Page:Physical Geography of the Sea and its Meteorology.djvu/409

Rh the left are as denoted by the line of arrows (Plate VI.); but, after this great sun-swing, the waters on the left side begin to lose their heat, grow cold, become heavy, and press the hot waters of this stream into the channel marked out for them. Thus it acts like a pendulum, slowly propelled by heat on one side and repelled by cold on the other. In this view, it becomes a chronograph for the sea, keeping time for its inhabitants, and marking the seasons for the great whales; and there it has been for all time vibrating to and fro, once every year, swinging from north to south, and from south to north again, a great self-regulating, self-compensating pendulum, beating time in the sea to the seasons of the year.

722. Sea and land climates contrasted.—In seeking information concerning the climates of the ocean, it is well not to forget this remarkable contrast between its climatology and that of the land, namely: on the land February and August are considered the coldest and the hottest months; but to the inhabitants of the sea, the annual extremes of cold and heat occur in the months of March and September. On the dry land after the winter "is past and gone," the solid parts of the earth contimie to receive from the sun more heat in the day than they radiate at night, consequently there is an accumulation of caloric, which continues to increase until August. The summer is now at its height; for, with the close of this month, the solid parts of the earth's crust and the atmosphere above begin to dispense with their heat faster than the rays of the sun can impart fresh supplies, and consequently, the climates which they regulate grow cooler and cooler until the dead of the winter again. But at sea a different rule seems to prevail. Its waters are the store-houses in which the surplus heat of summer is stored away against the severity of winter, and its waters continue to grow warmer for a month after the weather on shore has begun to get cool. This brings the highest temperature to the sea in September, the lowest in March. Plate IV. is intended to show the extremes of heat and cold to which the waters—not the ice—of the sea are annually subjected, and therefore the isotherms of 40°, 50°, 60°, 70°, and 80° have been drawn for March and September, the months of extreme heat and extreme cold to the inhabitants of the " great deep." Corresponding isotherms for any other month will fall between these, taken by pairs. Thus the isotherm 70° for July will fall