262 Mr. J. Croll on the Physical Cause of Ocean-currents. 



to reduce the temperature of its whole mass below the freezing- 

 point of fresh water " (§ 22) . Here Dr. Carpenter not only 

 admits that the greater heating effect of the tropical sun is for 

 the most part confined to the " uppermost stratum/' but he goes 

 further, and admits that its effect " may here be practically dis- 

 regarded." But it seems to me that if the heating of the upper 

 stratum be practically disregarded, then, so far as his theory 

 is concerned,- every thing else may be also practically disregarded. 

 For, according to his theory, difference of density is due to dif- 

 ference of temperature; but, on the other hand, the temperature of 

 the sea in intertropical regions differs from that of the sea in polar 

 regions only so far as the warm upper stratum is concerned. 

 The tropical sea is warmer than the polar, just because it receives 

 more heat from the sun than the polar. But the heat of the sun is 

 received on the surface of the sea; it is " surface-heat." If this 

 warm upper stratum in tropical regions- be left out of account, 

 then there is actually no difference whatever between the tem- 

 perature of the sea in tropical and polar regions. And if there 

 is no difference in temperature, there can be no difference in 

 specific gravity, and consequently nothing, according to Dr. Car- 

 penter's theory, that can possibly move the water. Cold is not 

 a something positive imparted to the polar waters giving them 

 motion, and of which the tropical waters are deprived. If we 

 dip one hand in a basin filled with tropical water at 80° and the 

 Other in a basin filled with polar water at 32°, referring to our 

 sensations, we call the water in the one hot and the water in the 

 other cold ; but so far as the water itself is concerned, heat and 

 cold simply mean difference in the amounts of heat possessed. 

 Both the polar and the tropical water possess a certain amount of 

 energy in the form of heat, only the polar water does not possess 

 so much of it as the tropical. 



But we have more than a mere statement that the excess of heat 

 in equatorial regions is chiefly confined to the upper stratum of 

 the ocean. Dr. Carpenter affords us positive evidence on the sub- 

 ject. From a series of three observations made in the Mediter- 

 ranean, he found that the superheating produced by the direct 

 action of the sun upon the surface " is almost entirely limited to 

 a stratum fifty fathoms deep, the descent of the thermometer 

 being most marked in the first twenty fathoms" (§ 40). Fortu- 

 nately one of the observations was made at a place where the 

 temperature of the water at the surface was as high as 77°, 

 which is within 3° of what I have in my calculations assumed to 

 to be the surface-temperature of the sea in equatorial regions. 

 The following Table will show the rate at which Dr. Carpenter 

 found the temperature to sink from the surface to the depth of 

 100 fathoms : — 



