234 LIGHT SCIENCE FOR LEISURE HOURS. 



tropics, the existence of which is also generally ad- 

 mitted. 



But now we again approach a subject of controversy, 

 and one which is certainly better worthy of discussion 

 than that which we considered above. It relates, in 

 fact, to the question how this wonderful system of 

 oceanic circulation is brought about. 



Passing over several crude theories which have long 

 since been disposed of, we come first to the theory that 

 the system of oceanic circulation is due to the action of 

 the trade-winds. This theory has been maintained by 

 Franklin and others in former times, by Sir John 

 Herschel in our own, and is warmly advocated in the 

 present day, by many whose opinions are not to be 

 rashly contradicted. 



Against this theory it has been urged by Captain 

 Maury c with singular wrongheadedness ' accordirg 

 to the Edinburgh Eeviewer, but as it seems to me with 

 no small degree of reason that the trade-winds are 

 neither powerful enough nor persistent enough to ac- 

 count for the great equatorial currents, or therefore for 

 the Grulf Stream. Maury says, ' with the view of ascer- 

 taining the average number of days during the year 

 that the north-east trade-winds of the Atlantic operate 

 upon the water between the equator and 25 degrees 

 north latitude, log-books containing no less than 

 380,284 observations on the force and direction of the 

 wind in that ocean were examined. The data thus 

 afforded were carefully compared and discussed. The 

 results show that within these latitudes and on the 



