? climate is also a portion of the earth's surface, in cluded between two parallels of latitude, and of such a breadth, that the longest clay under the parallel nearest the pole, is half an hour longer than under the other. There are twenty-four such climates between the equator and either of the polar circles. Between the polar circle and the pole, there are six climates of such a breadth that the longest day under the two parallels varies by a month. There are thus thirty climates in all on each side of the equator.
The principle that has already been employed, in ex plaining the division of the earth into zones, may serve also to illustrate the nature of climates. When the sun is vertical to Q, (Plate CCLXV. Fig. 2 ,) the circle of illu mination coinciding with the meridian N S, divides the equator 2F.Q, and every parallel of latitude, into two equal parts, or the clay and night are equal all over the globe. When the sun passes to either side of Q, as northward wards L, the equator is still divided into two equal parts by the circle of illumination, as it must always be by a great circle ; but the parallels of latitude are divided unequally, the greater arch of the northern parallels being above, and of the southern below, the circle of illumination ; that is, the day and night are still equal at the equator, but in northern latitudes the day is longer than the night, and in southern the night is longer than the day. This inequality continues to increase in all latitudes, as the sun approaches L ; and of two given parallels, as c d, e 1, at any timeohe inequality is greatest at that which is nearest the pole. When time sun reaches L, or at the summer solstice, the day is longest in all northern, and shortest in all southern latitudes. At the polar circle it is 24 hours, the whole of the parallel a b being above the circle of illumination. At the parallel c d, the clay is to the night as twice p d to twice p c, or as p d to p c ; at e L it is as q L to g e, icc. ; and at the equator they arc equal. It is obvious, therefore, that between the equator and the polar circle, the length of the longest day varies from 12 to 24 hours, and that conse quently 24 parallels may be found t.t such distances from each other, that the longest clay under the one, shall exceed that under the other, by half an hour. With regard to places within the polar circle, it is evident that the parallel which comes to be wholly above the circle of illumination, half a month before the sun reaches the summer solstice L, or two months and a half after be passes Q, will conti nue to be a month above time circle, viz. a month be fore the sun reaches L, and half a month till he returns to the same position. In like manner, the parallel mat to be wholly illuminated one month before the sun reaches the solstice, or two months after the vernal equinox, will continue to be so for two months, and so of others. As
the pole rises above the circle of illumination at the equi nox, or three months before the solstice, it continues, as was formerly observed, to be illuminated for six months. Hence, between the polar circles and the poles, six paral lels may be found such, that the longest day under the one, shall he a month longer than under the other. The slime reasoning will apply to the southern hemisphere doling the sun's progress from Q to b; and, in general, it is to be observed, that at any time the length of the day, at a given latitude in one hemisphere, is always equal to the night, at the same latitude in the opposite hemisphere. The climates, as above defined, are contained in the follow ing Table, where the first column spews the number of the climate, the second the length of the day under the highest parallel, or that nearest the pole, the third the la titude of that parallel, and the fourth the breadth of the climate.
Besides dividing the earth into different climates, the ancients also employed certain terms to distinguish the in habitants of particular countries, which it may be useful to notice. Those who live under the same meridian and parallel of latitude, but on opposite sides of the equator, wete called relatively to one another ?lnteccii, from an„ OPPOSite to ; and ciY.ICe, a habitation. They have always the same hour of the day, but opposite seasons of the yea r. Those who live on the same side of the equator, and under the same parallel of latitude, but differ 180° in longitude, were called Perirecii, from 7rEel, about, and OiY.ltt, a habita tion. They have always the same seasons, but opposite hours of the day. The inhabitants of places under the same parallel of latitude, but on opposite sides of the equator, and differing in longitude were called the ;dial/lodes of each other, from avrt, opposite to, and the foot. They have always opposite hours of the day, as well as contrary seasons of the year. The inhabitants of the different zones were also distinguished according to the projection of their shadows. Thus the inhabitants of the torrid zone were called ?imphiscii, from aver ; around. and a shadow ; because their shadow is projected sometimes towards the north, and at other times towards the south; or ?scii, from a, without, and G-xia, because they sometimes have no shadow. The inhabitants of the temperate zones were called Heteroscii, from ;TEeoz, differ ent, and csia; because their shadows are always projected in opposite directions, or towards the poles. And the in habitants of the frigid zones were named Periscii, from meet, about, and csra ; because, during their longest day, their shadows describe a circle round them.