204 LIGHT SCIENCE FOR LEISURE HOURS. 



great currents are set in motion. The agents which 

 derange equilibrium in the waters of the sea, by altering 

 specific gravity, reach from the equator to the poles, 

 and in their operations they are as ceaseless as heat and 

 cold ; consequently, they call for a system of perpetual 

 currents to undo their perpetual work.' c Other causes 

 help to cause currents,' he says, c but the currents created 

 by them are ephemeral? 



Here we have what is ' a very pretty quarrel as it 

 stands.' Each of the disputants points to causes of 

 acknowledged importance, and also (whether efficient 

 or not in the particular matter under question) of 

 acknowledged general efficiency. Each has much to 

 say in favour of his own view, and each considers his 

 antagonist's agent as utterly insufficient for the work 

 ascribed to it. Each has heard his opponent's argu- 

 ments, and reiterates his own statement. Nor can it be 

 said that the opponents are unequally matched ; for, if 

 we must place Sir John Herschel far before Maury as a 

 mathematician and physicist, and if we must undoubt- 

 edly look upon the former as the more practised 

 reasoner, yet we must remember, in turn, the special 

 attention which Captain Maury has given to the subject 

 under discussion, and the practical acquaintance with it 

 which his experience as a seaman has given to him. 



Let us briefly state the arguments adduced by 

 Herschel against Maury's view, and by Maury against 

 Herschel's. 



Sir John Herschel asserts that, inasmuch as the sun's 

 heat warms the surface of the ocean most intensely, so 



