THE GULF STREAM. 



205 



that the water of least specific gravity is already upper- 

 most, there can be no tendency to motion. For the 

 heated water cannot descend, being buoyant; nor 

 ascend, being uppermost ; nor move laterally, having 

 no impulse to motion of that sort, and being only able 

 to move laterally ' by reason of a general declivity of 

 surface, the dilated portion occupying a higher level.' 

 He then applies to this declivity the test of quanti- 

 tative analysis. Taking a column of water at the 

 equator having at the base a temperature of 39 (at 

 which temperature fresh water attains its greatest 

 density, and which is also the temperature of water 

 7,200 feet beneath the surface at the equator), while its 

 top has a temperature of 84 (the warmth of equatorial 

 surface-water), he finds that such a column is 10 feet 

 higher than a similar column in latitude 56, where 

 39 is the surface temperature. And since from 

 the equator to latitude 56 the distance is 3,360 

 geographical miles, we have a declivity of barely one- 

 twenty-eighth of an inch per geographical, or one- 

 thirty-second of an inch per statute mile. Such a 

 declivity is utterly insufficient to account for the 

 existence of a strong current from the equator towards 

 the tropics ; while, so far from giving any account of 

 the motion of the equatorial current from east to west, 

 it would tend to form a north-easterly current. 



This seems strongly opposed to Maury's view, and I 

 do not find that he does much to get over the force of 

 Herschel's reasoning. He points out, indeed, that sea- 

 water does not attain its greatest density at a tempera- 



