THE TIDAL WAVE. 293 



wheel, resistance is overcome, and the given rotatory effect 

 diminished. 



Water-mills have been turned by the action of the tides ; 

 the effects produced by such an arrangement are distinguished 

 in a remarkable manner from those of a mill turned by a 

 mountain-stream. The one obtains the vis viva with which 

 it works from the earth's rotation, the other from the sun's 

 radiation. 



Various causes combine to incessantly maintain, partly in 

 an undulatory, partly in a progressive motion, the waters of 

 the ocean. Besides the influence of the sun and the moon on 

 the rotating earth, mention must be made of the influence of 

 the movement of the lower strata of the atmosphere on the 

 surface of the ocean, and of the different temperatures of the 

 sea in various climates ; the configuration of the shores and 

 the bed of the ocean likewise exercise a manifold influence on 

 the velocity, direction, and extent of the oceanic currents. 



The motions in our atmosphere, as well as those of the 

 ocean, presuppose the existence and consumption of vis viva 

 to overcome the continual resistances, and to prevent a state 

 of rest or equilibrium. Generally speaking, the power neces- 

 sary for the production of aerial currents may be of threefold 

 origin. Either the radiation of the sun, the heat derived 

 from a store in the interior of the earth, or, lastly, the rota- 

 tory effect of the earth may be the source. 



As far as quantity is concerned the sun is by far the most 

 important of the above. According to Pouillet's measure- 

 ments, a square metre of the earth's surface receives on the 

 average 4*408 units of heat from the sun per minute. Since 

 one unit of heat is equivalent to 367 Km, it follows that one 

 square metre of the surface of our. globe receives per minute 

 an addition of vis viva equal to 1620 Km, or the whole of the 

 earth's surface in the same time 825,000 billions of Km. A 

 power of 75 Km per second is called a horse-power. Ac- 

 cording to this, the effect of the solar radiation in mechanical 



