THE TIDAL WAVE. 291 



VII THE TIDAL WAVE. 



IN almost every case the forces and motions on the sur- 

 face of the earth may be traced back to the rays of the sun. 

 Some processes, however, form a remarkable exception. 



One of these is the tides. Beautiful, and in some re- 

 spects exhaustive researches on this phenomenon have been 

 made by Newton, Laplace, and others. The tides are caused 

 by the attraction exercised by the sun and the moon on the 

 moveable parts of the earth's surface, and by the axial rota- 

 tion of our globe. 



The alternate rising and falling of the level of the sea 

 may be compared to the ascent and descent of a pendulum ' 

 oscillating under the influence of the earth's attraction. 



The continual resistance, however weak it may be, which 

 an instrument of this nature (a physical pendulum) suffers, 

 constantly shortens the amplitude of the oscillations which it 

 performs ; and if the pendulum be required to continue in 

 uniform motion, it must receive a constant supply of vis viva 

 corresponding to the resistance it has to overcome. 



Clocks regulated by a pendulum obtain such a supply, 

 either from a raised weight or a bent spring. The power 

 consumed in raising the weight or in bending the spring, 

 which power is represented by the raised weight or the bent 

 spring, overcomes for a time the resistance, and thus secures 

 the uniform motion of the pendulum and clock. In doing so, 

 the weight sinks down or the spring uncoils, and therefore 

 force must be expended in winding the clock up again, or it 

 would stop moving. 



Essentially the same holds good for the tidal wave. The 

 moving waters rub against each other, against the shore, and 

 against the atmosphere, and thus, meeting constantly with re- 

 sistance, would soon come to rest if a vis viva did not exist 

 competent to overcome these obstacles. This vis viva is the 



