I think, Sir, when you left me last night you told me our next business would be to explain the nature of the seasons?

Tutor. I did so, and am persuaded you will find no great difficulty in comprehending it.—Cast your eye on the little drawing I gave you, (Plate III. fig. 2.) where the earth is represented as situated at the four quarters of the year, namely, Spring, Summer, Autumn, and Winter.—But before we proceed to an explanation it will be necessary to remark, that, in the little scheme the eye is supposed to be elevated above the plane of the earth’s orbit, and that we see it very obliquely. The orbit by this means appears very elliptical; and, the enlightened hemisphere, or that half of the earth which is turned to the sun in the spring, and the darkened hemisphere, or that turned from him in the autumn, are there represented.

Pupil. This I understand.

Tutor. Well then, we will begin with the spring.—In this situation of the earth the equator is exactly opposed to the sun: and, as he always enlightens a hemisphere, or half of its surface, his rays will reach to both the poles: whence, from the diurnal revolution of the earth, the day and night are equal all over the globe.

Pupil. This I remember you told me happened when the sun was in Aries and Libra. The sun is now entering Aries: and, as we are in the rays of the sun one half of the diurnal revolution, and in the shadow of the earth, or dark, the other half, the day and night must be equal.

Tutor. Certainly. And as the sun enters Aries in the equinoctial, it is then called the Vernal, that is, Spring Equinox. When the sun enters the opposite sign Libra, the same effects are produced, and it is then called the Autumnal Equinox.

Pupil. You have passed on from Spring to Autumn.

Tutor. I have so.—We will now return, and trace the earth in its orbit from spring to summer.—You have already seen that the north and south poles are both enlightened, and that the day and night are equal at the equinoxes. If the axis of the earth were perpendicular to the plane of the earth’s orbit, this would constantly be the case, and we should have no diversity of seasons: for, the sun being over the equator, the poles must be perpetually enlightened, and of course we should have equal day and night at all times of the year.

Pupil. That is plain. I suppose then that it is to the inclination of the earth’s axis we are indebted for the increase and decrease of days.

Tutor. It is occasioned by the inclination of the earth’s axis and its preserving its parallelism, which I explained to you last evening.—As the sun is now in the first point of Aries, the earth you know must be in the beginning of Libra, it being the opposite sign.—Now fix your attention on the scheme, and imagine the earth to be advancing in its orbit through Libra, Scorpio, and Sagittarius: and at the first degree of Capricorn give me your opinion of the earth’s position.

Pupil. The north pole is turned to the sun, the south pole from him, and the tropic of Cancer is opposite to him.

Tutor. How many degrees are the tropics from the equator, or, in other words, what is the inclination of the earth’s axis?

Pupil. Twenty-three degrees and a half.

Tutor. And so far are the rays of the sun cast beyond the north pole, and fall short of the south pole: so that the whole of the arctic circle is enlightened, and the antarctic circle involved in darkness.

Pupil. What conclusion am I to draw from this?

Tutor. That in the northern half of the globe it is the longest day, or summer, and in the southern half the shortest, or winter, whilst under the equator the days and nights are equal.

Pupil. I used to think that when it was winter or summer here it was so in every part of the world.

Tutor. You now find your mistake. For as the earth is making its progress from Libra, the north pole is approaching the sun, and the south pole receding from him: consequently the length of the day is increasing in the northern hemisphere and decreasing in the southern.—The sun has now been three months above the horizon of the north pole, and the same time below that of the south pole, and in three months more, when the earth arrives at Aries, the scene will be reversed: the sun will be over the equator, both poles will be again enlightened, and the day and night will be equal in every part of the globe. The sun will now be rising to the south and setting to the north pole. This is our Autumn.

Pupil. And as the earth is advancing towards winter, the south pole will be turning to the sun, and the north pole from him, whence I conclude that when the earth is in Cancer it must be summer, south of the equator, when it is our winter.

Tutor. Most assuredly. For you see that the sun is over the tropic of Capricorn, which you know is as much south of the equator as the tropic of Cancer is north of it, where the sun was in our summer. The antarctic circle is now enlightened, and the arctic obscured in shade; but, under the equator there is neither increase nor decrease, the days and nights being each twelve hours.

Pupil. It is now our winter, the sun has been three months above the horizon of the south pole, and will continue so till the vernal equinox, when he will again rise to the north pole, and so on in regular succession.

Tutor. It must be plain then to you that there can be but one day and one night at each of the poles, reckoning the time the sun is above or below their respective horizons; under the arctic and antarctic circles, the longest day is twenty-four hours, and in the shortest the sun is just visible in the horizon at noon. The longest day decreases in length the nearer we approach the equator, where I before observed there is no variation, because the circle bounding light and darkness, in every position of the earth, divides the equator into two equal parts; and, it must be observed, that the longest day and longest night are equal to each other in every part of the globe.

Pupil. If the longest day under the arctic circle be just twenty-four hours, the sun must rise in the north.

Tutor. He does so, makes a complete circle and sets in the ^[15]north again. From the arctic circle to the equator, he rises north of the east and sets north of the west: at the equator he rises due east and sets due west, thence southward to the antarctic circle, he rises south of the east, and sets south of the west: and under the antarctic circle, as I observed just now, he is visible in the horizon in the south at noon.

Pupil. We usually say, the sun rises in the east and sets in the west.

Tutor. At the equinoxes it must be so in all parts of the globe, the poles excepted: in every other situation, except under the equator, there is a continual change. What I have now told you, respecting the northern hemisphere, will be reversed at our shortest day: that is, in the northern hemisphere the sun will rise south of the east and set south of the west; and, in the southern hemisphere the contrary, the sun will be in the horizon, at noon, under the arctic circle, and the day will be twenty-four hours under the antarctic circle.

Pupil. Pray Sir, are the regions within the polar circles inhabited? If they are, their situation, in winter, must, I think, be dreadful.

Tutor. It is foreign to my present purpose to speak of the inhabitants of the earth, as that more properly belongs to Geography. Thus much however I shall tell you, that, although it must be very cold and dreary, they are not so long deprived of light as you may imagine; for, even under the poles, when the sun is hidden from them, they are but a short time in total darkness, for, you must recollect, that the twilight continues till the sun is eighteen degrees below the horizon; and the sun’s greatest depression, you know, can be but twenty three degrees and a half, equal to the inclination of the earth’s axis. Besides this, the moon is above the horizon of the poles a fortnight together; being half her period north, and the other half south, of the equator; and, as the moon at full is in the sign opposite to the sun, the tropical full moons must be twenty-four hours above the horizon at the polar circles.

Pupil. This description is very pleasing, as I had no idea of their being favoured with so much light in the absence of the sun: and, I find, as the sun is longer above the horizon in summer than in winter, the moon, on the contrary, continues longer with us in winter, when we most need her assistance, than she does in summer.

Tutor. As you seem to understand what I have been explaining, I shall shew you, that the reason why it is hottest when we are farthest from the sun is, that in winter when we are nearest to him the days are shorter, his rays sail very obliquely on us, and are more dispersed than they are in summer, when he not only remains longer above the horizon, but being higher, his rays fall more direct on us, by which means the earth becomes so much heated that it has not time in the short nights to get cold again.—When the earth is nearest the sun it is summer in the southern hemisphere, therefore it is reasonable to suppose that the heat there must far exceed ours in the same latitude; but to counteract this their summer is shorter by eight days than ours: and it is well known that it is much colder near the poles in the southern than in the northern hemisphere: but this is accounted for from there being more land to retain the heat in the latter than in the former.

Pupil. My doubts on this head being now removed, I must beg you to give me such other information as you may think proper.

Tutor. As there are different degrees of heat and cold, the earth has been divided into five zones, namely, one torrid, two temperate, and two frigid zones.

Pupil. How are they distinguished?

Tutor. The torrid zone is all that space surrounding the globe contained between the tropics, having the equator running through the middle of it. It is so called on account of its excessive heat, for, twice every year the sun is vertical to the inhabitants, that is, he shines directly on their heads, and casts no shadow, but under their feet, at noon.

Pupil. We find it sometimes extremely hot here in our summer; surely, in the torrid zone it must be almost insupportable?

Tutor. They are inured to it from their infancy.—But we are departing from our subject.—The temperate zones are comprehended between the tropics and polar circles, that between the tropic of Cancer and the arctic circle is called the north temperate zone, and that between the tropic of Capricorn and the antarctic circle the south temperate zone.

Pupil. I suppose they are called temperate because the heat is not so intense as in the torrid zone?

Tutor. True. Neither is the cold so severe as in the frigid zones, which are those regions comprized within the polar circles, and are denominated north and south, as they are contiguous to the north or south poles.

Pupil. Why are they called frigid?

Tutor. They are called frigid or frozen zones, because near the poles there are perpetual fields of ice, the heat of the sun, even in summer, being insufficient to dissolve it.—Now try if you can tell me the breadth of each zone in degrees.

Pupil. The torrid zone being twenty-three degrees and a half on each side the equator must be forty-seven degrees, which must also be the breadth of the frigid zones, as the polar circles are distant twenty-three degrees and a half from the poles, which are their centers. And, as from the equator to either pole is ninety degrees, from the equator to the tropics twenty-three and a half, and from the polar circle to the pole twenty-three and a half, if the sum of these, that is, forty-seven, be taken from ninety, the remainder, forty-three, will be the breadth of each of the temperate zones.

Tutor. Very well.

Pupil. From what you have told me I have no doubt but that the earth is globular, but I have no proof of it: I must therefore beg your assistance.

Tutor. That it cannot be an extended plane, as some have imagined, is very evident; for, if it were, the angle made with that plane and the north pole star would be always equal, for reasons I have before given you: neither can it be cylindrical, that is like a garden roller, as others have supposed.—If a person travel northward the pole star becomes more elevated, and if he could penetrate to the north pole of the earth the star would be in the zenith, or directly over his head: on the contrary, if he travel southward, it is more and more depressed till he arrives at the equator, where the star is in the horizon; as he proceeds it disappears, and other stars rise to his view, invisible to us. Here then you see it must be circular northward and southward.

Pupil. I am convinced it must be so.

Tutor. And it is as certain that it is so east and west: for, navigators have often sailed round it steering the same course: that is, if they sail an easterly or westerly course at setting off, by continuing the same course they will return to the port whence they departed. This you know they could not do if it were not round, any more than an insect could, by crossing a round table, arrive at the place it set out from; but, by going round the edge it would be still going forward and come again to the point it had left.

Pupil. It is very evident.

Tutor. Again. In every direction, if a ship be seen at a distance, the first things observed are the top-mast and rigging, whilst the hull or body of the ship is hidden behind the convexity, that is roundness of the water, just as you would see a man coming over a hill, you would first see his head, he would be rising more and more to your view till he arrived at the top, where he would be full in sight.

Pupil. I am at a loss to account for the convexity of the water. How can its surface be round?

Tutor. Have you never observed the drops of water falling from the eaves of a house?

Pupil. Often, Sir.

Tutor. Of what shape were they?

Pupil. Globular.—But what is the cause of their being so?

Tutor. Attraction.—For as every particle of water which composes the drop tends to the same center, every part of the surface must be equidistant from the center, it must therefore be spherical. In like manner if you separate quicksilver, each portion will form itself into a globe.

Pupil. All this is very clear. And, for the same reason, the water in the ocean must be convex; for, I remember you told me that it gravitated towards the center of the earth.

Tutor. Once more.—I think you must have seen an eclipse of the moon.

Pupil. I have, Sir.

Tutor. Of what figure was the darkened part?

Pupil. Circular.

Tutor. Take this ball, and hold it before the candle between your finger and thumb, so that the shadow may be thrown on the wall, and in all positions you will find it circular.

Pupil. It is so.

Tutor. Apply this crown piece in the same manner, with the flat side to the candle.

Pupil. It is a circle.

Tutor. Turn it a little obliquely.

Pupil. It is now an ellipsis.

Tutor. Now turn the edge to the candle.

Pupil. The shadow is a strait line.

Tutor. You now see that no other body than that of a globe can in all positions cast a circular shadow.

Pupil. I do, Sir.

Tutor. The darkness on the disc of the moon at the time of an eclipse is the shadow of the earth, which in all situations is circular; the earth, therefore, which casts the shadow, must be a globe.

Pupil. It must be so.—But——

Tutor. The earth is mountainous.—It is so: but remember that the highest mountain bears no greater proportion to the bulk of the earth than the small irregularities on the peel of an orange bears to that fruit: that objection therefore is soon removed. And yet it is not a true sphere.

Pupil. What then?

Tutor. A spheroid, that is, it is a little flattened at the poles, and is in shape not unlike an orange or a turnip. This you will not be surprized at when I tell you that the equatorial parts are about four thousand miles from the center of motion.

Pupil. I suppose then you infer that as the centrifugal force is greater the farther it is removed from the center, that the parts near the poles have a tendency to fly off towards the equator.

Tutor. I do. And as we have finished this part of our subject, I shall take leave of you.