m Fluids. 331 



at the surface of the sea, and that at the depth of 3600 

 feet has been found to amount to no less than 3 1 de- 

 grees ; the temperature above or at the surface being 

 84°, and at the given depth below no more than c^^ .^ 



It appears to me to be extremely difficult, if not quite 

 impossible, to account for this degree of cold at the bot- 

 tom of the sea in the torrid zone on any other supposition 

 than that of cold currents from the poles ; and the util- 

 ity of these currents in tempering the excessive heats of 

 those climates is too evident to require any illustration. 



These currents are produced, as we have already seen, 

 in consequence of the difference in the specific gravity of 

 the sea water at different temperatures ; their velocities 

 must therefore be in proportion to the change produced 

 in the specific gravity of water by any given change of 

 temperature ; and hence we see how much greater they 

 must be in salt water than they could possibly have been 

 had the ocean been composed of fresh water. 



It is not a little remarkable that the water of all great 

 lakes is fresh, and nearly so in all inland seas (like the 

 Baltic) in cold climates, and which communicate with the 

 ocean by narrow channels. We shall find reason to con- 

 clude that this did not happen without design, when we 

 consider what consequences would probably ensue should 

 the waters of a large lake in an inland situation, in a cold 

 country (such as the lake Superior, for instance, in 

 North America), become as salt as the sea. 



Though the cold winds which blow over the lake in 

 the beginning of winter would be more warmed, and the 

 temperature of the air on the side of the lake opposite 

 to the quarter from whence these winds arrive would be 

 rendered somewhat milder than it now is; yet, as the 



* Phil. Transactions, 1752. 



