96 MECHANICAL RECREATIONS [CH. V 



solar heat, and the tides are among the more ohvious of such 

 sources. 



There was at Paris in the latter half of the eighteenth 

 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 disconnecting apparatus, so 

 that the contraction due to a fall of temperature produced no 

 effect, and there was a similar arrangement to prevent over- 

 winding. I believe that a rise of eight or nine degrees 

 Fahrenheit was sufficient to wind up the clock for twenty-four 

 hours. 



By utilizing the rise and fall of the barometer, James Cox, 

 a London jeweller of the eighteenth century, produced, in an 

 analogous way, a clock f which ran continuously without 

 winding up. 



I have in my possession a watch which produces the 

 same effect by somewhat different 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 raised by the 

 action of the person who has it in his pocket in taking a 

 step, and in falling it 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 winds 

 up the watch disconnects automatically, so as to prevent over- 

 winding 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. 



* Ozanam, 1803 edition, vol. n, p. 105 ; 1840 edition, p. 238. 

 \ A full description of the mechanism will be found in the English Mechanic, 

 April 30, 1909, pp. 288—289. 



