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



ocean at the equator decreases at a uniform rate from the sur- 

 face downwards, which is far from being the case. The rate of 

 decrease is most rapid at the surface, and decreases as we de- 

 scend. The principal part of the decrease of temperature takes 

 place within no very great depth from the surface ; consequently 

 the greater part of the excess of temperature at the equator 

 over that at the poles affects the sea to no great depth. But 

 there is another reason why the expansion of the waters at the 

 equator cannot amount to near 18 feet. It is this : the rate at 

 which water expands as its temperature rises is not uniform, 

 but increases with the temperature. Sea-water, according to 

 Muncke's Table, in rising for example from 32° to 42° expands 

 •00047, whereas in rising from 70° to 80° it expands no less 

 than •00152. But these higher temperatures affect only a small 

 quantity of water near the surface ; the great depth of water 

 below is affected by the lower temperatures, which do not pro- 

 duce much expansion. As no reliable observations, so far as I 

 am aware, have been made to ascertain the rate at which the 

 temperature of the waters at the equator decreases from the 

 surface downwards to great depths, it is impossible to determine 

 with any thing like accuracy the height at which the ocean, in 

 virtue of higher temperature, should stand above the level of the 

 ocean at the poles. But one thing we are certain of is that it 

 must be very much under 18 feet, and that the force acting on 

 the waters of the ocean to impel them forward as a current 

 resulting from the difference of specific gravity between the 

 sea in intertropical and polar regions, is very much under one- 

 fourth of a grain per cubic foot. And if the sea in intertropical 

 regions is much Salter than the sea in polar regions, as Maury 

 strongly insists, then this will make the force still less ; for 

 this will go so far to neutralize the effects due to difference of 

 temperature between the waters of equatorial and polar regions. 



It is perfectly evident that a pressure of one-fourth of a grain 

 on the cubic foot of water, were it even so great as that, would 

 be totally inadequate to overcome the mere molecular resistance 

 of the water to go into motion, far less to produce the great 

 currents of the ocean. It is therefore certain that ocean-cur- 

 rents are in no way whatever due to differences of specific 

 gravity. 



But it must be observed that this force of one-fourth of a 

 grain per cubic foot would affect only the water at the surface; 

 a very short distance below the surface the force would be abso- 

 lutely insensible. 



If water were perfectly fluid, and offered no resistance to mo- 

 tion, it would not only flow down an incline, however small it 

 might be, but would flow down with an accelerated motion. 



