32 Baron N. Schilling on the Constant Currents 



If, then, difference of temperature can only call forth incon- 

 siderable winds by the expansion of the air, it is clear that in 

 water, so much less expansible, heated to only a proportionally 

 slight depth, no great current can be generated by this cause. 

 If this force were sufficient to occasion a considerable current, it 

 would extend over the entire surface of the ocean, and not merely 

 show itself in a narrow strip at its margin. 



We nevertheless allow that the heating of the water may, in 

 certain cases, have an important influence on the maintenance 

 and extension of an already existing current. If, e. g., a cur- 

 rent arising from other causes strikes upon a coast, it usually 

 takes the direction of this coast, along which it continues until 

 it gets beyond the sphere of the action to which it owes its de- 

 velopment. But if it accumulates at the coast the heated sur- 

 face-water of the sea, the mass of warmer and lighter water, 

 continually replaced, will perpetually exhibit the endeavour to 

 spread over the heavier water of colder regions. Difference of 

 temperature will therefore in this case have an essential influence 

 on the continuance and the direction of the currents toward 

 higher latitudes, but cannot independently generate the currents. 

 This, then, explains to us also how it is that warm and cold cur- 

 rents are found preeminently at sea-coasts. The first impulse, 

 however, to the flowing which collects the heated water does not 

 arise from difference of temperature, but always from other 

 causes. The ascertaining of these initiating causes is of very 

 great importance for the foundation of a theory; for without 

 accurate knowledge of the fundamental laws, we can get no 

 account of the action of the accessory causes. 



Let us now consider the possible influence of evaporation. As 

 already said, the evaporation of water is in close connexion with 

 heat ; for with a rise of temperature the capability of the air to 

 take up aqueous vapour is also increased. Hence much more 

 water is evaporated in the equatorial regions than in higher 

 latitudes; and the vapours are driven by air-currents into other, 

 cooler regions, where, on the cooling of the air, they are given 

 back to the sea as an atmospheric precipitate. Evidently from 

 this cause must arise a sea-current toward the equator, though 

 only a very inconsiderable one. Muhry calculates that in the 

 tropics about 15 feet of water evaporate yearly, therefore 

 about half an inch daily. Perhaps half of this evaporation is 

 returned to the tropical seas as rain and river-w 7 ater, and only 

 the other half (J inch daily) returns by sea-currents from higher 

 latitudes. But a current which in the course of 24 hours re- 

 places only a layer of water a quarter of an inch in thickness, 

 must be imperceptible. This slight current flows at the surface, 

 and is directed to the equator; it thus counteracts the current 



