more
Name of Place.
Country, etc. Latitude. Longitude. spaces to the north and south of the equator; there is, however, no necessity for doing this, as it has been done in our map
7° 26'N 2° 3E. merely for the sake of filling up a given space, namely, that of
Egypt 2 page of the POPULAR EDUCATOR. The parallels of 40° north Accra
6W. and south will serve very well as the inner line of the border of Agulhas (Cape). Cape Colony
57 E. the map at top and bottom, and define the limits of the map to Alexandria
9 N. the north and south. It will now be necessary to insert the Algiers
Algeria . dotted lines representing the Tropic of Cancer and the Tropic Anamaboo
5 14 0 53W. of Capricorn, which must be drawn parallel to the equator Angra Pequena (Cape) South Africa .
26 35 s.
Abyssinia through points at the distance of 23° 30' from it on either side Antalo
13 17 N.
Apollonia (Cape) of it, north and south.
2 32W. Axum
14 17 In order to draw the meridians, because at the equator the
38 47 E. Bambara Soudan.
2 42W. degrees of longitude are equal in length to those of latitude, we Bathurst
33 26 S. 26 45 E. must again open our compasses to the extent of the line assumed Bengazi
32 18 N. 20 10E, as equal to a space of five degrees of latitude, and set off eight Berbera
45 7 of these spaces east and west, or right and left, of the central Biban
33 15 perpendicular line which represents the meridian of 15° E. Birbeh
Egypt from Greenwich. The points thus obtained will be those through
37 19 which the meridians must pass at the equator; those on the right
9 48 Sahara Bojador (Cape).
14 27W. of the central meridian being points through which the meri.
48 s. 21 29 E. dians of 20°, 25°, 30°, 35°, 400, 450, 500, 550 east from Green. Bon (Cape)
11 N. wich will pass; while those to the left hand are those through Bona
36 51 which the meridians of 10°, 5o east from Greenwich, 0°, or the Brava
43 57 meridian of Greenwich itself, and 50, 100, 150, 20°, 25° west Cabes
Tunis from Greenwich will pass. The points through which the meri. Cairo
Egypt. dians of 55° east from Greenwich and 25° west from Greenwich Cantin (Cape)
14W. Upper Guinea
5 15 paes may be taken as points through which to draw straight Cape Coast Castle
18 27 E. lines parallel to the central meridian, to form the limits of the Cape Town
52N. 18W. map to the east and west and the inner line of the border of
5 s. 18 57 E. the map on either side.
6 50 N. 2 16W. As the distance between each meridian decreases gradually Corrientes (Cape) Mozambique
24 10 S. 35 12 E. from the equator to the poles, means must now be taken to Cossire, or Kosseir Egypt
5 N determine the relative distance of every fifth meridian from the Cyrene
32 47 21 47 central meridian along each parallel drawn in our projection.
26 15 37 16
11 31 9
19 40 48
1 3 33 51
3 32 32
10 31 8 0