210 LIGHT SCIENCE FOR LEISURE HOURS. 



account as available to generate the easterly equatorial 

 current. 



And now let us consider for a moment the relation 

 which holds between cause and effect in the case sup- 

 posed by Herschel. We have more than a fourth part 

 of the Atlantic Ocean in a state of perpetual motion, 

 and it is assumed that the air immediately above the 

 ocean is responsible for this circulation. Now even if 

 we suppose that the whole of the vis viva in the aerial 

 circulation is imparted to the waters, and neglect all 

 consideration of the fact that for a large portion of the 

 year the winds do not act in the manner available for 

 the production of the currents we are considering, yet 

 even then, I apprehend that we shall find the vis viva 

 of the aerial very far below that of the aqueous circula- 

 tion. The volume of moving water is, of course, far 

 less than that of the moving air, and the mean velocity 

 of the water-currents is less than that of the air-cur- 

 rents ; but, on the other hand, the specific gravity of 

 water is some 830 or 840 times greater than that of 

 air, and this difference far more than counterbalances 

 the others. 



But now, when we come to consider the forces called 

 into action in producing changes of temperature, etc., 

 we no longer find such a disproportion between cause 

 and effect. The sun's action on the equatorial and 

 tropical regions of the Atlantic not only produces a 

 great change in the density of the water, but also 

 raises immense masses by evaporation. Now the buoy- 

 ancy caused by increase of temperature is partly 



