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



will circulate in consequence of difference of specific gravity re- 

 sulting from difference of temperature. But this does not require 

 proof; for no physicist denies it. The point which requires to 

 be proved is this . Is the difference of specific gravity which 

 exists in the ocean sufficient to produce the supposed circulation? 

 Now his mode of experimenting will not prove this, unless he 

 makes the conditions of his experiment agree with what actually 

 exists in the ocean. These conditions I have already stated at 

 considerable length in my last paper*. If his trough be as 

 much as 1 inch in depth, it will require to be upwards of 120 

 feet in length. Let the surface-temperature of the water at one 

 end of such a trough be 80°, decreasing from the. surface down- 

 wards till at the depth of half an inch it is as low as 30° or 32°, 

 and let the water at the other end be kept at 32°, or as low as 

 it can be kept without freezing. If the experiment succeeds 

 under these conditions, his point will be established. 



But I most decidedly object to the water being heated in the 

 way in which it has been done by him in his experiment before 

 the Royal Geographical Society ; for I feel pretty confident that 

 in this experiment the circulation resulted not from difference of 

 specific gravity, as was supposed, but rather from the way in 

 which the heat was applied. In that experiment the one half of 

 a thick metallic plate was placed in contact with the upper sur- 

 face of the water at one end of the trough ; the other half, pro- 

 jecting over the end of the trough, was heated by means of a spirit- 

 lamp. It is perfectly obvious that though the temperature of 

 the great mass of the water under the plate might not be raised 

 over 80° or so, yet the molecules in contact with the metal would 

 have a very high temperature. These molecules, in consequence 

 of their expansion, would be unable to sink into the cooler and 

 denser water underneath, and thus escape the heat which was 

 being constantly communicated to them from the heated plate. 

 But escape they must, or their temperature would continue to 

 rise until they would ultimately burst into vapour. They can- 

 not ascend, neither can they descend, but will be expelled by the 

 heat from the plate in a horizontal direction. The next layer of 

 molecules from beneath would take their place and would be ex- 

 pelled in a similar manner, and this process would continue so 

 long as the heat was applied to the plate. A circulation would 

 thus be established by the direct expansive force of vapour, and 

 not in any way due to difference of specific gravity, as Dr. Car- 

 penter supposes. 



The case referred to by him of the heating-apparatus in the 

 London University is also unsatisfactory. The water leaves the 

 boiler at 120° and returns to it at 80°. The difference of spe- 

 * Phil. Mag. Oct. 1870, p. 254. 



