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



present the equatorial and polar columns respectively, Dr. Car- 

 penter says : — 



" The force which will thus lift up the entire column of water 

 in W is that which causes the descent of the entire column in 

 C, namely the excess of gravity constantly acting in C, — the levels 

 of the two columns, and consequently their heights, being main- 

 tained at a constant equality by the free passage of surface-water 

 from W to C." 



"The whole of Mr. Croll's discussion of this question, how- 

 ever," he continues, "proceeds upon the assumption that the 

 levels of the polar aud equatorial columns are not kept at an 

 equality , &c." (§ 30). And again, "Now, so far from assert- 

 ing (as Captain Maury has done) that the trifling difference 

 of level arising from inequality of temperature is adequate to 

 the production of ocean-currents, I simply affirm that as fast as 

 the level is disturbed by change of temperature it will be restored 

 by gravity" (§ 23). See also to the same effect Brit. Assoc. 

 Report, 1872, p. 50. 



In order to understand more clearly how the circulation under 

 consideration cannot take place without a difference of level, 

 let WE (fig. 1) represent the equatorial column, and CP the 



Fig. 1. 



w 



polar column. The equatorial column is warmer than the polar 

 column because it receives more heat from the sun than the 

 latter ; and the polar is colder than the equatorial column be- 

 cause it receives less heat from the sun than the latter. The 

 difference in the density of the two columns results from their 

 difference of temperature ; and the difference of temperature re- 

 sults in turn from the difference in the quantity of heat received 

 from the sun by each. Or, to express the matter in other 

 words, the difference of density (and consequently the circula- 

 tion under consideration) is due to the excess of heat received 

 from the sun by the equatorial over that received by the polar 

 column ; so that to leave out of account the superheating of 

 the intertropical waters by the sun is to leave out of account the 

 very thing of all others that is absolutely essential to the exist- 



