434 



SWAN. 



guard-rings and the diaphragms to follow the motion of the latter. 

 The amplitude of vibration of the tyne of the fork was measured by 

 observing through a microscope provided with a micrometer eyepiece 

 the edge of a razor blade attached to the tyne. 



The fork F was driven electromagnetically by a second fork D 

 situated in a distant room to remove it from the range of hearing. On 



FIGURE I 



account of unequal changes of temperature in the neighborhood of the 

 two forks, it was necessary to tune the driving fork frequently to keep 

 it in unison with the natural period of the sounding fork. The driving 

 fork was operated without intermission, the sounding fork only at the 

 pleasure of the observer in the Constant Temperature Room. The 

 latter fork was controlled by a key K in the hands of the observer 

 which closed a circuit through a relay R, causing the driving current 



