192

CONVERSATION IX.

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, SIDEREAL, 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 have 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 VIII. 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.

The names of these lines would have conveyed ideas of the figures they were designed to express, though the use of these figures 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 ideas should be treasured up, the application of which we may learn when the understanding is more developed. It is, I think, a very mistaken notion,

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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, though I am convinced that appropriate and scientific terms would have conveyed more precise and accurate ideas; but I was afraid of not being understood.

EMILY.

You may depend upon our learning the names of these lines thoroughly, Mrs. B. But, before we commit them to memory, will you have the goodness to explain them to us?

MRS. B.

Most willingly. This 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 CD, which divides the globe into two equal parts between the poles, 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 EF, which surrounds the north pole, is called the arctic circle; that, GH, which surrounds the south pole, the antarctic circle. There are two intermediate circles, between the polar circles and the equator; that to the north, IK, called the tropic of Cancer; that to the south, LM, 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, or plain, is an even level surface. Let us suppose a smooth thin solid plane cutting the sun through the centre, extending out as far as the fixed stars, and terminating in a circle, which passes through the middle of the zodiac. In this plane the earth would move in its revolution round the sun : it is therefore called the plane of the earth's orbit,

and the circle in which this plane cuts the signs of the zodiac is the ecliptic. Let the fig. 1. Plate IX. represent such a plane, S the sun, E the earth with its orbit, and ABCD the ecliptic passing through the middle of the zodiac.

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 the inclination of the earth's axis to the plane of its orbit. But to return to fig. 2. Plate VIII.

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.

The several lines which, you observe, are drawn from one pole to the other, cutting the equator at right angles, are called meridians. When any one of these meridians is exactly opposite the sun, it