202 LIGHT SCIENCE FOE LEISURE HOURS. 



as much as one and three-quarter knots. The men in 

 the boat could not repress exclamations of surprise, for 

 it really appeared as if some monster of the deep had 

 hold of the weight below, and was walking off with it.' 



Lastly, we may mention that Captain Wilkes, of the 

 United States Exploring Expedition, established the 

 existence of a cold under-current no less than two hun- 

 dred miles broad at the equator. 



We may assume, then, that a complete system of cir- 

 culation, vertical as well as horizontal, exists throughout 

 the whole of the waters contained within the great 

 Atlantic valley. 



Where are we to look for the origin of this vast series 

 of movements ? The actual ' work done ' in the Atlantic 

 Ocean is so enormous in other words, the transfer of 

 such large volumes of water represents so enormous a 

 force, that we might well expect to be able at once to 

 assign the motive-power of this great machinery. For 

 it would seem that the giant which works such wonders 

 could not readily hide himself from our recognition. 



It has not been found, however, that the solution of 

 the problem has been so simple as was to have been 

 anticipated. 



Passing over the earlier guesses which marked the 

 Gulf Stream as the offspring of the Mississippi Kiver, 

 of the sun's motion in the ecliptic (a mysterious inter- 

 pretation of the phenomena), and of the tidal wave, we 

 may remark that but two explanations of the Atlantic 

 currents seem to merit discussion. 



Sir John Herschel is the principal exponent of the 



