20i THYSICAL GEOGllAPHY OP TKE SEA, AND ITS METEOROLOGY. 



out heat, i-aising the temperature, and changing the specific 

 gravity of like quantities in the various thermal strata through 

 Avhich it has to pass. Thus heat is conveyed from the top to the 

 bottom of the sea, there to bo libeiated and impart to its waters 

 dynamical force for their upward movement. This is the power 

 Ave paused to search for : whatever 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. Assisted hy its salts. — Now, but for the salts of the sea 

 this process could not go on so long as the laws of thermal 

 dilatation remain as they are for sea water. Unlike fresh water, 

 Avhich expands as it is cooled below 39°. 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 near the 

 surface within the tropics would, by reason of its warmth and 

 thermal dilatation, remain near 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. The origin of currents. — If we except the tides, and tho 

 partial currents of the sea, 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 water 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 equilibrium, and currents are 

 the consequences. The heavier water goes towards the lighter, 

 and the lighter whence the heavier comes ; for two fluids differ- 

 ing 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 tempera- 

 ture, hy the matter held in solution, or by any other thing ; the 

 effect is the same, namely, a current. 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 



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



