GO LIGHT SCIENCE FOR LEISURE HOURS. 



turics than she should do according to the lunar 

 theory. 



But in this difficulty, small as it seems, we are not 

 left wholly without resource. We are not only able 

 to say that the discrepancy is probably due to a 

 gradual retardation of the earth's rotation-movement, 

 but we are able to place our finger on a very sufficient 

 cause for such a retardation. One of the most firmly- 

 established principles of modern science is this that 

 where work is done, force is, in some way or other, 

 expended. The doing of work may show itself in a 

 variety of ways in the generation of heat, in the 

 production of light, in the raising of weights, and so 

 on ; but in every case an equivalent force must be 

 expended. If the brakes are applied to a train in 

 motion, intense heat is generated in the substance of 

 the brake ; now, the force employed by the brakesman 

 is not equivalent to the heat generated. "Where, then, 

 is the balance of force expended ? We all know that 

 the train's motion is retarded, and this loss of motion 

 represents the requisite expenditure of force. !N"ow, is 

 there any process in Nature resembling, in however 

 remote a degree, the application of a brake to check 

 the earth's rotation ? There is. The tidal wave, which 

 sweeps, twice a day, round the earth, travels in a di- 

 rection contrary to the earth's motion of rotation. 

 That this wave " does work," no one can doubt who 

 has watched its effects. The mere rise and fall in 



