lower temperature over the ocean, the total amount of vapour deposited by any given volume of atmosphere, as it is blown from the vicinity of the tropical towards that of the polar regions, is greater than that which is taken up again. This is an interesting and important fact.
724. The effects of night and day upon the temperature of sea water.—Having, therefore, more precipitation in. high than in low latitudes at sea, we should have more clouds; and therefore it requires a longer time for the sun, with his feeble rays, to raise the temperature of the cold water which, from September to January, has brought the isotherm of 60° from latitude 56° down to the parallel of 40°, than it did for those cool surface currents to float it down. After this southwardly motion of the isotherm of 60° has been checked in December by the cold, and after the sources of the current which have brought it down have been bound in fetters of ice, it pauses in the long nights of the northern winter, and scarcely commences its return till the sun recrosses the equator, with increased powers both as to intensity and duration. Thus, in studying the physical geography of the sea, we must take cognizance of its actinometry also, for here we have the effects of night and day, of clouds and sunshine, upon. its currents and its climates, beautifully developed. These effects are modified by the operations of certain powerful agents which reside upon the land; nevertheless, feeble though those of the former class may be, a close study of this plate will indicate that they surely exist.
725. A belt of uniform temperature at sea.—Now, returning towards the south: we may, on the other hand, infer that the mean atmospherical temperature for the parallels between which the isotherm of 80° fluctuates is below 80°, at least for the nine months of its slow motion. This vibratory motion suggests the idea that there is probably, somewhere between' the isotherm of 80° in August and the isotherm of 60° in January, a line or belt of invariable or nearly invariable temperature, which extends on the surface of the ocean from one side of the Atlantic to the other. This belt or band may have its cycles also, but they are probably of a long and uncertain period.
726. The western half of the Atlantic warmer than the eastern.—The fact has been pretty clearly established by the discoveries to which the wind and current charts have led, that the western half of the Atlantic Ocean is heated up, not by the Gulf Stream
