elliptical orbit, of which the sun is one of the foci, and consequently, that the earth is nearer him, in one part of her orbit than in another. From the great difference we experience between the heat of summer and that of winter, we should be led to suppose that the earth must be much nearer the sun in the hot season, than in the cold. But when we come to inquire into this subject, and to ascertain the distance of the sun at different seasons of the year, we find that the great source of heat and light, is nearest us during the cold of winter, and at the greatest distance during the heat of summer. It has been explained, under the article Optics, that the angle of vision depends on the distance at which a body of given dimensions is seen. Now on measuring the angular dimension of the sun, with accurate instruments, at different seasons of the year, it has been found that his dimensions increase and diminish, and that these variations correspond exactly with the supposition, that the earth moves in an elliptical orbit. If, for instance, his apparent diameter be taken in March, and then again in July, it will be found to have diminished, which diminution is only to be accounted for, by supposing that he is at a greater distance from the observer in July than in March. From July, his angular diameter gra dually increases, till January, when it again diminishes, and continues to diminish, until July. By many observations, it is found, that the greatest apparent diameter, of the sun, and therefore his least distance from us, is in January, and his least diameter, and therefore his greatest distance, is in July. The actual difference is about three millions of miles, the sun being that distance further from the earth in July than in January. This however, is only about one sixtieth of his mean distance from us, and the difference we should experi ence in his heat, in consequence of this difference of distance, will therefore be very small. Perhaps the effect of his proximity to the earth, may diminish, in some small degree, the severity of winter.

The heat of summer, and the cold of winter, must therefore

At what season of the year is the sun at the greatest, and at what season the least distance, from the earth? How is it ascertained that the earth moves in an elliptical orbit, by the appearance of the sun? When does the sun appear under the greatest apparent diameter, and when under the least? How much farther is the sun from us in July, than in January? What effect does this difference produce on the earth? How is the heat of summer, and the cold of winter accounted for?

arise from the difference in the meridian altitudes of the sun, and in the time of his continuance above the horizon. In summer, the solar rays fall on the earth, in nearly a perpendicular direction, and his powerful heat is then constantly accumulated by the long days and short nights of the season. In winter, on the contrary, the solar rays fall so obliquely on the earth, as to produce little warmth, and the small effect they do produce during the short days of that season, is almost entirely destroyed by the long nights which succeed. The dif ference between the effects of perpendicular and oblique rays, seems to depend, in a great measure, on the different extent of surface over which they are spread. When the rays of the sun are made to pass through a convex lens, the heat is increased, because the number of rays which naturally covered a large surface, are then made to cover a smaller one, so that the power of the glass depends on the number of rays thus brought to a focus. If, on the contrary, the rays of the sun are suffered to pass through a concave lens, their natural heating power is diminished, because they are dispersed, or spread over a wider surface than before.

SummerRays

Now, to apply these different effects to the summer and winter rays of the sun, let us suppose that the rays falling perpendicularly on a given extent of surface, impart to it a Eig. 199. certain degree of heat, then it is obvious, that if the same number of rays be spread over twice that extent of surface, their heating power would be diminished in proportion, and that only half the heat would be imparted. This is the effect produced by the sun's rays in the winter. They fall so obliquely on the earth, as to occupy nearly double the space that the

Winter Rays

Horizon

same number of rays do in the summer.

Why do the perpendicular rays of summer produce greater effects than the oblique rays of winter? How is this illustrated by the convex and concave lenses? How is the actual difference of the summer and winter rays shewn?

This is illustrated by fig. 199, where the number of rays, both in winter and summer, are supposed to be the same. But it will be observed, that the winter rays, owing to their oblique direction, are spread over nearly twice as much surface as those of summer.

It may, however, be remarked, that the hottest season is not usually at the exact time of the year, when the sun is most vertical, and the days the longest, as is the case towards the end of June, but some time afterwards, as in July and August.

To account for this, it must be remembered, that when the sun is nearly vertical, the earth accumulates more heat by day than it gives out at night, and that this accumulation continues to increase after the days begin to shorten, and conse. quently, the greatest elevation of temperature is some time after the longest days. For the same reason, the thermometer generally indicates the greatest degree of heat at two, or three o'clock on each day, and not at 12 o'clock, when the sun's rays are most powerful.

Figure of the Earth.

Astronomers have proved that all the planets, together with their satellites, have the shape of the sphere or globe, and hence, by analogy, there was every reason to suppose, that the earth would be found of the same shape; and several phenomena tend to prove, beyond all doubt, that this is its shape. The figure of the earth is not, however, exactly that of a globe, or ball, because its diameter is about 34 miles less, from pole to pole, than it is at the equator. But that its general figure is that of a sphere, or ball, is proved by many cir

cumstances.

When one is at sea, or standing on the sea shore, the first part of a ship seen at a distance, is its mast. As the vessel advances, the mast rises higher and higher above the horizon, and finally the hull, and whole ship become visible. Now, were the earth's surface an exact plane, no such appearance would take place, for we should then see the hull long before the mast, or rigging, because it is much the largest object.

Why is not the hottest season of the year at the period when the days are longest, and the sun most vertical? What is the general figure of the earth? How much less is the diameter of the earth at the poles than at the equator? How is the convexity of the earth proved, by the approach of a ship at sea?

Fig. 200,

The Earths Convexity

a

It will be obvious by fig. 200, that were the ship, a, elevated, so that the hull should be on a horizontal line with the eye, the whole ship would be visible instead of the topmast, there being no reason, except the convexity of the earth, why the whole ship should not be visible at a, as well as at b.

We know, for the same reason, that in passing over a hill, the tops of the trees are seen, before we can discover the ground on which they stand; and that when a man approaches from the opposite side of a hill, his head is seen before his feet.

It is a well known fact, also, that navigators have set out from a particular port, and by sailing continually westward, have passed around the earth, and again reached the port from which they sailed. This could never happen, were the earth an extended plain, since then the longer the navigator sailed in one direction, the further he would be from home.

Another proof of the spheroidal form of the earth, is the figure of its shadow on the moon, during eclipses, which shadow is always bounded by a circular line.

These circumstances prove beyond all doubt, that the form of the earth is globular, but that it is not an exact sphere, and that it is depressed, or flattened at the poles, is shown by the difference in the lengths of pendulums vibrating seconds at the poles and at the equator.

Under the article pendulum, it was shown, that its vibrations depend on the attraction of gravitation, and that as the centre of the earth is the centre of this attraction, so the nearer this instrument is carried to this point, the stronger will be the attraction, and consequently the more frequent its vibrations. From a great number of experiments, it has been found

Explain fig. 200. What other proofs of the globular shape of the earth are mentioned? How is it proved by the vibrations of the pendulum, that the earth is flattened at the poles?

that a pendulum, which vibrates seconds at the equator, has its number of vibrations increased, when it is carried towards the poles, and as its number of vibrations depends upon its length, a clock which keeps accurate time at the equator, must have its pendulum lengthened at the poles. And so on the contrary, a clock going correctly at, or near the poles, must have its pendulum shortened, to keep exact time, at the equator. Hence the force of gravity is greatest at the poles, and least at the equator.

Fig. 201.

The manner in which the figure of the earth differs from that of a sphere, is represented by fig. 201, where n is the north pole, and s the south pole, the line from one of these points to the other, the axis of the earth, and the line crossing this the equator. It will be seen, by this figure, that the surface of the earth, at the poles, is nearer its centre, than the surface at the equator. The actual difference between

the polar and equatorial diameters is in the proportion of 300 to 301. The earth is therefore called an oblate spheroid, the word oblate-signifying the reverse of oblong, or shorter in one direction than in another.

state.

The compression of the earth at the poles, and the consequent accumulation of matter at the equator, is probably the effect of its diurnal revolution, while it was in a soft, or plastic If a ball of soft clay, or putty, be made to revolve rapidly, by means of a stick passed through its centre, as an axis, it will swell out in the middle, or equator, and be depressed at the poles, assuming the precise figure of the earth. This figure is the natural and obvious consequence of the centrifugal force, which operates to throw the matter off, in proportion to its distance from the axis of motion, and the rapidity with which the ball is made to revolve. The parts about the equator would therefore tend to fly off, and leave

In what proportion is the polar, less than the equatorial diameter? What is the earth called, in reference to this figure? How is it supposed that it came to have this form? How is the form of the earth illustrated by experiment? Explain the reason why a plastic ball will swell at the equator, when made to revolve.

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