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.