44 LIGHT SCIENCE FOR LEISURE HOURS. 



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 gene- 

 rated 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. Now, 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 direction 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 open ocean may not be strikingly 

 indicative of * work done ; ' but when we see the 

 behaviour of the tidal wave in narrow channels, when 

 we see heavily-laden ships swept steadily up our tidal 

 rivers, we cannot but recognise the expenditure of force. 

 Now, where does this force come from ? Motion being 

 the great < force-measurer,' what motion suffers that the 

 tides may work? We may securely reply, that the 

 only motion which can supply the requisite force is the 

 earth's motion of rotation. Therefore, it is no mere 

 fancy, but a matter of absolute certainty, that, though 

 slowly, still very surely, our terrestrial globe is losing 

 its rotation-movement. 



Considered as a time-piece, what are the earth's 



