26 CHAP. III. THE BALANCED CRESCOGRAPH 



subsidence of the plant-holder which balances the rate of 

 growth is, as already stated, proportional to the rate of 

 rotation of the vertical spindle with the dependent train 

 of revolving clock-wheels. A striker is attached to one of 

 the wheels and a bell is struck at each complete revolution. 

 The clockwork is adjusted by the governor to revolve at 

 a medium speed, the bell striking 35 times in a minute. 

 A microscope-micrometer is focused on a mark made on 

 the plant-holder, and the amount of subsidence of the mark 

 is determined during one minute ; this was found to be 

 0-0525 mm. As this fall occurred when the bell was 

 striking 35 times in a minute, the subsidence per stroke 

 was 0-0015 mm. From this it is possible to determine the 

 absolute rate of growth. 



Determination of the absolute rate of growth. — Supposing 

 that balance occurred at N strokes of bell per minute, the 

 rate of balancing subsidence = N X • 0015 mm. per minute ; 

 = N X 0-000025 mm. per second; = N x 0-025 [i per 

 second. 



Experiment 7. — The growth of a specimen of Zea Mays 

 was found to be balanced when the number of strokes of 

 the bell was 20 per minute (absolute rate of growth 

 = 20 X 0-025 [-^ = 0'5 y- per second) ; the length thus 

 measured is equal to the wave-length of sodium light. This 

 affords some idea of the sensitiveness of the Crescographic 

 Method of Balance. 



Growth-scale. — All necessity for calculation is obviated 

 by the scale attached to the apparatus. The speed of the 

 clockwork which brings about the balance of growth is 

 determined by the position of the adjusting screw S, the 

 gradual lowering of which produces a continuous diminution 

 of speed. A particular position of the screw therefore 

 indicates a definite rate of subsidence for balancing the 

 growth. The up or down movement of the screw causes 

 rotation of an index pivoted at the centre of a circular 

 scale. Each division of the scale is calibrated by counting 

 the corresponding number of strokes of the bell per minute 



