126 LIGHT SCIENCE FOR LEISURE HOURS. 



over our whole land. But as it thus reaches a region 

 of comparative cold the vapour is condensed that is, 

 turned into fog, or mist, or cloud, according to circum- 

 stances. It is during this change that it gives out the 

 heat it has brought with it from the Gulf Stream. 

 For precisely as the evaporation of water is a process 

 requiring heat, the change of vapour 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 

 south-westerly 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 south-westerly 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 

 enquire (see p. 166). 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 

 to time respecting this system of circulation, and even 

 now the scientific world is divided between two oppos- 

 ing theories. 



Of old the Mississippi River was supposed to be the 

 parent of the Gulf Stream. It was noticed that the 

 current flows at about the same rate as the Mississippi, 



