Tutor. Because first, in the summer, the sun rises to a much greater height above our horizon, and therefore, its rays coming more perpendicularly, a greater number of them, as I showed you yesterday, must fall upon the surface of the earth, and they come also with greater force; which are the principal causes of our great summer's heat. Secondly, in the summer, the days are very long, and the nights short; therefore the earth and air are heated by the sun in the day more than they are cooled in the night.

James. Why have we not, then, the greatest heat at the time when the days are longest?

Tutor. The hottest season of the year is certainly a month or two after this, which may be thus accounted for. A body once heated

does not become cold again instantaneously, but gradually; now, as long as more heat comes from the sun in the day than is lost in the night, the heat of the earth and air will be daily increasing, and this will evidently be the case for some weeks after the longest day, both on account of the number of rays which fall on a given space, and also from the perpendicular direction of those rays.

James. Will you now explain to us in what manner the seasons are produced?

Tutor. By referring to the last figure you will observe, that in the month of June, the north pole of the earth inclines towards the sun, and consequently brings all the northern parts of the globe more into light than at any other time in the year.

Charles. Then to the people in those parts it is summer?

Tutor. It is: but in December, when the earth is in the opposite part of its orbit, the north-pole declines from the sun, which occasions the northern places to be more in the dark than in the light; and the reverse at the southern places.

James. Is it then summer to the inhabitants of the southern hemisphere?

Tutor. Yes, it is; and winter to us. In the months of March and September, the axis of the earth does not incline to, or decline from the sun, but is perpendicular to a line drawn from its centre. And then the poles are in the boundary of light and darkness, and the sun being directly vertical to, or over the equator, makes equal day and night

at all places. Now trace the annual motion of the earth in its orbit for yourself, as it is represented in the figure.

Charles. I will, sir; about the 20th of March the earth is in Libra, and consequently to its inhabitants the sun will appear in Aries, and be vertical to the equator.

Tutor. Then the equator and all its parallels are equally divided between the light and dark.

Charles. Consequently the days and nights are equal all over the world. As the earth pursues its journey from March to June, its northern hemisphere comes more into light, and on the 21st of that month, the sun is vertical to the tropic of Cancer.

Tutor. You then observe, that all the circles parallel to the equator

are unequally divided; those, in the northern half, have their greater portions in the light, and those in the southern half, have their larger portions in darkness.

Charles. Yes; and, of course, it is summer to the inhabitants of the northern hemisphere, and winter to the southern.

I now trace it to September, when I find the sun vertical again to the equator, and, of course, the days and nights are again equal. And following the earth in its journey to December, or when it has arrived at Cancer, the sun appears in Capricorn, and is vertical to that part of the earth called the tropic of Capricorn, and now the southern pole is enlightened, and all the circles on that hemisphere have their larger parts in light, and, of course, it is