Mr. J. Croll on the Physical Cause of Ocean- currents. 253 



is in whole or in part restored. Take the latter supposition, and 

 suppose that water has flowed down till an addition of 2 feet of 

 water is made to the polar column, and the difference of level, of 

 course, diminished by 2 feet. The surface of the ocean in this 

 case will now be represented by the dotted line P' E, and the 

 slope reduced from 18 feet to 16 feet. Let us then suppose a 

 pound of water to leave E and flow down to P' ; 16 foot-pounds 

 will be the amount of work performed. The polar column being 

 now too heavy by the extent of the mass of water P'P 2 feet thick, 

 its extra pressure causes a mass of water equal to P' P to flow off 

 laterally from the bottom of the column. The column there- 

 fore sinks down 2 feet till P' reaches P. Now the pound of 

 water in this vertical descent from V to P has 2 foot-pounds of 

 work performed on it by gravity; this, added to the 16 foot- 

 pounds derived from the slope, gives a total of 18 foot-pounds 

 in passing from E to P' and then from P' to P. This is the 

 same amount of work that would have been performed had it 

 descended directly from E to P. In like manner it can be 

 proved that 18 foot-pounds is the amount of work performed in 

 the descent of every pound of water of the mass P' P. The first 

 pound which left E flowed down the slope directly to P, and per- 

 formed 18 foot-pounds of work. The last pound flowed down 

 the slope E P', and performed only 16 foot-pounds; but in de- 

 scending from P' to P it performed the other 2 foot-pounds. A 

 pound leaving at a period exactly intermediate between the two 

 flowed down 17 feet of slope and descended vertically 1 foot. 

 Whatever path a pound of water might take, by the time that it 

 reached P 18 foot-pounds of work would be performed. But no 

 further work can be performed after it reaches P. 



But some will ask, in regard to the vertical movement, is it 

 only in the descent of the water from P' to P that work is per- 

 formed ? Water cannot descend from P' to P, it will be urged, 

 unless the entire column P underneath descend also. But the 

 column P descends by means of gravity. Why, then, it will 

 be asked, is not the descent of the column a motive power as real 

 as the descent of the mass of water P' P ? 



Does Dr. Carpenter suppose that motive power is derived from 

 the descent of the polar column P ? Unless he does so, it is 

 difficult to understand what he means by saying that, according 

 to his theory, ' ' the deep efflux of polar water is considered as the 

 primum mobile of the General Oceanic Vertical Circulation" 

 (§ 29). Again, unless he considers that the descent of the water 

 below the level of P is a motive power, what grounds can he have 

 for asserting that I have ignored the primum mobile of the whole 

 affair ? Gravity cannot perform any more work upon the water 

 above the level of P than what is derived from the slope ; and 



