256 



LECTURES TO SCIENCE TEACHERS. 



has been made to pass through the electro-magnet B, and 

 it has attracted the steel rod bearing the pencil. You will 

 also observe that the pencil remains in this new position 



.Fie. 3. 



for some time in fact, for just so long a time as the 

 pressure of the toothed wheel keeps the spring in contact 

 with the binding screw b, for when this pressure is removed 

 the pencil resumes its original position. We see then that 

 each of these depressions which we observe in the tracing 

 corresponds to the time which is required for the rotation 

 of a single tooth of the wheel. 



This is made more evident when the tracing on the disc 

 is tabulated in the manner illustrated by Fig. 4. In this 

 the Arabic figures indicate hours, the Roman figures the 

 depressions in the disc-tracing, or, in other words, divisions 

 of the toothed wheel. 



Fig. 5 is the tracing, similarly tabulated, taken during the 

 twelve hours between 6 A.M. and 6 P.M. 



Fig. 6 is a tabulated arrangement of the two preceding 

 figures, by means of which the curve of the velocity of 

 growth during the twenty-four hours is obtained. In this 

 the smallest velocity is taken as the unit of measurement 

 for the ordinates. 



So much then for the mechanism of this apparatus, and 

 for the mode of using it. Let us now go on to consider 



