46 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



the yellow one, and the vapor of a liquid contained in the 

 former is elevated to a higher temperature than in the latter. 

 As the result, the vapor leaves the one ball, and being con- 

 densed in the other, this becomes the heavier, disturbs the 

 equilibrium of the system, and in so doing liberates a weight, 

 giving motion to a clock-work attachment. In the sun-dial 

 here referred to a pair of these balls is fixed at every hour- 

 mark. When the shadow of the gnomon reaches any par- 

 ticular hour-mark one of the balls is shaded, a preponderance 

 of liquid enters the ball, the lever tilts, the mechanism is set 

 going, and a gong sounded as often as the number of the 

 hour to be indicated. It is necessary, however, that the sun 

 should shine when the hour- mark is being passed by the 

 shadow, or the time will not be struck. 



