THK BALANCED CRBSCOGRAPH 263 



million times. In this method a very delicate astatic 

 system of magnets undergoes deflection by the movement 

 of a magnetised lever in its neighbourhood. A spot of 

 light reflected from a small mirror attached to the astatic 

 system, thus gives the highly magnified movement of the 

 rate of growth, which may easily be raised to ten million 

 times. I shall in the following describe the results obtained 

 with this easily managed magnification of ten million 

 times. 



Determination of sensitiveness .- Experiment 99. — A seed- 

 ling of Zea Mays was placed on the Crescographic Balance ; 

 and the magnetic amplification, as stated above, was ten 

 million times. With 18 strokes of the bell per minute 

 the spot of light had a drift of + 266 cm. per minute to 

 the right ; this is because the growth was underbalanced. 

 With faster rate of clock movement, i.e., 21 strokes in 68 

 seconds or 18*53 strokes per minute, the drift of the spot 

 of light, owing to overbalance, was to the left at the 

 rate of - 530 cm. per minute. Thus 



(1) 18 strokes per minute caused a drift of + 266 



cm. per minute. 



(2) 18"53 strokes per minute caused a drift of - 530 



cm. per minute. 



Hence by interpolation the exact balance is found to 

 correspond to. 18*177 strokes per minute. 



Therefore the absolute rate of growth 



= 18-177 X 0-025 fi per second. 



= 0-45/i, per second. 



= 0-000018 inch per second. 



We learn further from (1) and (2) that a variation of 



ii-— — ^^ produces a change of drift of the spot of light 



18-177 ^ f & 



from + 266 to - 530 cm., i.e., of 796 cm. per minute. 



22 



