CONVERSATION XXXI.

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 depend, 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 (Plate vi. Fig. 6.) were perpendicular to a line c E drawn through the centres of the sun and earth; for then 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 its 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 to him the sun would never appear to rise, or set, but always be in the horizon.

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

Tutor. They would: and that part would be EQ 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 forty or fifty degrees on each side of that line and the poles, would be desolated by an unceasing winter :

-Some say the sun

Was bid turn reins from th' equinoctial road
Up to the Tropic Crab; thence down amain
By Leo, and the Virgin, and the Scales
As deep as Capricorn, to bring in change
Of seasons to each clime: else had the spring
Perpetual smil❜d on earth with verdant flowers,
Equal in days and nights, except to those
Beyond the polar circles; to them day
Had unbenighted shone, while the low sun
To recompense his distance, in their sight
Had rounded still th' horizon.

PAR. LOST, Book x. 1. 672.

James. In what manner is this prevented? Tutor. By the axis of the earth N s (Plate VI. Fig. 7.) being inclined or bent about twentythree degrees and a half out of the perpendicular as it is described by Milton :

He bid his angles turn askance

The poles of earth twice ten degrees and more
From the sun's axis.

In this case you observe, that all the parallel circles, except the equator, are divided into two unequal parts, having a greater or less portion of their circumferences in the enlightened, than in the dark hemisphere, according to their situation with respect to N the north, or s the south pole.

Charles. At what season of the year is the earth represented in this figure ?

Tutor. At our summer season: for you observe that the parallel circles in the northern hemisphere have their greater parts enlightened and their smaller parts in the dark. If D L represent that circle of latitude on the globe in which Great Britain is situated, it is evident that about two-thirds of it is in the light and only one-third in darkness.

You will remember that parallels of latitude are supposed circles on the surface of the earth, and are shown by real circles on its representa

tive the terrestrial globe, drawn parallel to the equator.

James. Is that the reason why our days towards the middle of June are sixteen hours long, and the nights but eight hours?

Tutor. It is and if you look to the parallel next beyond that marked D L, you will see a still greater disproportion between the day and night, and the parallel more north than this is entirely in the light.

Charles. Is it then all day there?

Tutor. To the whole space between that and the pole it is continual day for some time, the duration of which is in proportion to its vicinity to the pole; and at the pole there is a permanent day-light for six months together.

James. And during that time it must, I suppose, be night to the people who live at the south-pole?

Tutor. Yes, the figure shows that the southpole is in darkness; and you may observe, that to the inhabitants living in equal parallels of latitude, the one north, and the other south, the length of the days to the one will be always equal to the length of the nights to the other.

Charles. What then shall we say of those who live at the equator, and consequently who have no latitude?

Tutor. To them the days and nights are always equal, and of course twelve hours each in

length, and this is also evident from the figure, for in every position of the globe one half of the equator is in the light and the other half in darkness.

James. If, then, the length of the days is the cause of the different seasons, there can be no variety in this respect, to those who live at the equator.

Tutor. You seem to forget that the change in the seasons depends upon the position of the earth with respect to the sun, that is, upon the perpendicularity with which the rays oflight fall upon any particular part of the earth; as well as upon the length of days.

Charles. Does this make any material difference with regard to the heat of the sun?

Tutor. It does; let A B (Plate vi. Fig. 8.) represent a portion of the earth's surface, on which the sun's rays fall perpendicularly; let B C represent an equal portion on which they fall obliquely or aslant. It is manifest that в C, though it be equal to A в, 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.