that make any material difference with regard to the heat of the sun? Tutor. It does, let A B represent a portion of the

earth's surface

on which the sun's rays fall 3

perpendicularly; let B C represent an equal portion on which they fall obliquely or aslant. It is manifest that BC in the position of the figure, though it be equal to A B, receives but half the light and heat that A B does Moreover, by the sun's rays coming more perpendicularly, they come with greater force, as well as in greater numbers, on the same place.

CONVERSATION XI.

Of the Seasons.

TUTOR. If you now take a view of the earth in its annual course round the sun, considering its axis as inclined 23 degrees to a line perpendicular to its orbit, and keeping, through its whole journey, a direction parallel to itself, you will find that, according as the earth is in different parts of its orbit, the rays of the sun are presented perpendicularly to the equator, and to every point of the globe, within 23 degrees of it both north and south.

This figure represents the earth in

four different parts of its orbit, or as it is situated with respect to the sun in the months of March, June, September, and December.

Charles. The earth's orbit is not made circular in the figure.

Tutor. No; but the orbit itself is nearly circular: we are, however, supposed to view it from the side B D, and therefore, though almost a circle, it appears to be long ellipse.

For the

All circles appear elliptical in an oblique view, as is evident by looking obliquely at the rim of a bason at some distance from you. true figure of a circle can only be seen when the eye is directly over its centre. You observe that the sun is not in the centre.

James. I do; and it appears nearer to the earth in the winter than in the

summer.

Tutor. We are indeed more than three millions of miles nearer to the sun in December than we are in June.

Charles.

Is this possible, when our winter is so much colder than the summer?

Tutor. Notwithstanding this, it is a well known fact: for it is as'certained that our summer, that is, the time that passes between the

vernal and autumnal equinoxes, is nearly eight days longer than our winter, or the time between the autumnal and vernal equinoxes. Consequently the motion of the earth is slower in the former case than in the latter, and therefore, as we shall see, it must be at a greater distance from the sun. Again, the sun's apparent diameter is greater in our winter than in summer; but the apparent diameter of any object increases in proportion as our distance from the object is diminished, and therefore we conclude, that we are nearer the sun in winter than in summer. The sun's apparent diameter on January 1st, is 32.35′′; on July 1st, 31'. 30".

James. But if the earth is farther from the sun in summer than in winter, why are our winters so much colder than our summers?