154 



REVOLVING DRUM. 



[CH. VI 



measures 25 cm. in height and 512 mm. in circumference: 

 it rotates on a vertical steel spindle, and by means of a 

 clockwork escapement makes a short sudden rotation at 

 intervals of one hour or at shorter intervals if required. 

 At each rotation the index writes a short horizontal mark 



FIG. 28. Exp. 181. 

 From the Cambridge Instrument Company's Catalogue. 



on the smoked paper covering the drum : between whiles 

 the index moves downwards over the surface of the drum 

 at a rate proportional to the rate of growth. The figure 

 traced by the index is thus a series of steps of which the 



