Obrazy na stronie

GENERAL APPEARANCE OF THE HEAVENS. It matters not what part of the world be chosen as the place

of experiment; the result is almost exactly the same every Let us suppose a man to be totally ignorant of Astronomy, where, and the amount of the bending of the surface of and to turn his attention from events occurring upon earth, the ocean may be illustrated thus :-If we had a piece of to those which are presented by the heavens. He sees a

string four miles long, and were to apply both ends to the brilliant and glowing body, the Sun, rise in the east, at

surface of still water, and could possibly draw the string from four to eight o'clock in the morning, (according to the

into a perfectly straight line, the middle of the string would season of the year.). This body gradually attains a con

be about sixteen inches below the surface of the water; siderable altitude in the heavens, and continues to rise until thus showing that the water is not quite flat. noon ;-it then gradually descends, and in its descent, bends

Thus might be supposed to arise the first conjecture that towards the west, where it sinks, (apparently below the

the earth is round, like a ball; a fact which was clearly earth,) some time between four and eight o'clock in the proved by Captain Cook, who was the first to perform a afternoon. The spectator now loses sight of the glorious voyage completely round the earth, about the year 1769. orb, which does not again become visible for several hours ; He left a given spot, and arrived again at that spot by an and when it does again appear, it is not at the point where

opposite course. There is an abundance of proof, derived it escaped from view, but at the opposite side of the heavens, from other sources, that the earth is a globe ; but those will namely, at the point where it first appeared on the preceding open upon us more plainly, as we advance. day. On watching the progress of the sun, he finds that

We have hitherto endeavoured to trace what would prothe path, before noticed, is again trarelled over by the luminary, which becomes lost to his view, as before, in the bably be the feelings and opinions of one who, without west. A third and a fourth day the same phenomenon previous instruction in Astronomy, should note the appear.

ances presented by the sun. But that golden orb is not occurs; and the question naturally suggests itself to the

seen to be a solitary inhabitant of the sky: another lumiobserver's mind, “Is it the same cheering and dazzling

nous body about as large in apparent size as the sun, would visiter which I see day after day, or are they different

soon attract the notice of the gazer. He would see it rise bodies, resembling one another, and following one after

in the east, soar aloft, and then sink in the west. After another?" Were he to confine his attention to the occur

the lapse of a few hours it would again appear in the east, rences of only a few days, he would, perhaps, think the attain a height from which latter supposition to be correct, viz., that they were different

With a boundless tide bodies which thus appeared to him day after day. But if

of silver radiance, trembling round the world, he were to continue his observations for weeks, months, years, or the greater part of his life, and to find that the it would again descend, to sink as before in the west. The daily appearance of such a body still continued, he could

train of reasoning which would lead the observer to conclude hardly fail to conclude that it was the same body which thus that the sun revolved round the earth, would lead him to a so frequently attracted his attention and admiration. But similar conclusion regarding the moon. But here the re how ?" (he might say,) "do I not see this ruddy disc of semblance terminates. The sun always presents a perfect ligbt dip into the ground in the west, and appear to me circle to the eye of the observer day after day, and month again in the east on the following morning: what becomes after month; but a few evenings suffice to show that such of it in the meanwhile?" To answer this question, or to is not the case with the moon: at one time a crescent only obtain the means of answering it, he would, perhaps, trace is seen, whose hollow side is towards the left of the observer; more particularly the path which the sun followed from at another time the hollow side is towards the right: the morning to evening, which he would find to be a semicircle, crescent sometimes enlarges to a perfect circle ; and at or a curve not differing much from it. If, likewise, he others it contracts from a circle to a crescent. If, then, the were to take note of the time occupied in these occurrences, moon be a ball or globular body, shedding light upon the he would soon find the time that the sun is above the surface earth, there is great difficulty in conceiving what can occaof the earth to be, on an average, equal to the time that it sion the change in its apparent shape; but, if we were to is invisible.

consider the moon to be an opaque or non-luminous body, From these two facts, it would be by no means unreason we should find the means of explaining the change in its able to arrive at the conclusion, that the sun moves in a apparent figure, by supposing that the sun shines upon the circle; and that, while he is invisible to us, he is passing moon, and that it is only by the reflection of the sun's light round the earth, in a direction from west to east, at which from the moon that the moon becomes visible to us. If we latter point he arrives at the moment that the observer sees place a large round ball on a table, in a place where the sun him rise in the morning.

is shining, we shall see the ball more or less illuminated, Here a difficulty arises, which it may well be supposed, according to our position with regard to the ball and the would perplex the uninitiated observer. What can be sun: in one position, the ball will appear to be equally meant by going round the earth, if the earth be, as it seems divided by a boundary line, into two semicircles, one illuto our eyes, a tlat and extended surface of ground, or water, minated and the other in the shade: in another position, which seems to touch the sky at the farthest points which the illuminated portion will be only a crescent, all the rest the eye can reach :-how then can the sun pass round being shaded: from another point again it will all appear this ?' There can scarcely be a more reasonable question, illuminated, except a small crescent of shade. If the obso long as we judge merely by what meets the eye; but, server watch the relative positions of the sun and moon, if we extend our observations to certain appearances on the he will see that the shape of the bright part of the moon ocean, we begin to see proof that the surface of the water depends on its relative position in respect of the sun; just is not quite flat. On land, we have not the means of as the bright portion of the illuminated ball depended on making a similar observation, because of the intervention the position in which he viewed it. of mountains and valleys. The proof that the surface of We should in this way find the means of accounting for water is not quite flat, is derived from the following circum- the change in the shape of the moon. We shall, by and bye, stance. If we are at sea, on a clear day, and watch the show that these suppositions are really true. But another approach of a vessel from a distance, we find that the masts point would attract the notice of the observer, independently are visible sooner than the hull: we first see the top of the of the change in apparent form. If the two brilliant bodies, masts: then the whole height of the masts gradually comes the sun and moon, were observed to be near each other on a into view; and finally, we see the hull. These different certain day when the latter appeared as a thin crescent to appearances of the ship, as it approaches towards the the left of the sun, they would be seen the next day further observer, are represented in the following figure. This removed, and on the following day, at a still greater distance could not occur if the earth were perfectly fat, because then from each other: if, therefore, the motion of the moon the hull and masts would come into view at the same time. round the earth be admitted, it is necessary to suppose

that motion to be slower than the sun's motion round the earth.

But in addition to the glowing light of the sun and moon, a glittering assemblage of smaller bodies meets the eye of the observer : a crowd of little spangles adorns the sky when the sun has left it, and softens the dreary darkness which results from his absence. These stars are seen to resemble the sun and moon in the circumstance of rising in or near the east, attaining a certain altitude, and setting in the west; from which circumstance the observer infers



that the stars, like the sun and moon, revolve round the of the observer, and do not return till after a long absence. earth. But, by a careful attention to different stars, he To such bodies we give the name of Comets. would find that the same remark does not apply to all. Such then are the equally sublime and facinating ap Those which rise exactly in the east, set exactly in the pearances which present themselves to the eye of an west; those which rise to the south of east, set south of observer, when that eye is directed towards the heavens. west; and many which rise somewhat north of east, set That the invigorating and fructifying warmth and light somewhat north of west; but in looking northward he sees shed by the sun,—the serene, quiet light of the moon,stars which appear neither to rise nor to set, but which and the diamond-like glittering of the stars,-should invite perform a complete circle round a particular point of the men to a study of the laws which, under their Divine heavens. For example,—there are seven stars, which, to Creator, govern the motions of such exquisite globes of most persons who pay any attention to the appearance of light, is what we are not only prepared to expect, but fancy the heavens, are known under the name of the Great Bear. that we should feel disappointment in finding it otherwise. These stars never rise or set to an inhabitant of London. Man is not, by nature, the cold heartless being who can let If they become invisible, it is either because clouds obscure such beauties pass unheeded; and if he approach the study them, or the superior brilliancy of the sun drowns their with the humility which true self-knowledge is calculated comparatively feeble light. The same may be said of five to engender, he becomes more and more able to appreciate conspicuous stars known by the name of Cassiopeia's the surpassing grandeur and power of the great Being who Chair.

made and who rules all. Well, indeed, may we direct our If we watch the progress of these stars, and others in attention to the phenomena which we have briefly detheir vicinity, we shall find that they describe a circle round scribed, and to which Milton so exquisitely alludes in the a star called the Pole-star. The position of this star we following lines :shall find to be nearly this: if we suppose the distance from

First in his East the glorious lamp was seen, the north point of our horizon (which is the line in which

Regent of day, and all th' horizon round the surface of the earth appears to touch the sky) to the

Invested with bright rays, jocund to run zenith (which is the point immediately over our heads) to

His longitude through Heaven's high road; the gray

Dawn, and the Pleiades * before him danced, be divided into five equal parts, then at about the height of

Shedding sweet influence: less bright the moon, three of those parts from the earth, will be seen the Pole

But opposite in levelld West was set star.

His mirror, with full face borrowing her light All this would seem to show to an observer, that the

From him ; for other light she needed none Pole-star is the end of an axis round which all the stars

In that aspect, and still that distance keeps

Till night; then in the East her turn she shines, revolve, and that the reason why we cannot see the whole

Revolved on Heaven's great axle, and her reign circular path of any stars except of those in the vicinity of

With thousand lesser lights dividual holds, the Pole-star, is, that they pass round under the earth

With thousand thousand stars, that then appear'd during a part of their journey, and are therefore concealed Spangling the hemisphere : then first adorn'd from our view. Those stars which are further removed

With their bright luminaries that set and rose,

Glad evening and glad morn crown'd the fourth day. from the Pole-star describe a larger circle than those which

Par. Lost, b. vii. are nearer, while the Pole-star is almost stationary. This is exactly what takes place when we see a wheel turn round; the axle remains in one spot, but any particular THEORIES TO EXPLAIN TIIE MOTIONS OF THE HEAVENLY point on the outer edge of the wheel describes a larger circle than that which is described by any point between the axle and the circumference.

In very early ages, before Europe occupied a page in the If we consider the Pole-star to form one end of the axle history of nations, the phenomena of the heavens were or axis round which the stars revolve, then it is obvious studied with great attention by several nations of the that there must be an opposite end of the same axis, in East. The Chaldeans, the Indians, the Chinese, and the the contrary direction. If then we turn towards the south, Egyptians, have all left evidences of the industry and with the expectation of seeing such a point, we shall find ingenuity with which their observations were conducted. that it will not be realized; for none of the stars in that They constructed observatories,-invented instruments for quarter are seen to describe circles ; for they all rise and observing and measuring with accuracy,-separated the set. Still, however, the semicircles or portions of semi- stars into different groups, called Constellations, for the circles, commonly called arcs, which they describe, appear facility of finding any particular star,—gave particular to have a common centre, which is some distance below the names to most of the moving stars or planets, and noted horizon; and this centre may be considered as the southern the period which each took to move through its apparent end of the axis before spoken of.

path in the heavens; and, in many other ways, the ancients If we suppose the observer to have arrived at this amount helped to lay the foundation of that mass of astronomical of information respecting the stars, he will be prepared to knowledge which the men of later ages have brought to notice the uniformity of the positions of the stars with

more maturity. respect to one another. The Great Bear, for instance, Various opinions were formed respecting the motions of whether it be under or over the Pole-star, or at the right or the sun, moon, and stars of all kinds, both with reference the left of it, will always have its seven principal stars at

to one another, and also to the earth; but the first theory the same relative distances from each other. The changes which had attained a name and an importance in the early which take place in the distance of the sun from the moon,

ages of the world, was that of Ptolemy, a distinguished would lead to the opinion that these revolve round the earth Egyptian astronomer, who lived about one hundred and in unequal times ; and by similar reasoning, the constant thirty years after the birth of Christ. He conceived that maintenance of the same distance between any two stars,

the various bodies which had been distinguished by the would seem to imply that the stars revolve in equal times. appellation of “the heavenly host," were disposed in the

The seeming myriads of stars which present themselves order represented in the annexed diagram. to the notice of the observer all appear to follow this rule, He supposed, according to the popular opinion, that the of remaining at the same relative distances from one Earth was fixed as the centre of the universe, and that the another, with very few exceptions. These exceptions, are,

Sun, Moon, Planets, and Stars, revolved round it in the indeed, so few, that a constant watching of the same following order; namely—the Moon, Mercury, Venus, Sun, part of the heavens for a considerable period, would be Mars, Jupiter, and Saturn; the Moon being the nearest, necessary to determine that a star had actually changed and so on ; exterior to all of which, he supposed that a its place relatively to other stars. There have, however, great concave sphere in which all the stars were fixed, kept been discovered at different times ten stars, more or less on revolving round the earth. From the early history of brilliant, which change their relative distances from one Astronomy, we learn that before the time of Ptolemy, it another, and from other stars. These we know by the name had been conjectured by some, that the earth passed round of Planets; and by a careful attention to their movements, the sun, and not the sun round the earth; but ihe difficulty it is seen that each one travels in a curved path among the of believing a statement so contrary to appearances and to other stars, and returns again nearly to the point from the evidence of one's physical senses, led to the rejection whence it set out.

of this opinion; and although it was afterwards found to be At intervals, again, star-like bodies of another order present themselves, whose progress among the other stars These stars rise with the sun about the time of Spring, and our poet,

* Seven small stars clustered together in the constellation Taurus. is more rapid than that of the planets; and which at a

in this passage, intimates the old and common opinion that the period more or less brief, vanish altogether from the view | Creation took place in the Spring.


[blocks in formation]

correct, yet nearly two thousand years elapsed before such Copernicus, an eminent astronomer, who was born at a theory was generally admitted by philosophers.

Thorn, in Polish Prussia, in 1473. This distinguished After Ptolemy had promulgated the theory which bears individual perceived the unreasonable results which follow his name, he found that there were certain difficulties from the theory of Ptolemy. That all the planets, the which followed from the adoption of it. He conceived that sun, the moon, and all the stars and comets, should revolve the seven bodies mentioned before which revolved round round the earth, seemed to him much more unnatural and the earth, moved in the same general direction from West complex than that the earth should revolve on its own axis, to East. But on watching the progress of some of the and move in an orbit round the sun. All the appearances planets, he found that they did not appear to travel uni- of the heavenly bodies can be explained with much greater formly round the earth, but seemed to have, at certain ease and simplicity by the latter supposition than by the times, a retrograde or opposite motion, with reference to former; and Copernicus was thus led, after the study of the other planets; while at other times they seemed to be forty years, to the adoption of a theory which had been stationary. To account for this, he was obliged to suppose advocated by Pythagoras and Thales of Greece, five or six that those planets did not revolve in a perfect circle round hundred years before the time of Ptolemy. the earth, but that they described a peculiar path called an The frontispiece to this number is a representation of the epicycloid, of the nature of which some idea may be formed solar system, according to the theory of Copernicus. In from the following illustration :-if we had a large coach this system, the sun, the glorious source of light and heat wheel, and by any contrivance could make a smaller to us, is placed in the centre. Round him the planets wheel roll round the outside of it, at its circumference, revolve in the following order :-Mercury, Venus, Earth, then, any particular point on the small wheel would describe Mars, Jupiter, and Saturn. Since the time of Copernicus, that curve which is called an epicycloid : the axle of the five more planets have been discovered, namely, Uranus, small wheel would describe a perfect circle, but it is easy Pallas, Vesta, Juno, and Ceres. In order to account for to see that any point on the edge would not describe a the phenomena presented by the moon, Copernicus assumed, circle. Ptolemy was obliged to multiply these epicycloids (what has since been confirmed,) that the moon has a twoto a most perplexing extent, in order to account for the fold motion,-round the earth and round the sun: a small various appearances of the planets. It is to this circum- circle, therefore, surrounds the earth, which circle represents stance that Milton alluded, when he spoke of the shifts the path of the moon round the earth, while the earth and and difficulties which beset the progress of those who build the accompanying moon together revolve round the sun. their opinions on a wrong foundation :

The frontispiece of course represents the Copernican When they come to model Heaven

system with the addition of the five planets since disAnd calculate the stars, how they will wield

covered. The circles represent the orbits of the several The mighty frame; how build, unbuild, contrive,

planets; that is, the paths in which they travel in their To save appearances; how gird the sphere

progress round the sun. The distances of these circles With centric and eccentric scribbled o'er, Cycle and epicycle, orb in orb.Par. Lost, b. viii.

from the centre at which the sun is placed, could not con

veniently be in the same proportion one to another, as the There is nothing which more beautifully shows the power real distance of the planets is respectively from the sun, and force of truth, than the embarrassments which retard because the innermost orbits would be too small to be conthe progress of those who do not take truth for their guide. veniently seen. The real proportions, however, which exist In religion, in morals, in science, he whose steps are guided between the distances of the planets from the sun, admit by the light of truth, can arrive, by a short and pleasant of being easily understood. For instance, the earth, (as we path, at results which others can scarcely obtain by a com- shall hereafter explain more fully) is about ninety-five plex and wearying track. Thus Ptolemy was forced to millions of miles from the sun. If now we call that assume the existence of much unwieldly machinery in the distance 1, the distances of the other ten planets from the scheme of the heavens, in order to account for those mo- sun, will be represented by the following numbers, with tions which are most simple and beautiful; this was because sufficient nearness for our present purpose. he placed in his own imagination) the earth in the midst Mercury


21 of the heavenly bodies, all of which were made to revolve Venus

Pallas round it.

1 Jupiter

5} Great as were the difficulties which attended the adoption Mars.


9 of the Ptolemaic system, it yet retained its ground, with

2} Uranus

19 some slight modifications, until the time of Nicholas

Juno .


[ocr errors]



[ocr errors]
[ocr errors]


[ocr errors]
[ocr errors]


[ocr errors]
[ocr errors]

There was an astronomer of very great skill, who lived | Jupiter and Saturn; while at the same time, the Sun himabout the same time as Copernicus, who thought that a self, with all those planets, revolves round the Earth. It mean might be struck between the theories of Ptolemy and certainly appears strange that a man of such eminent Copernicus, by which the favourite idea of a celestial revo- abilities as Tycho was, should have preferred this pon. lution round the earth might still be retained, and so con- derous arrangement to the more simple theory advocated cession might be made both to the growing authority of by Copernicus. After the death of Tycho, his theory grascience, and to the persuasion of men's senses. This dually sank in the estimation of philosophers, who found, astronomer was Tycho Brahé, who was born in 1546, at in the arrangement advocated by Copernicus, the means of Knudstrup, in Denmark. The system advocated by him, explaining celestial phenomena by less complex reasonings and which is named from the inventor the Tychonic system, than by the theories either of Ptolemy or of Tycho Brahé. is represented in the annexed diagram.

Here, then, we arrive at an important part of our subject; In this system, which was published about the year that there are, as we shall hereafter show more clearly, 1586, the Sun is considered as a centre, round which five immense bodies revolving in orbits which measure millions of the planets revolve; namely, Mercury, Venus, Mars, / of miles across. The appearances presented to the eye

[merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

lead to an opinion that these bodies revolve round the But ought this circumstance to prevent us from studying earth; but a further inquiry into the accompanying cir- the nature and extent of the motions impressed upon these cumstances, have led men universally, at the present day, glorious bodies? Is it presumptuous in us to endeavour to to conclude that the earth and the other planets revolve become acquainted with the laws which, once known, will round the sun.

give us additional proofs of the wisdom and power of the What then can keep these splendid bodies in motion ? Divine architect of the hearens ? Assuredly not! The origin How did they first begin to move ? And why do they of those movements we know not, but the study of ihe revolve round the sun instead of moving in a straight line? nature and direction of them is a noble and fitting employThese questioris naturally occur to our minds, for our hum- ment for the human mind. ble faculties can give us but a faint idea of the powers In the century following the age of Copernicus and necessary to keep such immense masses in motion. If we Tycho Brahé, there sprang up a genius, who was destined see a carriage passing with great rapidity along a railway, to add more to the amount of human knowledge in this we can account for its motion, by tracing the action of path than any one, perhaps, who had previously existed. steam-pressure upon a piston ; which, by connecting This was Sir Isaac Newton, who was born in 1642, at machinery, makes the axles of the wheels' revolve, and Woolsthorpe, in Lincolnshire. This distinguished man thus sets the whole vehicle in motion. If we see a car- displayed from boyhood an ardent love for the study of riage passing along the street, we can assign a cause the natural phenomena around him. When a child, he for its motion, by the muscular efforts of the horse which made clepsydras, or clocks which told the hour by the is attached to it. If a ball be shot from a cannon, its flight descent of water through an orifice at the bottom of a is so rapid as to render it invisible to us, but we can account vessel. He also constructed a windmill, which was worked, for its motion by considering that a quantity of gunpowder, in part, by a mouse placed inside. As he grew towards small in bulk, suddenly enlarges to about two thousand manhood, his soaring mind directed itself to the sublime times its former dimensions, by being converted into gas, phenomena presented by the heavens, and to the nature when heat is applied ; and that the endeavour to obtain and composition of light. That part of his career which two thousand times as much room as it before occupied, more particularly concerns us at present, we will now acts with such pressure against the ball as to force it out consider. with fearful velocity. All this we can understand, because

When Newton was about twenty-ihree years of age,

he we can trace the progress of the occurrences step by step: was forced to leave Cambridge where he had been residing, But when we turn to the heavens, we are lost in wonder ! on account of the appearance of the plague at that town. Our means of judging, admit of no such details of com- | He retired to Woolsthorpe, where, sitting one day in his parison as those which before assisted us; and we are garden, he saw an apple fall from a tree at his feet. The brought to the conclusion that the Almighty, for his own falling of an apple might have been noticed often enough ; wise purposes, impressed upon the heavenly bodies those but Newton's searching mind directed itself to this inquiry : motions which we know they possess,—which we can cal Why does the apple fall, when it is loosened from the culate and measure,—but the origin of which we can here trce ?". Some may laugh at such a question,--some bave

1 never know.

laughed at it, and yet the only answer which the laughers

could gire, would be," Because it is the nature of things to space; and why do not all the planets get close together fall, when unsupported." This vague explanation did not by virtue of the attraction which draws them one to another ? satisfy the sagacious Newton. He set himself to consider These questions we cannot answer without a previous the nature of the occurrence which had attracted his notice, attention to other particulars, which we must shortly and of others similar to it, and he finished by framing explain. that train of opinions which, under the name of the Theory If we stand at the top of a high tower, and throw a stone of Gravitation, laid the foundation of all correct knowledge forward to the ground, we shall find that the stone will not of the motions of the heavenly bodies, and which is now proceed on in a straight line, but that it will soon assume received by all who deserve the name of philosophers. a bending path and approach towards the earth. The All intellectual eye, our solar round

direction in which this bending occurs, is such as to bring First gazing through, he by the blended power

the stone to the earth in a more perpendicular direction Of gravitation and projection saw

than it had when it set out. Now the reason why the The whole in silent harmony revolve.-Tromsox. stone is thus forced to change the direction in which it first We shall here find it convenient to consider the nature began to move, is, because the earth attracts it and hastens of this force of gravitation, and will endeavour to do so in its descent. If we suppose it possible that a hole could be the simplest way possible. Every particle of matter in the bored through the earth from side to side, we should find creation has a tendency to attract, or draw towards itself, the mass of matter would be more accumulated in the every other particle, however distant it may be. We know direction which passes through the centre of the earth than not what this attraction is; we can therefore only judge of in any other direction; and as bodies attract other bodies it by its effects. This attraction is of different degrees of in proportion to their mass of matter, we see reason to force according to the size or density of the attracting believe that a falling stone is attracted more powerfully body. A small loadstone or magnet will take up a little towards the central direction of the earth, than towards key, whereas a larger magnet will take up a larger key: any other part. When, therefore, the stone has left the so it is with attraction; a lump of lead weighing one pound hand, it gradually tends to a direction perpendicular to the only, attracts a distant body with half the force which is surface of the earth, or it gets into a direction which, if exerted by another piece of lead weighing two pounds. continued, would lead to the centre of the earth. Again, the force of this invisible agency is subject to vari Now if there were no such force as gravitation, the stone ation with respect to the distance between the attracting would proceed in the same straight line in which it was first and attracted bodies. If a mass of lead be the attracting propelled. If it were thrown upward from the surface of the body, and two equal-sized bodies be at some distance from earth, it would coutinue to travel upwards, without crer it, the one which is the nearer of the two will be attracted again descending to the earth. Here we should remember with greater force than that which is further removed. that there is in nature no such thing as up and down; but The diminution of attractive power from increase of dis- that up means simply a departure from the centre of the tance is much more rapid than from the lessening of the earth, down an approaching towards that centre. size of the attracting body. For instance, if any mass of We all know with what amazing velocity a cannon-ball matter attract another mass with such force as to make it moves, after it has left the mouth of the cannon; yet it more through twenty feet in a second of time; then, if soon begins to decline towards the earth, whether it be the first mass be removed to double its former distance, originally shot from the cannon in a horizontal or in an the strength of the attraction will be so much weakened, upward direction. But still the curvature of its path that the second body will move only five feet in a second, towards the earth is much less rapid than in the case of or with one-fourth of its former rapidity.

the stone, because the latter moved so much more slowly. These simple principles, when applied to such immense The mortar in St. James's Park can propel a bomb to a bodies as the planets, produce results of a gigantic kind. distance of about four miles, before it will fall to the It may be asked, “If a pound of lead will attract a piece earth; whereas a stone thrown by a very powerful arm of cork more strongly at the distance of one than of two would certainly fall to the ground within a few hundred feet, how is it that we cannot see the proofs of such a fact yards. Suppose, now, that it were possible to increase to in practice ?" The reason is, that the earth itself forms an indefinite extent the velocity with which a cannon-ball such an enormous mass of matter that it completely neu- would move; say that it should travel ten or twenty miles tralizes the effects of the comparatively small bodies at its before it touched the ground; a greater velocity would surface. In strict truth, if an apple be suspended from the carry it one hundred miles before it fell: and we may go branch of a tree, it tends to attract the earth upwards to on in the same train to any extent we please, always bear. meet it; but the attraction of the earth for the apple is so in mind that, the quicker the body moves, the greater incalculably more powerful, that the moment the apple is distance will it travel before it falls to the ground. Now, loosened from the tree, the earth draws it down to meet it, if we assume such a velocity that the ball would travel and that is what occurs when we say that the apple falls. twenty-five thousand miles before it fell to the earth, we Suppose that, instead of an apple, a mass of lead weighing shall arrive at a very curious result. The diameter of the a thousand pounds, were for a monient suspended in the earth is almost eight thousand miles, which gives, for the air; the lead would tend to attract the earth upwards circumference, about twenty-five thousand miles. The ball towards itself with much greater force than the apple had would, therefore, have gone completely round the earth done; but still the earth's attraction would so completely before it fell to the ground. If the velocity were still overpower the lead, that it would be drawn down to the greater it would not reach the ground at the completion of earth; while the latter would rise to meet it by a quantity, this circuit, but would go on to describe part of another wholly inappreciable by mortal sense, though not by com revolution round the earth. When this condition is once putation.

attained, the ball might, by a due increase of the projectile But let us now suppose a body placed in the universe, force, continue to revolve for ever about the earth, and we equal in size to the earth, but at some distance from it. should thus have a cast-iron satellite moving around us. Here we perceive that there is no reason why the earth Let not the reader smile at the absurdity of supposing a should attract the other body more strongly than the latter cannon-ball to travel twenty-five thousand miles; for it will attracts the earth; accordingly, if they were both free to lead us to important results. If we suppose that the sun move, they would naturally approach towards each other, were stationary in the universe, and that a body very much each one moving through half the distance which separated smaller than the sun were to be projected with immense them. If, therefore, we were to suppose that the universe velocity in a direction at right angles to a line joining the contained many such bodies, each as large as the earth, sun and the other body, then the latter (which would proeach one would attract all the others, with a force varying ceed in a straight line if the sun were not present) is drawn only as the several distances varied. But if one body by the sun into a curved path, the concave or hollow side were very much larger than any of the others, it would of which is always towards the sun. If the velocity with attract each of the others more powerfully than itself could which this body were propelled were below a certain limit, be attracted; and wouid not, therefore, have to move it would move in a spiral which would gradually end at the through so great a distance to meet any of the other bodies, sun himself, to which, therefore, the body would fall: if as they would have to move through, in order to meet it. the velocity were beyond a certain limit, the body would

All this admits of being impressed upon the mind with describe a spiral round the sun, but gradually receding tolerable clearness, so long as we consider the bodies to be from it, and the body would continue through infinite ages in the first instance stationary, and then receiving an im- to recede farther and farther from the sun: but if a certain pulse. But how can we explain the curved path which velocity, of a corresponding ratio with the attractive power each of the heavenly bodies describes in its progress through of the sun, were imparted to this body in the first instance,

« PoprzedniaDalej »