PERPETUAL MOTION 39 



The changes which are constantly taking place in the . 

 temperature of all bodies, and the expansion and contrac- 

 tion which these variations produce, afford a very efficient 

 power for clocks and small machines. Professor W. W. R. 

 Ball tells us that " there was at Paris in the latter half of 

 last century a clock which was an ingenious illustration of 

 such perpetual motion. The energy, which was stored up 

 in it to maintain the motion of the pendulum, was provided 

 by the expansion of a silver rod. This expansion was 

 caused by the daily rise of temperature, and by means of a 

 train of levers it wound up the clock. There was|a dis- 

 connecting apparatus, so that the contraction due to\ a fall 

 of temperature produced no effect, and there was a similar 

 arrangement to prevent overwinding. I believe that a m$ 

 of eight or nine degrees Fahrenheit was sufficient to wind v 

 up the clock for twenty-four hours." 



Another indirect method of winding a watch is thus 

 described by Professor Ball: 



" I have in my possession a watch, known as the Lohr 

 patent, which produces the same effect by somewhat differ- 

 ent means. Inside the case is a steel weight, and if the 

 watch is carried in a pocket this weight rises and falls at 

 every step one takes, somewhat after the manner of a 

 pedometer. The weight is moved up by the action of the 

 person who has it in his pocket, and in falling the weight 

 winds up the spring of the watch. On the face is a small 

 dial showing the number of hours for which the watch is 

 wound up. As soon as the hand of this dial points to fifty- 

 six hours, the train of levers which wind up the watch dis- 

 connects automatically, so as to prevent overwinding the 

 spring, and it reconnects again as soon as the watch has 

 run down eight hours. The watch is an excellent time- 

 keeper, and a walk of about a couple of miles is sufficient 

 to wind it up for twenty-four hours." 



