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



and light, by the time it reaches the pole becomes cold and 

 dense. But unless it be denser than the polar water under- 

 neath, it will not sink down through it*. But why should it be 

 colder than the " whole mass " underneath, which, according to 

 Dr. Carpenter, is cooled by polar cold ? He does not explain 

 how this becomes the case. But that he does suppose it to sink 

 to the bottom in consequence of its contraction by cold would 

 seem from the following quotation : — 



" Until it is clearly apprehended that sea-water becomes more 

 and more dense as its temperature is reduced, and that it con- 

 sequently continues to sink until it freezes, the immense motor 

 power of polar cold cannot be apprehended. But when this 

 has been clearly recognized, it is seen that the application of 

 cold at the surface is precisely equivalent as a moving power to 

 that application of heat at the bottom by which the circulation of 

 water is sustained in every heating-apparatus that makes use of 

 it." (§ 25.) 



Here he says that the application of cold at the surface is 

 equivalent as a motor power to the application of heat at the 

 bottom. But the way in which heat applied to the bottom of a 

 vessel produces circulation is by convection. It makes the mo- 

 lecules at the bottom expand, and they, in consequence of buoy- 

 ancy, rise through the water in the vessel. Consequently if the 

 action of cold at the surface in polar regions is equivalent to that 

 of heat, the cold must contract the molecules at the surface and 

 make them sink through the mass of polar water beneath. But 

 assuming this to be his meaning, how much colder is this sur- 

 face-water than the water beneath ? Suppose there is one degree 

 of difference. How much work, then, will gravity perform upon 

 this one pound of water which is one degree colder than the 

 mass beneath supposed to be at 32° ? The force with which the 

 pound of water will sink will not be proportional to its weight, 

 but to the difference of weight between it and a similar bulk of 

 the water through which it sinks. The difference between the 

 weight of a pound of water at 31° and an equal volume of 

 water at 32° is 29000 °^ a P oun( l. Now this pound of water 

 in sinking to a depth of 10,000 feet, which is about the depth 

 at which a polar temperature is found at the equator, would per- 

 form only one third of a foot-pound of work. And supposing 

 it were three degrees colder than the water beneath, it would in 

 sinking perform only one foot-pound. This would give us only 



* It is a well-established fact that in polar regions the temperature of 

 the sea decreases from the surface downwards ; and the German Polar Ex- 

 pedition found that the water in very high latitudes is actually less dense 

 at the surface than at considerable depths, thus proving that the surface- 

 water could not sink in consequence of its greater density. 



