196 PHYSICAL GEOGRAPHY OF THE SEA, AXD ITS METEOEOLOGY. 



rated and impart to its waters djraamical force for their upward 

 movement. This is the 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 Nature 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, on SO loug as the kws of thermal dilatation remain 

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

 it is cooled below 39^.5, sea water contracts until it has passed its 

 freezuig-point and attained the temperatm'e of 25^.6.* Were it 

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

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

 the smface. Vertical ch'culation would be confined to polar seas, 

 and many of the living creatures that inhabit its waters would 

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



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

 The origin of currents, such as thosc that may bc created by the wind, we 

 may lay it do^vn as a rule (§ 103) that all the cmTents of the ocean 

 owe then' origin to difference of specific gravity between sea water 

 at one place and sea water at another ; for wherever there if? 

 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 towards 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 cmTent. That the sea^ 

 in all parts, holds in solution the same kind of solid matter ; that 

 its waters in this place, where it never rains, are not Salter than 

 the strongest brine ; and that in another place, where the rain is 

 incessant, they are not entirely 'v\Tthout salt, may be taken as evi- 

 dence in proof of a system of currents or of cnculation in the sea, 

 by which its waters are shaken up and kept mixed together as 

 though they were in a phial. 



407. CuRREXTS OF THE ATLANTIC. — The principal currents of 

 the Atlantic have been described in the chapter on the Gulf 



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



