APPARATUS OF VEGETABLE PHYSIOLOGY. 255 



the plant under observation, it passes over a pulley, and at 

 its other end a small weight is suspended. It is clear that 

 any elongation of the plant will cause a rotation of the 

 pulley in consequence of the friction of the thread upon it. 

 In close connection with this pulley is a toothed wheel, 

 and any movement of the pulley will cause a rotation of 

 the toothed wheel G. The teeth of the wheel as they rotate 

 come into contact with a small steel spring, and force it 

 against one or other of two small projecting binding screws 

 (a) and (6), from which it springs back when the pressure of 

 the tooth is removed by the further rotation of the wheel. 

 By means of a third binding screw (c) the spring is con- 

 nected with the wire coming from one pole of a galvanic 

 battery C, and wires connect the binding screws (a) and (6) 

 with the two electro-magnets A and B, which are in con- 

 nection with the other pole of the battery. It is evident 

 that when the spring touches the binding screw (a) or (6), 

 a current will be sent through the corresponding electro- 

 magnet A or B, in consequence of which it will attract 

 towards itself a steel rod P, which is supported between the 

 electro-magnets, and will keep it in that position until the 

 current passing through the electro-magnet is broken by 

 the escape of the lever from the pressure of the toothed 

 wheel, when the steel rod P springs back to its place. This 

 steel rod bears a pencil at one end which marks upon a disc 

 D, made to revolve once in twenty-four hours by means of 

 clockwork E. It is clear that every movement of the steel 

 rod bearing the pencil will be registered upon the surface 

 of the disc, and as these movements depend upon the 

 rotation of the toothed wheel, and this finally upon the 

 growth of the plant, the marks of the pencil of the disc 

 will give an indication of the growth of the plant. 



The accompanying sketch (Fig. 3) of a tracing taken 

 upon the disc will make this explanation somewhat more 

 intelligible. Only one half of the disc is represented, and 

 it is divided into twelve segments corresponding to the 

 twelve hours between 6 P.M. and 6 A.M. You will observe 

 that at 6 P.M. the pencil was describing a circle upon the 

 disc, and it continued to do so until just before seven, when 

 the tracing shows a sudden break. This indicates that, 

 owing to the pressure of the toothed wheel, the steel spring 

 has come into contact with the binding screw I, a current 



