220 The Radiograph and its Uses. [April, 
tube, and thus displaces the centre of gravity. In conse- 
quence, the right arm descends till the pencil F touches an 
hour-dial placed below it, and made to revolve by clockwork. 
So long as the sun shines the pencil remains in contadt with 
the paper on the dial, and leaves a mark as the latter re- 
volves, as shown in the outer line a a a, Fig. 2. 
As soon as the sun is overcast the air in t', adting like a 
spring, drives the mercury back to the left ; the balance 
