AUTOMATIC CRESCOGRAPH 7 



Weight of the levers. — The tension exerted by the weight 

 of even low magnification auxanometers is found to modify 

 the normal rate of growth. In the compound system of 

 levers employed in my apparatus, the first lever has to be 

 made rigid in order to exert a pull on the second without 

 undergoing any bending. In order to secure the rigidity 

 of the first lever, it must have a certain minimum cross- 

 section. This entails not only an increase of weight and 

 of tension on the plant, but also increased friction at the 

 fulcrum. The conditions necessary to overcome these 

 difficulties are : (i) construction of a very light lever 

 possessing sufficient rigidity, and (2) arranging the two 

 levers in such a way that the tension on the plant may be 

 reduced to any extent desired, so as practically to eliminate 

 it. It may be stated here that the second lever serving 

 as the writer can be made of fine glass fibre of extreme 

 lightness. 



For the construction of the first lever I use ' navaldum,' 

 the well-known alloy of aluminium which possesses con- 

 siderable rigidity: A thin strip of this material about 

 30 cm. in length is taken, rigidity being imparted to it by 

 giving it a T-shape. A specially prepared thin strip of 

 bamboo has been found to be a satisfactory substitute for 

 the metal. 



Each of the two levers is nearly balanced by a counter- 

 poise W (fig. i) . The horizontal fulcrum-axis of each lever 

 is supported in a fork provided with appropriate jewel- 

 bearings. 



Attacher and Connector. — The first of these terms 

 designates the contrivance for connecting the plant with 

 the first lever ; and the second, the flexible connection 

 between the first and the second lever. For the Attacher 

 I use a thin glass hook, which does not stretch, nor does 

 it undergo any appreciable change of length on variation 

 of temperature. The upper end of the hook can be placed 

 on notches in various positions on the first lever, greater 

 magnification being produced when the hook is brought 



