L>4 



EXPERIMENTAL PHYSIOLOGY 



that the spring s does not touch the writing surface. With \v in the position 

 drawn, the oscillations are at the rate of two per second. When w is moved 



Fig. 29. — A Spring Chronograph. 



to the transverse mark a the rate becomes four per second, and when at d 

 eight per second. As the weight is heavy, when once the spring is started 

 vibrating it keeps on for a sufficient length of time for most experiments. 

 In a recording cylinder such great differences of speed are required for 



Fig. 30. —Lower Part of Drum to show Method of Driving the Cylinder at 



Different Eates. 



different purposes that it is often difficult on the same drum to obtain suf- 

 ficient variations. For rapid rates of movement a common plan is to drive a 



