124 LIGHT SCIENCE FOR LEISURE HOURS. 



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, are matters inquired into farther 

 on (see p. 164). The subject is full of interest, but 

 need not here detain us. 



It would seem that a mechanism involving the mo- 

 tion 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 contradictory 

 hypotheses have been put forward from time to time re- 

 specting this system of circulation, and even now the 

 scientific world is divided between two opposing 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, 

 and this fact was considered sufficient to support the 

 strange theory that a river can give birth to an ocean- 

 current. 



It was easy, however, to overthrow this theory. 

 Captain Livingston showed that the volume of water 

 which is poured out of the Gulf of Mexico in the form 

 of an ocean stream is more than a thousand times 



