128 LIGHT SCIENCE FOR LEISURE HOURS. 



of undoubted efficiency. We know that as the trade- 

 winds travel towards the equator they lose their 

 westerly motion. It is reasonable to suppose that this 

 is caused by friction against the surface of the ocean, 

 to which, therefore, a corresponding westerly motion 

 must have been imparted. 



There is a simplicity about Franklin's theory which 

 commends it favourably to our consideration. But 

 when we examine it somewhat more closely, several 

 very decided flaws present themselves to our attention. 



Consider, in the first place, the enormous mass of 

 water moved by the supposed agency of the winds. 

 Air has a weight volume for volume which is less 

 than one eight-hundredth part of that of water. So 

 that, to create a water-current, an air-current more 

 than eight hundred times as large and of equal velocity 

 must expend the whole of its motion. Now the trade- 

 winds are gentle winds, their velocity scarcely exceed- 

 ing in general that of the more swiftly-moving portions 

 of the Gulf Stream. But even assigning; to them a 



o o 



velocity four times as great, we still want an air- 

 current two hundred times as large as the water- 

 current. And the former must give up the whole of 

 its motion, which, in the case of so elastic a substance 

 as air, would hardly happen, the upper air being un- 

 likely to be much affected by the motion of the lower. 

 But this is far from being all. If the trade-winds 

 blew throughout the year, we might be disposed to 

 recognise their influence upon the Gulf Stream as a 

 paramount, if not the sole one. But this is not the 



