208 LIGHT SCIENCE FOR LEISURE HOURS. 



illustration, the Gulf Stream may be likened to the jet, 

 and the Atlantic to the pool. We remember to have 

 observed, as children, how soon the mill-tail loses its 

 current in the pool below ; or we may now see at any 

 time, and on a larger scale, how soon the Niagara, cur- 

 rent and all, is swallowed up in the lake below.' 



Franklin, who was the originator of the theory sup- 

 ported by Herschel, had unnecessarily introduced the 

 supposition that the trade-winds maintain a 6 head of 

 water ' in the Gulf of Mexico, and that the Gulf Stream 

 flows downwards like a river from this ' head,' as a 

 fountain or source. Maury rightly attacks this view, 

 which is undoubtedly a mistaken one ; but in doing so, 

 he falls into an error which exhibits his weakness in 

 the treatment of hydrodynamical problems. He points 

 out that, inasmuch as the Gulf Stream grows wider as 

 it crosses the Atlantic, it necessarily grows shallower, 

 so that the water-bed in which the stream flows has a 

 higher level under the shallow than under the deep 

 part of the current, and therefore, says Maury, c the 

 current runs up hill. 9 Herschel terms this a strange 

 perversion of language, but perhaps it would be more 

 correct to speak of it as a strange blunder. The stream 

 could, of course, only be said to run up hill if its surface 

 were seeking a higher level, which does not and cannot 

 happen. That the spreading out of the water of the 

 current, so as to form a wider and shallower stream, 

 does not correspond to an upward flow, is evident from 

 this, that it happens often with rivers, which no one 

 will suspect of running up hill. 



