CH. VI] 



EEVOLVING DRUM. 



141 



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 

 on the smoked paper covering the drum. Between whiles 



Fig. 26. Exp. 181. 

 (Instrument Company's Catalogue.) 



the index writes a vertical line representing the growth 

 of the plant: the figure traced by the index is thus a 

 series of steps of which the " fall " or vertical line is 

 proportional to the growth per hour. The slight shock 

 due to the sudden rotation of the drum is an advantage 



