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matter four times denser than that of the sun; and hence the quantity of matter in the sun is between two and three hundred thousand times greater than that which is contained in the earth.

Charles. Then for the momenta of these two bodies to be equal, the velocity of the earth must be between two and three hundred thousand times greater than that of the

sun.

Tutor. Just so and to effect this, the centre of gravity between the sun and earth, will be as much nearer to the centre of the sun than it is to the centre of the earth, as the former body contains a greater quantity of matter than the latter and hence it is found to be several thousand miles within the surface of the

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James, I now clearly perceive, that since one of these bodies revolves about the other in the space of a year, and that they both move round their common centre of gra vity, that it must of necessity be the earth which revolves about the sun, and not the sun round the earth.

Tutor. Your inference is just. To suppose that the sun moves round the earth is as absurd as to maintain that a mill-stone could be made to move round a pebble.

CONVERSATION X.

Of the Seasons.

TUTOR. I will now show you how the different seasons are produced by the annual motion of the earth.

James. Upon what do they de pend, sir?

Tutor. The variety of the seasons depends (1) upon the length of the days and nights, and (2) upon the position of the earth with respect to

the sun.

Charles. But if the earth turn round its imaginary axis every 24 hours, ought it not to enjoy equal days and nights all the year?

Tutor.

This would be the case if the axis of the earth N S were perpen

dicular to a line C E drawn through the centres of the sun and earth; for thẹn, as the sun always enlightens one half of the earth by its rays, and as it is day at any given place on the globe, so long as that place continues in the enlightened hemisphere, every part, except the two poles, must, during it rotation on its axis, be one half of its time in the light and the other half in darkness: or, in other words, the days and nights would be equal to all the inhabitants of the earth, excepting to those, if any, who live at the poles?

James. Why do you except the people at the poles ?

Tutor. Because the view of the spectator situated at the poles N and s, must be bounded by the line C E; consequently the sun to him would never appear to rise or set, but would always be in the horizon.

Charles. If the earth were thus situated, would the rays of the sun always fall vertically on the same part of it?

Tutor. They would: and that part would be E Q the equator; and, as we shall presently show, the heat excited by the sun being greater or less in proportion as its rays come more or less perpendicularly upon any body, the parts of the earth about the equator would be scorched up, while those beyond 40 or 50 degrees on each side of that line and

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