Page:Popular Science Monthly Volume 33.djvu/837

Rh of the ocean there has also been acquired an altogether new series of facts bearing on its temperature, and its capacity for supporting life. The variations of heat and cold, due to change of season or to day and night, which affect the surface, descend to a comparatively small depth, being greatly reduced in the first 100 fathoms, and below that depth for the most part eliminated, so that at 300 or 400 fathoms an approximately uniform temperature is met with. With increased temperature at the surface, there is increased evaporation, followed by greater density, by reason of which the surface water sinks, and the higher surface temperature is partially communicated to the subjacent strata. From the mobility of water, and its high specific heat, which is almost four times that of the materials composing the land-surface, the sea surface can never acquire a very high temperature. At the same time, the evaporation which is constantly going on from the whole surface of the ocean leads to a large quantity of the heat it receives from the sun becoming latent, and powerfully aids in preventing an accumulation of heat. These facts render the ocean one of the most important factors of terrestrial existence; it furnishes to the atmosphere the moisture which is one of the essentials of life, and serves by the circulation of its waters, and the diffusion of vapor derived from it, to equalize the temperature of the globe, by moderating the extremes both of heat and cold. Hence the greater or less proximity of the sea directly affects all conditions of climate. The circulation of the waters of the ocean, which is set up chiefly by the action of winds on the surface, but in part by variations of temperature and of density, and by the effects of evaporation, is controlled in all its details by geographical features.

Among the influences which give to the earth the characteristics that most immediately affect its fitness for occupation by man and the support of life generally, those due to the atmosphere are, without doubt, the most prominent. These, under the designation of climate, are constantly affecting us. But of all recognized branches of science, that which treats of the atmosphere—meteorology—is at the present time certainly the most backward. The reasons are not far to seek. The air is invisible, and in its upper regions inaccessible. The changes it undergoes are difficult to observe, and, from their great complexity, difficult to grasp, while what we know of them is almost wholly confined to the immediate proximity of the earth. It is pretty certain that the most important among the causes which operate on the atmosphere are changes of temperature; but the application of mathematical reasoning to the movements of an elastic fluid such as the air, charged with watery vapor, when submitted to changes of temperature upon a rotating sphere, presents very serious difficulties,