200 THE PHYSICAL GEOGRAPHY OF THE SEA. 



the winds at tlie surface, to have its saltness so increased as to al- 

 ter its specific gravity to sinking : Like the air, it is compressed, 

 and contracts in its descent, giving out heat, raising the tempera- 

 ture, and changing the specific gravity of like quantities in the va- 

 rious thermal strata through which it has to pass. Thus heat is 

 conveyed from the top to the bottom of the sea, there to be liber- 

 ated and impart to its waters dynamical force for their upward 

 movement. This is tlie power we paused to search for: what- 

 ever be its amount it is in the nature of a vera causa, and we must 

 therefore recognize it, if not as the sole agent, nevertheless as one 

 of the principal agents which Kature employs in the system, of 

 vertical circulation that has been ordained for the waters of the 

 sea. 



405. Now, but for the salts of the sea, this process could not go 

 Assisted by its salts, ou SO long as the laws of thermal dilatation remain 

 as they are for sea water. Unlike fresh water, which expands as 

 it is cooled below 89°. 5, sea water contracts until it has passed its 

 freezing point and attained the temperature of 25°.6." Were it 

 not for its salts, sea water once on the surface within the tropics 

 would, by reason of its warmth and thermal dilatation, remain on 

 the surface. Vertical circulation would be confined to polar seas, 

 and many of the living creatures that inhabit its waters would 

 perish for the lack of currents to convey them their food. 



406. If we except the tides, and the partial currents of the sea, 

 The origin of currents, such as those that may be created by the wind, we 

 may lay it down as a rule (§ 103) that all the currents of the ocean 

 owe their origin to difference of specific gravity between sea wa- 

 ter at one place and sea water at another ; for wherever there is 

 such a difference, whether it be owing to difference of temperature 

 or to difference of saltness, etc., it is a difference that disturbs equi- 

 librium, and currents are the consequence. The heavier water 

 goes toward the lighter, and the lighter whence the heavier comes ; 

 for two fluids differing in specific gravity (§ 106), and standing 

 at the same level, can no more balance each other than unequal 

 weights in opposite scales of a true balance. It is immaterial, as 

 before stated, whether this difference of specific gravity be caused 

 by temperature, by the matter held in solution, or by any other 

 thing; the effect is the same, namely, a current. That the sea, 



* See Prof. Hubbard's experiments, vol. i., Sailing Directions. 



