140 LIGHT SCIENCE FOR LEISURE HOURS. 



ration is going on at a greater rate than over the neigh- 

 boring seas, because the waters of the current are 

 warmer than those which surround them. The vapor 

 thus rising above the Gulf Stream is presently wafted 

 by the southwesterly winds to our shores and over our 

 whole land. But as it thus reaches a region of com- 

 parative cold the vapor is condensed that is, turned 

 into fog, or mist, or cloud, according to circumstances. 

 It is during this change that it gives out the heat it 

 has brought with it from the Gulf Stream. For pre- 

 cisely as the evaporation of water is a process requiring 

 heat, the change of vapor into water whether in the 

 form of fog, mist, cloud, or rain is a process in which 

 heat is given out. Thus it is that the southwesterly 

 wind, the commonest wind we have, brings clouds and 

 fogs and rain to us from the Gulf Stream, and with 

 them brings the Gulf-Stream warmth. 



"Why the southwesterly winds should be so common, 

 and how it is that over the Gulf Stream there is a sort 

 of air-channel along which winds come to us as if by 

 their natural pathway, we have not space here to 

 inquire (see p. 183). The subject is full of interest, but 

 it does not belong to the question we are considering. 



It would seem that a mechanism involving the 

 motion of such enormous masses of water as the 

 current-system of the Atlantic should depend on the 

 operation of very evident laws. Yet a variety of con- 

 tradictory hypotheses- have been put forward from time 



