SEASONS. In the article EARTH the motions of the earth on which the changes of the seasons ultimately depend are explained. The chief cause of the greater heat of summer and cold of winter is that the rays of the sun fall more obliquely on the earth in the latter season than in the former. See CLIMATE. Another concurrent cause is the greater length of the day in summer, and of the night in winter. Within the tropics the sun's rays have at no time so much obliquity as to make one part of the year very sensibly colder than another. There are, therefore, either no marked seasons, or they have other causes altoge,her, and are distinguished as the wet and dry seasons. This is explained in the article RAIN. But iu all the temperate parts of the globe the year is naturally divided into four seasons—spring, summer, autumn, and winter. In the arctic and antarctic regions, spring and autumn are very brief, and the natural division of the yeat is simply into summer and winter, the whiter being long and the summer,short; and this is very much the case also in regions of the temperate zones lying near the arctic and antarctic circles. In subtropical regions the distinction of four seasons is in like man ner very imperfectly marked. This distinction is everywhere arbitrary as to the periods, of the year included in each season, which really vary according to latitude, and partly according to the other causes which influence climate; time seasons passing one into another more or less gradually, and their conunencement and close not being determined by precise astronomical or other phenomena. The greatest heat of summer is never
reached till a considerable time after the summer solstice, when the sun's rays are most nearly vertical, and the day is longest; the greatest cold of winter is in like manner after the winter solstice, when the day is shortest, and the sun's rays are most oblique; the reason in the former case being that as summer advances the earth itself becomes more heated by the continued action of the sun's rays; in the latter, that it retains a portion of the heat which it has imbibed during slimmer, just as the warmest part of the day is somewhat after midday, and the coldest part of the night is toward morning. The four seasons of temperate regions are distinguished by the phenomena of nature which characterize them, anal w deli are of the greatest importance in relation to the wants and labors of man. But the renewal of vegetative activity in spring is not to be ascribed entirely to the increasing warmth of the sun's rays. Plants are so constituted that a period of rest is followed by new activity, and this new activity very generally begins im the fresh circulation of sap and enlargement of buds while the cold of winter still con tinues unabated, or before it has reached its greatest intensity. A similar remark may be made with regard to some of the phenomena of animal life, which may as well be said to herald the approach of spring as to attend its first days of genial weather.