Plate vii.
Caroline.A planet has frequently been pointed out to me in the heavens; but I could not perceive that its motion differed from that of the fixed stars, which only appear to move.
Mrs. B.The great distance of the planets, renders their apparent motion so slow, that the eye is not sensible of their progress in their orbits, unless we watch them for some considerable length of time: but if you notice the nearness of a planet to any particular fixed star, you may in a few nights perceive that it has changed its distance from it, whilst the stars themselves always retain their relative situations. The most accurate idea I can give you of the situation and motion of the planets in their orbits, will be by the examination of this diagram, (plate 7. fig. 1.) representing the solar system, in which you will find every planet, with its orbit delineated.
Emily.But the orbits here are all circular, and you said that they were elliptical. The planets appear too, to be moving round the centre of the sun; whilst you told us that they moved round a point at a little distance from thence.
Mrs. B.The orbits of the planets are so nearly circular, and the common centre of gravity of the solar system, so near the centre of the sun, that these deviations are too small to be represented. The dimensions of the planets, in their proportion to each other, you will find delineated infig. 2.
Mercury is the planet nearest the sun; his orbit is consequently contained within ours; his vicinity to the sun, prevents our frequently seeing him, so that very accurate observations cannot be made upon Mercury. He performs his revolution round the sun in about 87 days, which is consequently the length of his year. The time of his rotation on his axis is not known; his distance from the sun is computed to be 37 millions of miles, and his diameter 3180 miles. The heat of this planet is supposed to be so great, that water cannot exist there but in a state of vapour, and that even quicksilver would be made to boil.
Caroline.Oh, what a dreadful climate!
Mrs. B.Though we could not live there, it may be perfectly adapted to other beings, destined to inhabit it; or he who created it may have so modified the heat, by provisions of which we are ignorant, as to make it habitable even by ourselves.
Venus, the next in the order of planets, is 68 millions of miles from the sun: she revolves about her axis in 23 hours and 21minutes, and goes round the sun in 244 days, 17 hours. The orbit of Venus is also within ours; during nearly one-half of her course in it, we see her before sun-rise, and she is then called the morning star; in the other part of her orbit she rises later than the sun.
Caroline.In that case we cannot see her, for she must rise in the day time?
Mrs. B.True; but when she rises later than the sun, she also sets later; so that we perceive her approaching the horizon after sun-set: she is then called Hesperus, or the evening star. Do you recollect those beautiful lines of Milton?
Now came still evening on, and twilight grayHad in her sober livery all things clad;Silence accompanied; for beast and bird,They to their grassy couch, these to their nestsWere slunk, all but the wakeful nightingale;She all night long her amorous descant sung;Silence was pleas'd; now glowed the firmamentWith living sapphires. Hesperus that ledThe starry host, rode brightest, till the moonRising in clouded majesty, at lengthApparent queen unveil'd her peerless light,And o'er the dark her silver mantle threw.
Now came still evening on, and twilight grayHad in her sober livery all things clad;Silence accompanied; for beast and bird,They to their grassy couch, these to their nestsWere slunk, all but the wakeful nightingale;She all night long her amorous descant sung;Silence was pleas'd; now glowed the firmamentWith living sapphires. Hesperus that ledThe starry host, rode brightest, till the moonRising in clouded majesty, at lengthApparent queen unveil'd her peerless light,And o'er the dark her silver mantle threw.
The planet next to Venus is the Earth, of which we shall soon speak at full length. At present I shall only observe that we are 95 millions of miles distant from the sun, that we perform our annual revolution in 365 days 5 hours and 49 minutes; and are attended in our course by a single moon.
Next follows Mars. He can never come between us and the sun, like Mercury and Venus; his motion is, however, very perceptible, as he may be traced to different situations in the heavens; his distance from the sun is 144 millions of miles; he turns round his axis in 24 hours and 39 minutes; and he performs his annual revolution, in about 687 of our days: his diameter is 4120 miles. Then follow four very small planets, Juno, Ceres, Pallas and Vesta, which have been recently discovered, but whose dimensions, and distances from the sun, have not been very accurately ascertained. They are generally called asteroids.
Jupiter is next in order: this is the largest of all the planets. He is about 490 millions of miles from the sun, and completes his annual period in nearly 12 of our years. He turns round his axis in about ten hours. He is above 1200 times as big as our earth; his diameter is 86,000 miles. The respective proportionsof the planets cannot, therefore, you see, be conveniently delineated in a diagram. He is attended by four moons.
The next planet is Saturn, whose distance from the sun, is about 900 millions of miles; his diurnal rotation is performed in 10 hours and a quarter: his annual revolution is nearly 30 of our years. His diameter is 79,000 miles. This planet is surrounded by a luminous ring, the nature of which, astronomers are much at a loss to conjecture: he has seven moons. Lastly, we observe the planet Herschel, discovered by Dr. Herschel, by whom it was named the Georgium Sidus, and which is attended by six moons.
Caroline.How charming it must be in the distant planets, to see several moons shining at the same time; I think I should like to be an inhabitant of Jupiter or Saturn.
Mrs. B.Not long I believe. Consider what extreme cold must prevail in a planet, situated as Saturn is, at nearly ten times the distance at which we are from the sun. Then his numerous moons are far from making so splendid an appearance as ours; for they can reflect only the light which they receive from the sun; and both light, and heat, decrease in the same ratio or proportion to the distances, as gravity. Can you tell me now how much more light we enjoy than Saturn?
Caroline.The square of ten is a hundred; therefore, Saturn has a hundred times less—or to answer your question exactly, we have a hundred times more light and heat, than Saturn—this certainly does not increase my wish to become one of the poor wretches who inhabit that planet.
Mrs. B.May not the inhabitants of Mercury, with equal plausibility, pity us for the insupportable coldness of our situation; and those of Jupiter and Saturn for our intolerable heat? The Almighty power which created these planets, and placed them in their several orbits, has no doubt peopled them with beings, whose bodies are adapted to the various temperatures and elements, in which they are situated. If we judge from the analogy of our own earth, or from that of the great and universal beneficence of Providence, we must conclude this to be the case.
Caroline.Are not comets, in some respects similar to planets?
Mrs. B.Yes, they are; for by the reappearance of some of them, at stated times, they are known to revolve round the sun; but in orbits so extremely eccentric, that they disappear for a great number of years. If they are inhabited, it must be by a species of beings very different, not only from the inhabitants ofthis, but from those of any of the other planets, as they must experience the greatest vicissitudes of heat and cold; one part of their orbit being so near the sun, that their heat, when there, is computed to be greater than that of red-hot iron; in this part of its orbit, the comet emits a luminous vapour, called the tail, which it gradually loses as it recedes from the sun; and the comet itself totally disappears from our sight, in the more distant parts of its orbit, which extends considerably beyond that of the furthest planet.
The number of comets belonging to our system cannot be ascertained, as some of them are several centuries before they make their reappearance. The number that are known by their regular reappearance is, I believe, only three, although their whole number is very considerable.
Emily.Pray, Mrs. B., what are the constellations?
Mrs. B.They are the fixed stars; which the ancients, in order to recognise them, formed into groups, and gave the names of the figures, which you find delineated on the celestial globe. In order to show their proper situations in the heavens, they should be painted on the internal surface of a hollow sphere, from the centre of which you should view them; you would then behold them as they appear to be situated in the heavens. The twelve constellations, called the signs of the zodiac, are those which are so situated, that the earth, in its annual revolution, passes directly between them, and the sun. Their names are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, Pisces; the whole occupying a complete circle, or broad belt, in the heavens, called the zodiac. (plate 8. fig. 1.) Hence, a right line drawn from the earth, and passing through the sun, would reach one of these constellations, and the sun is said to be in that constellation at which the line terminates: thus, when the earth is at A, the sun would appear to be in the constellation or sign Aries; when the earth is at B, the sun would appear in Cancer; when the earth was at C, the sun would be in Libra; and when the earth was at D, the sun would be in Capricorn. You are aware that it is the real motion of the earth in its orbit, which gives to the sun this apparent motion through the signs. This circle, in which the sun thus appears to move, and which passes through the middle of the zodiac, is called the ecliptic.
Caroline.But many of the stars in these constellations appear beyond the zodiac.
Plate viii.
Mrs. B.We have no means of ascertaining the distance of the fixed stars. When, therefore, they are said to be in the zodiac, it is merely implied that they are situated in that direction, and that they shine upon us through that portion of the heavens, which we call the zodiac.
Emily.But are not those large bright stars, which are called stars of the first magnitude, nearer to us, than those small ones which we can scarcely discern?
Mrs. B.It may be so; or the difference of size and brilliancy of the stars may proceed from their difference of dimensions; this is a point which astronomers are not enabled to determine. Considering them as suns, I see no reason why different suns should not vary in dimensions, as well as the planets belonging to them.
Emily.What a wonderful and beautiful system this is, and how astonishing to think that every fixed star may probably be attended by a similar train of planets!
Caroline.You will accuse me of being very incredulous, but I cannot help still entertaining some doubts, and fearing that there is more beauty than truth in this system. It certainly may be so; but there does not appear to me to be sufficient evidence to prove it. It seems so plain and obvious that the earth is motionless, and that the sun and stars revolve round it;—your solar system, you must allow, is directly in opposition to the evidence of our senses.
Mrs. B.Our senses so often mislead us, that we should not place implicit reliance upon them.
Caroline.On what then can we rely, for do we not receive all our ideas through the medium of our senses?
Mrs. B.It is true that they are our primary source of knowledge; but the mind has the power of reflecting, judging, and deciding upon the ideas received by the organs of sense. This faculty, which we call reason, has frequently proved to us, that our senses are liable to err. If you have ever sailed on the water, with a very steady breeze, you must have seen the houses, trees, and every object on the shore move, while you were sailing.
Caroline.I remember thinking so, when I was very young; but I now know that their motion is only apparent. It is true that my reason, in this case, corrects the error of my sight.
Mrs. B.It teaches you, that the apparent motion of the objects on shore, proceeds from your being yourself moving, and that you are not sensible of your own motion, because you meet with no resistance. It is only when some obstacle impedes our motion, that we are conscious of moving; and if you were toclose your eyes when you were sailing on calm water, with a steady wind, you would not perceive that you moved, for you could not feel it, and you could see it only by observing the change of place of the objects on shore. So it is with the motion of the earth: every thing on its surface, and the air that surrounds it, accompanies it in its revolution; it meets with no resistance: therefore, like the crew of a vessel sailing with a fair wind, in a calm sea, we are insensible of our motion.
Caroline.But the principal reason why the crew of a vessel in a calm sea do not perceive their motion, is, because they move exceedingly slow, while the earth, you say, revolves with great velocity.
Mrs. B.It is not because they move slowly, but because they move steadily, and meet with no irregular resistances, that the crew of a vessel do not perceive their motion; for they would be equally insensible to it, with the strongest wind, provided it were steady, that they sailed with it, and that it did not agitate the water; but this last condition, you know, is not possible, for the wind will always produce waves which offer more or less resistance to the vessel, and then the motion becomes sensible, because it is unequal.
Caroline.But, granting this, the crew of a vessel have a proof of their motion, which the inhabitants of the earth cannot have,—the apparent motion of the objects on shore, or their having passed from one place to another.
Mrs. B.Have we not a similar proof of the earth's motion, in the apparent motion of the sun and stars? Imagine the earth to be sailing round its axis, and successively passing by every star, which, like the objects on land, we suppose to be moving instead of ourselves. I have heard it observed by an ærial traveller in a balloon, that the earth appears to sink beneath the balloon, instead of the balloon rising above the earth.
It is a law which we discover throughout nature, and worthy of its great Author, that all its purposes are accomplished by the most simple means; and what reason have we to suppose this law infringed, in order that we may remain at rest, while the sun and stars move round us; their regular motions, which are explained by the laws of attraction, on the first supposition, would be unintelligible on the last, and the order and harmony of the universe be destroyed. Think what an immense circuit the sun and stars would make daily, were their apparent motions, real. We know many of them, to be bodies more considerable than our earth; for our eyes vainly endeavour to persuade us, that theyare little brilliants sparkling in the heavens; while science teaches us that they are immense spheres, whose apparent dimensions are diminished by distance. Why then should these enormous globes daily traverse such a prodigious space, merely to prevent the necessity of our earth's revolving on its axis?
Caroline.I think I must now be convinced. But you will, I hope, allow me a little time to familiarise to myself, an idea so different from that which I have been accustomed to entertain. And pray, at what rate do we move?
Mrs. B.The motion produced by the revolution of the earth on its axis, is about seventeen miles a minute, to an inhabitant on the equator.
Emily.But does not every part of the earth move with the same velocity?
Mrs. B.A moment's reflection would convince you of the contrary: a person at the equator must move quicker than one situated near the poles, since they both perform a revolution in 24 hours.
Emily.True, the equator is farthest from the axis of motion. But in the earth's revolution round the sun, every part must move with equal velocity?
Mrs. B.Yes, about a thousand miles a minute.
Caroline.How astonishing!—and that it should be possible for us to be insensible of such a rapid motion. You would not tell me this sooner, Mrs. B., for fear of increasing my incredulity.
Before the time of Newton, was not the earth supposed to be in the centre of the system, and the sun, moon, and stars to revolve round it?
Mrs. B.This was the system of Ptolemy, in ancient times; but as long ago as the beginning of the sixteenth century it was generally discarded, and the solar system, such as I have shown you, was established by the celebrated astronomer Copernicus, and is hence called the Copernican system. But the theory of gravitation, the source from which this beautiful and harmonious arrangement flows, we owe to the powerful genius of Newton, who lived at a much later period, and who demonstrated its truth.
Emily.It appears, indeed, far less difficult to trace by observation the motion of the planets, than to divine by what powerthey are impelled and guided. I wonder how the idea of gravitation could first have occurred to sir Isaac Newton?
Mrs. B.It is said to have been occasioned by a circumstance from which one should little have expected so grand a theory to have arisen.
During the prevalence of the plague in the year 1665, Newton retired into the country to avoid the contagion: when sitting one day in an orchard, he observed an apple fall from a tree, and was led to consider what could be the cause which brought it to the ground.
Caroline.If I dared to confess it, Mrs. B., I should say that such an inquiry indicated rather a deficiency than a superiority of intellect. I do not understand how any one can wonder at what is so natural and so common.
Mrs. B.It is the mark of superior genius to find matter for wonder, observation, and research, in circumstances which, to the ordinary mind, appear trivial, because they are common; and with which they are satisfied, because they are natural; without reflecting that nature is our grand field of observation, that within it, is contained our whole store of knowledge; in a word, that to study the works of nature, is to learn to appreciate and admire the wisdom of God. Thus, it was the simple circumstance of the fall of an apple, which led to the discovery of the laws upon which the Copernican system is founded; and whatever credit this system had obtained before, it now rests upon a basis from which it cannot be shaken.
Emily.This was a most fortunate apple, and more worthy to be commemorated than all those that have been sung by the poets. The apple of discord for which the goddesses contended; the golden apples by which Atalanta won the race; nay, even the apple which William Tell shot from the head of his son, cannot be compared to this!
Questions1.(Pg.80) Into what two classes are the planets divided, and how are they distinguished?2.(Pg.80) By what reasoning do you prove that the sun contains a greater quantity of matter than any other body in the system?3.(Pg.81) What two circumstances govern the force with which bodies attract each other?4.(Pg.81) Were a planet removed to double its former distance from the sun, what would be the effect upon its attractive force?5.(Pg.81) Why would it be reduced to one-fourth?6.(Pg.81) What is meant by the square of a number, and what examples can you give?7.(Pg.81) What then would be the effect of removing it to three, or four times its former distance?8.(Pg.81) How is the rule upon this subject expressed?9.(Pg.81) Does this apply to any power excepting gravitation?10.(Pg.81) How is it that a secondary planet revolves round its primary, and is not drawn off by the sun?11.(Pg.82) What is said respecting the revolution of the moon, and of the earth, round a common centre of gravity?12.(Pg.82) By what law in mechanics is this explained?13.(Pg.82) What motions then has the earth, and are these remarks confined to it alone?14.(Pg.82) What effect have the planets upon the sun, and what is said of the common centre of gravity of the system?15.(Pg.83) What other motion has the sun, and how is it proved?16.(Pg.83) How may you observe the motion of a planet, by means of a fixed star?17.(Pg.83) What is represented byfig. 1. plate 7?18.(Pg.83) Why are the orbits represented as circular?19.(Pg.83) In what order do the planets increase in size as represented,fig. 2. plate 7?20.(Pg.83) What are we told respecting Mercury?21.(Pg.84) What respecting Venus?22.(Pg.84) When does Venus become a morning, and when an evening star?23.(Pg.84) What is said of the Earth?24.(Pg.84) What of Mars?25.(Pg.84) What four small planets follow next?26.(Pg.85) What is said of Jupiter?27.(Pg.85) What of Saturn?28.(Pg.85) What of Herschel?29.(Pg.85) Why do we conclude that the moons of Saturn afford less light than ours?30.(Pg.85) In what proportion will the light and heat at Saturn be diminished, and why?31.(Pg.86) What do the comets resemble, and what is remarkable in their orbits?32.(Pg.86) What is said of the number of comets?33.(Pg.86) What is a constellation?34.(Pg.86) How are the twelve constellations, or signs, called the zodiac, situated?35.(Pg.86) Name them.36.(Pg.86) What is meant by the sun being in a sign?37.(Pg.86) What causes the apparent change of the sun's place?38.(Pg.87) The stars appear of different magnitudes, by what may this be caused?39.(Pg.87) We are not sensible of the motion of the earth; what fact is mentioned to illustrate this point?40.(Pg.87) What does this teach us?41.(Pg.88) Would the slowness, or the rapidity of the motion, if steady, produce any sensible difference?42.(Pg.88) If we do not feel the motion of the earth, how may we be convinced of its reality?43.(Pg.89) Were we to deny the motion of the earth upon its axis, what must we admit respecting the heavenly bodies?44.(Pg.89) What distance is an inhabitant on the equator carried in a minute by the diurnal motion of the earth?45.(Pg.89) Why is not the velocity every where equally great?46.(Pg.89) What distance does the earth travel in a minute, in its revolution round the sun?47.(Pg.89) What was formerly supposed respecting the motion of all the heavenly bodies?48.(Pg.89) What do we mean by the Copernican system, and what is said respecting Copernicus and Newton?49.(Pg.90) What circumstance is said to have given rise to the speculations of Newton, on the subject of gravitation?
Questions
1.(Pg.80) Into what two classes are the planets divided, and how are they distinguished?
2.(Pg.80) By what reasoning do you prove that the sun contains a greater quantity of matter than any other body in the system?
3.(Pg.81) What two circumstances govern the force with which bodies attract each other?
4.(Pg.81) Were a planet removed to double its former distance from the sun, what would be the effect upon its attractive force?
5.(Pg.81) Why would it be reduced to one-fourth?
6.(Pg.81) What is meant by the square of a number, and what examples can you give?
7.(Pg.81) What then would be the effect of removing it to three, or four times its former distance?
8.(Pg.81) How is the rule upon this subject expressed?
9.(Pg.81) Does this apply to any power excepting gravitation?
10.(Pg.81) How is it that a secondary planet revolves round its primary, and is not drawn off by the sun?
11.(Pg.82) What is said respecting the revolution of the moon, and of the earth, round a common centre of gravity?
12.(Pg.82) By what law in mechanics is this explained?
13.(Pg.82) What motions then has the earth, and are these remarks confined to it alone?
14.(Pg.82) What effect have the planets upon the sun, and what is said of the common centre of gravity of the system?
15.(Pg.83) What other motion has the sun, and how is it proved?
16.(Pg.83) How may you observe the motion of a planet, by means of a fixed star?
17.(Pg.83) What is represented byfig. 1. plate 7?
18.(Pg.83) Why are the orbits represented as circular?
19.(Pg.83) In what order do the planets increase in size as represented,fig. 2. plate 7?
20.(Pg.83) What are we told respecting Mercury?
21.(Pg.84) What respecting Venus?
22.(Pg.84) When does Venus become a morning, and when an evening star?
23.(Pg.84) What is said of the Earth?
24.(Pg.84) What of Mars?
25.(Pg.84) What four small planets follow next?
26.(Pg.85) What is said of Jupiter?
27.(Pg.85) What of Saturn?
28.(Pg.85) What of Herschel?
29.(Pg.85) Why do we conclude that the moons of Saturn afford less light than ours?
30.(Pg.85) In what proportion will the light and heat at Saturn be diminished, and why?
31.(Pg.86) What do the comets resemble, and what is remarkable in their orbits?
32.(Pg.86) What is said of the number of comets?
33.(Pg.86) What is a constellation?
34.(Pg.86) How are the twelve constellations, or signs, called the zodiac, situated?
35.(Pg.86) Name them.
36.(Pg.86) What is meant by the sun being in a sign?
37.(Pg.86) What causes the apparent change of the sun's place?
38.(Pg.87) The stars appear of different magnitudes, by what may this be caused?
39.(Pg.87) We are not sensible of the motion of the earth; what fact is mentioned to illustrate this point?
40.(Pg.87) What does this teach us?
41.(Pg.88) Would the slowness, or the rapidity of the motion, if steady, produce any sensible difference?
42.(Pg.88) If we do not feel the motion of the earth, how may we be convinced of its reality?
43.(Pg.89) Were we to deny the motion of the earth upon its axis, what must we admit respecting the heavenly bodies?
44.(Pg.89) What distance is an inhabitant on the equator carried in a minute by the diurnal motion of the earth?
45.(Pg.89) Why is not the velocity every where equally great?
46.(Pg.89) What distance does the earth travel in a minute, in its revolution round the sun?
47.(Pg.89) What was formerly supposed respecting the motion of all the heavenly bodies?
48.(Pg.89) What do we mean by the Copernican system, and what is said respecting Copernicus and Newton?
49.(Pg.90) What circumstance is said to have given rise to the speculations of Newton, on the subject of gravitation?
OF THE TERRESTRIAL GLOBE. OF THE FIGURE OF THE EARTH. OF THE PENDULUM. OF THE VARIATION OF THE SEASONS, AND OF THE LENGTH OF DAYS AND NIGHTS. OF THE CAUSES OF THE HEAT OF SUMMER. OF SOLAR, SIDERIAL, AND EQUAL OR MEAN TIME.
MRS. B.
As the earth is the planet in which we are the most particularly interested, it is my intention this morning, to explain to you the effects resulting from its annual, and diurnal motions; but for this purpose, it will be necessary to make you acquainted with the terrestrial globe: you have not either of you, I conclude, learnt the use of the globes?
Caroline.No; I once indeed, learnt by heart, the names of the lines marked on the globe, but as I was informed they were only imaginary divisions, they did not appear to me worthy of much attention, and were soon forgotten.
Mrs. B.You supposed, then, that astronomers had been at the trouble of inventing a number of lines, to little purpose. It will be impossible for me to explain to you the particular effects of the earth's motion, without your having acquired a knowledge of these lines: inplate 8. fig. 2.you will find them all delineated: and you must learn them perfectly, if you wish to make any proficiency in astronomy.
Caroline.I was taught them at so early an age, that I could not understand their meaning; and I have often heard you say, that the only use of words, was to convey ideas.
Mrs. B.A knowledge of these lines, would have conveyed some idea of the manner in which they were designed to divide the globe into parts; although the use of these divisions, might at that time, have been too difficult for you to understand. Childhood is the season, when impressions on the memory are most strongly and most easily made: it is the period at which a large stock of terms should be treasured up, the precise application of which we may learn when the understanding is more developed. It is, I think, a very mistaken notion, that children should be taught such things only, as they can perfectly understand. Had you been early made acquainted with the terms which relate tofigure and motion, how much it would have facilitated your progress in natural philosophy. I have been obliged to confine myself to the most common and familiar expressions, in explaining the laws of nature; although I am convinced that appropriate and scientific terms, might have conveyed more precise and accurate ideas, had you been prepared to understand them.
Emily.You may depend upon our carefully learning the names of these lines, Mrs. B.; but before we commit them to memory, will you have the goodness to explain them to us?
Mrs. B.Most willingly. This figure of a globe, or sphere, represents the earth; the line which passes through its centre, and on which it turns, is called its axis, and the two extremities of the axis A and B, are the poles, distinguished by the names of the north and the south pole. The circle C D, which divides the globe into two equal parts between the poles, and equally distant from them, is called the equator, or equinoctial line; that part of the globe to the north of the equator, is the northern hemisphere; that part to the south of the equator, the southern hemisphere. The small circle E F, which surrounds the north pole, is called the arctic circle; that G H, which surrounds the south pole, the antarctic circle; these are also called polar circles. There are two circles, intermediate between the polar circles and the equator; that to the north I K, called the tropic of Cancer; that to the south, L M, called the tropic of Capricorn. Lastly, this circle, L K, which divides the globe into two equal parts, crossing the equator and extending northward as far as the tropic of Cancer, and southward as far as the tropic of Capricorn, is called the ecliptic. The delineation of the ecliptic on the terrestrial globe is not without danger of conveying false ideas; for the ecliptic (as I have before said) is an imaginary circle in the heavens, passing through the middle of the zodiac, and situated in the plane of the earth's orbit.
Caroline.I do not understand the meaning of the plane of the earth's orbit.
Mrs. B.A plane, is an even flat surface. Were you to bend a piece of wire, so as to form a hoop, you might then stretch a piece of cloth, or paper over it, like the head of a drum; this would form a flat surface, which might be called the plane of the hoop. Now the orbit of the earth, is an imaginary circle, surrounding the sun, and you can readily imagine a plane extendingfrom one side of this circle to the other, filling up its whole area: such a plane would pass through the centre of the sun, dividing it into hemispheres. You may then imagine this plane extended beyond the limits of the earth's orbit, on every side, until it reached those fixed stars which form the signs of the zodiac; passing through the middle of these signs, it would give you the place of that imaginary circle in the heavens, call the ecliptic; which is the sun's apparent path. Letfig. 1. plate 9, represent such a plane, S the sun, E the earth with its orbit, and A B C D the ecliptic passing through the middle of the zodiac.
Plate ix.
Emily.If the ecliptic relates only to the heavens, why is it described upon the terrestrial globe?
Mrs. B.It is convenient for the demonstration of a variety of problems in the use of the globes; and besides, the obliquity of this circle to the equator is rendered more conspicuous by its being described on the same globe; and the obliquity of the ecliptic shows how much the earth's axis is inclined to the plane of its orbit. But to return tofig. 2. plate 8.
The spaces between the several parallel circles on the terrestrial globe are called zones: that which is comprehended between the tropics is distinguished by the name of the torrid zone; the spaces which extend from the tropics to the polar circles, the north and south temperate zones; and the spaces contained within the polar circles, the frigid zones. By the term zone is meant a belt, or girdle, the frigid zones, however, are not belts, but circles, extending 231/2degrees from their centres, the poles.
The several lines which, you observe to be drawn from one pole to the other, cutting the equator at right angles, are called meridians; the number of these is unlimited, as a line passing through any place, directly to the poles, is called the meridian of that place. When any one of these meridians is exactly opposite to the sun, it is mid-day, or twelve o'clock in the day, at all the places situated any where on that meridian; and, at the places situated on the opposite meridian, it is consequently midnight.
Emily.To places situated equally distant from these two meridians, it must then be six o'clock.
Mrs. B.Yes; if they are to the east of the sun's meridian it is six o'clock in the afternoon, because they will have previously passed the sun; if to the west, it is six o'clock in the morning, and that meridian will be proceeding towards the sun.
Those circles which divide the globe into two equal parts, such as the equator and the ecliptic, are called greater circles; to distinguish them from those which divide it into two unequal parts, as the tropics, and polar circles, which are called lesser circles. All circles, you know, are imagined to be divided into 360 equal parts, called degrees, and degrees are again divided into 60 equal parts, called minutes. The diameter of a circle is a right line drawn across it, and passing through its centre; were you, for instance, to measure across this round table, that would give you its diameter; but were you to measure all round the edge of it, you would then obtain its circumference.
Now Emily, you may tell me exactly how many degrees are contained in a meridian?
Emily.A meridian, reaching from one pole to the other, is half a circle, and must therefore contain 180 degrees.
Mrs. B.Very well; and what number of degrees are there from the equator to one of the poles?
Caroline.The equator being equally distant from either pole, that distance must be half of a meridian, or a quarter of the circumference of a circle, and contain 90 degrees.
Mrs. B.Besides the usual division of circles into degrees, the ecliptic is divided into twelve equal parts, called signs, which bear the name of the constellations through which this circle passes in the heavens. The degrees measured on the meridians from the equator, either towards the north, or towards the south, are called degrees of latitude, of which there may be 90; those measured from east to west, either on the equator, or any of the lesser circles, are called degrees of longitude, of which there may be 180; these lesser circles are also called parallels of latitude. Of these parallels there may be any number; a circle drawn from east to west, at any distance from the equator, will always be parallel to it, and is therefore called a parallel of latitude.
Emily.The degrees of longitude must then vary in length, according to the dimensions of the circle on which they are reckoned; those, for instance, at the polar circles, will be considerably smaller than those at the equator?
Mrs. B.Certainly; since the degrees of circles of different dimensions do not vary in number, they must necessarily vary in length. The degrees of latitude, you may observe, never vary in length; for the meridians on which they are reckoned are all of the same dimensions.
Emily.And of what length is a degree of latitude?
Mrs. B.Sixty geographical miles, which is equal to 691/2English statute miles; or about one-sixth more than a common mile.
Emily.The degrees of longitude at the equator, must then be of the same dimensions, with a degree of latitude.
Mrs. B.They would, were the earth a perfect sphere; but it is not exactly such, being somewhat protuberant about the equator, and flattened towards the poles. This form proceeds from the superior action of the centrifugal power at the equator, and as this enlarges the circle, it must, in the same proportion, increase the length of the degrees of longitude measured on it.
Caroline.I thought I had understood the centrifugal force perfectly, but I do not comprehend its effects in this instance.
Mrs. B.You know that the revolution of the earth on its axis, must give to every particle a tendency to fly off from the centre, that this tendency is stronger, or weaker, in proportion to the velocity with which the particle moves; now a particle situated near to one of the poles, makes one rotation in the same space of time as a particle at the equator; the latter, therefore, having a much larger circle to describe, travels proportionally faster, consequently the centrifugal force is much stronger at the equator than in the polar regions: it gradually decreases as you leave the equator and approach the poles, at which points the centrifugal force, entirely ceases. Supposing, therefore, the earth to have been originally in a fluid state, the particles in the torrid zone would recede much farther from the centre than those in the frigid zones; thus the polar regions would become flattened, and those about the equator elevated.
As a large portion of the earth is covered with water, the Creator gave to it the form, denominated anoblate spheroid, otherwise the polar regions would have been without water,and those about the equator, would have been buried several miles below the surface of the ocean.
Caroline.I did not consider that the particles in the neighbourhood of the equator, move with greater velocity than those about the poles; this was the reason I could not understand you.
Mrs. B.You must be careful to remember, that those parts of a body which are farthest from the centre of motion, must move with the greatest velocity: the axis of the earth is the centre of its diurnal motion, and the equatorial regions the parts most distant from the axis.
Caroline.My head then moves faster than my feet; and upon the summit of a mountain, we are carried round quicker than in a valley?
Mrs. B.Certainly; your head is more distant from the centre of motion than your feet; the mountain-top than the valley; and the more distant any part of a body is from the centre of motion, the larger is the circle it will describe, and the greater therefore must be its velocity.
Emily.I have been reflecting, that if the earth is not a perfect circle——
Mrs. B.A sphere you mean, my dear: a circle is a round line, every part of which is equally distant from the centre; a sphere or globe is a round body, the surface of which is every where equally distant from the centre.
Emily.If, then, the earth is not a perfect sphere, but prominent at the equator, and depressed at the poles, would not a body weigh heavier at the equator than at the poles? For the earth being thicker at the equator, the attraction of gravity perpendicularly downwards must be stronger.
Mrs. B.Your reasoning has some plausibility, but I am sorry to be obliged to add, that it is quite erroneous; for the nearer any part of the surface of a body is to the centre of attraction, the more strongly it is attracted; because it is then nearest to the whole mass of attracting matter. In regard to its effects, you might consider the whole power of gravity, as placed at the centre of attraction.
Emily.But were you to penetrate deep into the earth, would gravity increase as you approached the centre?
Mrs. B.Certainly not; I am referring only to any situation on the surface of the earth. Were you to penetrate into the interior, the attraction of the parts above you, would counteract that of the parts beneath you, and consequently diminish the power of gravity in proportion as you approach the centre; and if youreached that point, being equally attracted by the parts all around you, the effects of gravity would cease, and you would be without weight.
Emily.Bodies, then, should weigh less at the equator than at the poles, since they are more distant from the centre of gravity in the former than in the latter situation?
Mrs. B.And this is really the case; but the difference of weight would be scarcely sensible, were it not augmented by another circumstance.
Caroline.And what is this singular circumstance, which seems to disturb the laws of nature?
Mrs. B.One that you are well acquainted with, as conducing more to the preservation than the destruction of order,—the centrifugal force. This we have just observed to be strongest at the equator; and as it tends to drive bodies from the centre, it is necessarily opposed to, and must lessen the power of gravity, which attracts them towards the centre. We accordingly find that bodies weigh lightest at the equator, where the centrifugal force is greatest; and heaviest at the poles, where this power is least: the weight being diminished at the equator, by both the causes mentioned.
Caroline.Has the experiment been made in these different situations?
Mrs. B.Louis XIV. of France, sent philosophers both to the equator, and to Lapland, for this purpose: the severity of the climate, and obstruction from the ice, have hitherto rendered every attempt to reach the pole abortive; but the difference of gravity at the equator, and in Lapland is very perceptible.
Caroline.Yet I do not comprehend how the difference of weight could be ascertained, for if the body under trial decreased in weight, the weight which was opposed to it in the opposite scale must have diminished in the same proportion. For instance, if a pound of sugar did not weigh so heavy at the equator as at the poles, the leaden pound which served to weigh it, would not be so heavy either; therefore they would still balance each other, and the different force of gravity could not be ascertained by this means.
Mrs. B.Your observation is perfectly just: the difference of gravity in bodies situated at the poles, and at the equator, cannot be ascertained by weighing them; a pendulum was therefore used for that purpose.
Caroline.What, the pendulum of a clock? how could that answer the purpose?
Mrs. B.A pendulum consists of a line, or rod, to one end of which a weight is attached, and by the other end it is suspended to a fixed point, about which it is made to vibrate. When not in motion, a pendulum, obeying the general law of attraction, hangs like a plumb line, perpendicular to the surface of the earth, but if you raise the pendulum, gravity will bring it back to its perpendicular position. It will, however, not remain stationary there, for the momentum it has acquired during its descent, will impel it onwards, and if unobstructed, it will rise on the opposite side to an equal height; from thence it is brought back by gravity, and is again forced upwards, by the impulse of its momentum.
Caroline.If so, the motion of a pendulum would be perpetual, and I thought you said, that there was no perpetual motion on the earth.
Mrs. B.The motion of a pendulum is opposed by the resistance of the air in which it vibrates, and by the friction of the part by which it is suspended: were it possible to remove these obstacles, the motion of a pendulum would be perpetual, and its vibrations perfectly regular; each being of equal distance, and performed in equal times.
Emily.That is the natural result of the uniformity of the power which produces these vibrations, for the force of gravity being always the same, the velocity of the pendulum must consequently be uniform.
Caroline.No, Emily, you are mistaken; the force is not every where the same, and therefore the effect will not be so either. I have discovered it, Mrs. B.; since the force of gravity is less at the equator than at the poles, the vibrations of the pendulum will be slower at the former place than at the latter.
Mrs. B.You are perfectly right, Caroline; it was by this means that the difference of gravity was discovered, and the true figure of the earth ascertained.
Emily.But how do they contrive to regulate their time in the equatorial and polar regions? for, since in our part of the earth the pendulum of a clock vibrates exactly once in a second, if it vibrates faster at the poles, and slower at the equator, the inhabitants must regulate their clocks in a manner different from us.
Mrs. B.The only alteration required is to lengthen the pendulumin one case, and to shorten it in the other; for the velocity of the vibrations of a pendulum depends on its length; and when it is said that a pendulum vibrates quicker at the pole than at the equator, it is supposed to be of the same length. A pendulum which vibrates seconds in this latitude is about 391/7inches long. In order to vibrate at the equator in the same space of time, it must be somewhat shorter; and at the poles, it must be proportionally lengthened.
The vibrations of a pendulum, resemble the descent of a body on an inclined plane, and are produced by the same cause; now you must recollect, that the greater the perpendicular height of such a plane, in proportion to its length, the more rapid will be the descent of the body; a short pendulum ascends to a greater height than a larger one, in vibrating a given distance, and of course its descent must be more rapid.
I shall now, I think, be able to explain to you the cause of the variation of the seasons, and the difference in the length of the days and nights in those seasons; both effects resulting from the same cause.
In moving round the sun, the axis of the earth is not perpendicular to the plane of its orbit. Supposing this round table to represent the plane of the earth's orbit, and this little globe, the earth; through this I have passed a wire, representing its axis and poles. In moving round the table, I do not hold the wire perpendicular to it, but obliquely.
Emily.Yes, I understand, the earth does not go round the sun in an upright position, its axis is slanting or oblique; and, it of course, forms an angle with a line drawn perpendicular to the plane of the earth's orbit.
Mrs. B.All the lines, which you learnt in your last lesson, are delineated on this little globe; you must consider the ecliptic as representing the plane of the earth's orbit; and the equator, which crosses the ecliptic in two places, then shows the degree of obliquity of the axis of the earth; which amounts to 231/2degrees, very nearly. The points in which the ecliptic intersects the equator, are called the equinoctial points.
But I believe I shall render the effects of the obliquity of the earth's axis clearer to you, by the revolution of the little globe round a candle, which shall represent the sun. (Plate IX. fig. 2.)
As I now hold it, at A, you see it in the situation in which it isin the midst of summer, or what is called the summer solstice, which is on the 21st of June.
Emily.You hold the wire awry, I suppose, in order to show that the axis of the earth is not upright?
Mrs. B.Yes; in summer, the north pole is inclined towards the sun. In this season, therefore, the northern hemisphere enjoys much more of his rays than the southern. The sun, you see, now shines over the whole of the north frigid zone, and notwithstanding the earth's diurnal revolution, which I imitate by twirling the ball on the wire, it will continue to shine upon it as long as it remains in this situation, whilst the south frigid zone is at the same time completely in darkness.
Caroline.That is very strange; I never before heard that there was constant day or night in any part of the world! How much happier the inhabitants of the north frigid zone must be than those of the southern; the first enjoy uninterrupted day, while the last are involved in perpetual darkness.
Mrs. B.You judge with too much precipitation; examine a little further, and you will find, that the two frigid zones share an equal fate.
We shall now make the earth set off from its position in the summer solstice, and carry it round the sun; observe that the pole is always inclined in the same direction, and points to the same spot in the heavens. There is a fixed star situated near that spot, which is hence called the north polar star. Now let us stop the earth at B, and examine it in its present situation; it has gone through one quarter of its orbit, and is arrived at that point at which the ecliptic cuts, or crosses, the equator, and which is called the autumnal equinox.
Emily.The sun now shines from one pole to the other, just as it would constantly do, if the axis of the earth were perpendicular to its orbit.
Mrs. B.Because the inclination of the axis is now neither towards the sun, nor in the contrary direction; at this period of the year, the days and nights are equal in every part of the earth. But the next step she takes in her orbit, you see, involves the north pole in darkness, whilst it illumines that of the south; this change was gradually preparing as I moved the earth from summer to autumn; the arctic circle, which was at first entirely illumined, began to have short nights, which increased as the earth approached the autumnal equinox; and the instant it passed that point, the long night of the north pole commences, andthe south pole begins to enjoy the light of the sun. We shall now make the earth proceed in its orbit, and you may observe that as it advances, the days shorten and the nights lengthen, throughout the northern hemisphere, until it arrives at the winter solstice, on the 21st of December, when the north frigid zone is entirely in darkness, and the southern has uninterrupted daylight.