368 



Sir J. C. Bose and Mr. G. Das. 



The Fulcrum. — The most serious difficulty was in connection with friction" 

 less support of the axes of the two levers. The horizontal axis was at first 

 supported on jewel bearings, with fine screw adjustment for securing lateral 

 support. Any sHght variation from absolute adjustment made the bearing 

 either too loose or too tight, preventing free play of the lever. When 

 perfect adjustment had been secured, the movement of the levers became 

 jerky after a few days. This I afterwards discovered was due to the deposit 

 of invisible particles of dust on the bearings. There were frequent interrup- 

 tions of work on account of the break of the fine points in the pivot, which 

 have to bear considerable strain in a lateral direction. These difficulties 

 forced me to work out a very perfect and at the same time a much simpler 

 device. The lever now rests on two fine vertical pin-points on conical agate 

 cups carried by the fork F. The axis of the lever passes through the points 

 of support. The friction of support is thus reduced to a minimum and the 

 lever is kept in place under the constant pressure of its own weight. The 

 excursion of the recording end of the lever, which represents magnified 

 movement of growth, was now found to be without jerk and quite uniform. 



It is possible to construct a still more sensitive instrument by means of a 

 compound system consisting of three levers. There is, however, a limit to the 

 number of levers that may be employed with advantage : for the slight over- 

 weight of the last lever becomes multiplied and exerts great tension on the 

 plant, thus interfering with its normal growth. 



2. Automatic Record of the Bate of Growth. 



Another great difficulty in obtaining an accurate record of the curve of 

 growth arises from the friction of contact of the bent tip of the writing 

 lever against the recording surface. This I was able to overcome by an 

 oscillating device in which the contact, instead of being continuous, is made 

 intermittent. The smoked glass plate, G, is made to oscillate to and fro at 

 regular intervals of time, say one second. The bent tip of the recording 

 lever comes periodically in contact with the glass plate during its extreme 

 forward oscillation. The record would thus consist of a series of dots, the 

 distance between successive dots representing magnified growth during a 

 second. 



The drawback in connection with the obtaining of a record on the oscilla- 

 ting plate lies in the fact that if the plate approaches the recording point 

 with anything like suddenness, then the stroke on the flexible lever causes an 

 after-oscillation ; the multiple dots thus produced spoil the record. In order 

 to overcome this, a special contrivance is necessary, by which the speed of 

 approach of the plate should be gradually reduced to zero at contact with the 



