WIT AT IS KNOWN OF THE EARTH. 817 



of tlie 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 compara- 

 tively 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 in- 

 creased 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 re- 

 ceives from the sun becoming latent, and powerfully aids in pre- 

 venting an accumulation of heat. These facts render the ocean 

 one of the most important factors of terrestrial existence ; it fur- 

 nishes to the atmosphere the moisture which is one of the essen- 

 tials 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 geograph- 

 ical 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 — meteor- 

 ology — 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 impor- 

 tant 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 tem- 

 perature upon a rotating sphere, presents very serious difficulties, 



VOL. XXXIII. — 52 



