Mr. J. Croll on the Physical Cause of Ocean-currents. 245 



cific gravity between the water leaving the boiler and the water 

 returning to it is supposed to produce the circulation. It seems 

 to me that this difference of specific gravity has nothing what- 

 ever to do with the matter. The cause of the circulation must be 

 sought for in the boiler itself, and not in the pipes. The heat is 

 applied to the bottom of the boiler, not to the top. What is the 

 temperature of the molecules in contact with the bottom of the 

 boiler directly over the fire, is a question which must be consi- 

 dered before we can arrive at a just determination of the causes 

 which produce circulation in the pipes of a heating-apparatus 

 such as that to which Dr. Carpenter refers. But, in addition 

 to this, as the heat is applied to the bottom of the boiler and not 

 to the top, convection comes into play, a cause which, as we 

 shall find, does not exist in the theory of oceanic circulation at 

 present under our consideration. 



But, be all this as it may, though I do not believe that in Dr. 

 Carpenter's experiments circulation is effected by difference of 

 specific gravity, still I freely admit that difference of specific 

 gravity will produce a circulation such as that supposed by 

 him. Neither do I deny that the thing can be shown expe- 

 rimentally. What I affirm is, that no experiment can show that 

 water will circulate under difference of specific gravity if the con- 

 ditions of the experiment be made to agree with what exists in 

 the ocean ; and unless these conditions are complied with, any 

 experiment, no matter what it may be, is useless so far as con- 

 cerns the question at issue. 



The Force exerted by Gravity. — Sir John Herschel, in proving 

 that difference of specific gravity could not be the cause of ocean- 

 currents, adopted the second of the three methods to which I 

 have referred ; viz. he showed that the force of gravitation, acting 

 on the waters of the ocean in virtue of specific gravity, is not 

 sufficient to produce the required motion. Sir John in his cal- 

 culations had taken 39° as the temperature of maximum den- 

 sity. The temperature of maximum density, however, is much 

 lower than this ; and as Dr. Carpenter maintained that all de- 

 terminations based upon the supposition that 39° is the tempe- 

 rature of maximum density gave too low an estimate of the effect 

 of difference of specific gravity in causing motion, I calculated 

 in my paper what would be the force of gravity, taking 32° 

 to be the temperature of the greatest density instead of 39°, 

 and found that the force of gravitation was about the same as 

 when the temperaturewas taken at 39°, — the reason being, that 

 when we take 32° as the temperature of maximum density, we 

 have 18 feet as the height of the ocean at the equator above the 

 place of maximum density, but then this place is the pole, 

 whereas when we take 39° as the temperature of maximum 



