OUR CHIEF TIME- PIECE LOSING TIME. 51 



expended. The doing of ivork 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. 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 e force- 

 rneasurer,' 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 idle dream, but a matter of 

 absolute certainty, that, though slowly, still very surely, 

 our terrestrial globe is losing its rotation-movement. 



