THE LAWS OF THEIR CHANGES. 22 8 



main direction of the current of air during the 

 summer is, therefore, towards the north, and thus 

 it is converted, by the slower motion of the 

 northern parts round the earth's axis, into a 

 south-west wind. During the winter the sea has 

 the advantage in temperature ; and so, again, the 

 relations of the Indian Ocean at this season, as 

 regards wind, form no exception to the universal 

 rule. 



Our knowledge of the laws of winds had made 

 very little progress from the time of Halley till 

 within the last twenty years, during which the 

 labours of Dove and of Eeid have placed the theory 

 of the winds on an entirely new footing. 



By a long course of laborious researches, Dove 

 was led to what he called the Law of Eotation ; in 

 establishing this law he showed that the trade- 

 winds, and the entangled relations of the winds of 

 the temperate and frigid zones, are but the neces- 

 sary and simple results of the same physical con- 

 ditions. 



If an observer is stationed at a spot, in the 

 northern hemisphere, close to the place where the 

 excitement of the wind begins, where, for instance, 

 currents of warm air are mounting up from the 

 heated soil ; and if by this agency a current is 

 made to set in from the north, he will see the 

 wind-vane settle itself at first in the direction of 

 due north and south. If, however the cause of 



