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 circ.u.mstance is said to have given rise to the speculations of Newton, on the subject of gravitation?

CONVERSATION VIII.

ON THE EARTH.

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: in plate 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 to figure 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 pa.s.ses 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, pa.s.sing through the middle of the zodiac, and situated in the plane of the earth's...o...b..t.

_Caroline._ I do not understand the meaning of the plane of the earth's...o...b..t.

_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 extending from one side of this circle to the other, filling up its whole area: such a plane would pa.s.s through the centre of the sun, dividing it into hemispheres. You may then imagine this plane extended beyond the limits of the earth's...o...b..t, on every side, until it reached those fixed stars which form the signs of the zodiac; pa.s.sing 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. Let fig. 1. plate 9, represent such a plane, S the sun, E the earth with its...o...b..t, and A B C D the ecliptic pa.s.sing through the middle of the zodiac.

[Ill.u.s.tration: 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...o...b..t. But to return to fig. 2. plate 8.

The s.p.a.ces 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 s.p.a.ces which extend from the tropics to the polar circles, the north and south temperate zones; and the s.p.a.ces 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 23-1/2 degrees 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 pa.s.sing 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 pa.s.sed 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 pa.s.sing 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 circ.u.mference.

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 circ.u.mference 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 pa.s.ses in the heavens. The degrees measured on the meridians from the equator, either towards the north, or towards the south, are called degrees of lat.i.tude, 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 lat.i.tude. 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 lat.i.tude.

_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 lat.i.tude, 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 lat.i.tude?

_Mrs. B._ Sixty geographical miles, which is equal to 69-1/2 English 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 lat.i.tude.

_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 s.p.a.ce 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 an _oblate 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.