440 On the Duration of Muscular Action. 



I have been led to infer the existence of these alternate 

 potions from a sensation perceptible upon inserting the 

 extfi'mity of the finger into the ear. A sound is then per- 

 ceived which resembles most nearh- thai of carriages at a 

 great distance passing rapidly over a pavement. 



The rapidity of the motion varies according to the degree 

 of force with which the finger is retained in its place. 

 The sound thus perceived is not at all dependent on the de- 

 gree of pressure upon the tympanum; for, on the contrary, 

 the vibratorv sound is most distinct v»hen that pressure is 

 slight, if the linger be at the same time rendered rigid by 

 the forcible action of antagonist muscles; and when the 

 ear is stopped with great Force without the presence of 

 muscular action, no such sound is produced. For instance, 

 if the head be rested upon the hand in such a position, as 

 to press with jts whole weight upon the ball of the ihun»b 

 applied to the ear, no noise is perceived, unless the extremity 

 of the thumb be at the same time pressed against the head, 

 or unless the action of some other muscles be communi- 

 cated to the ear, by any inadvertence in the method of cbii-r 

 ducting the experiment. 



When I endeavoured to estimate the frequency of these 

 vibratory alternations, they appeared to be in general be- 

 tween 20 and 30 in a second; but it is possible that the 

 method \ employed may be found defective, and it is to 

 be hoped that niy estimate may be corrected, by some 

 means better adapted to the determinaliou of intervals that 

 cannot actually be measured. 



It was by imitation alone that I was enabled to judge of 

 their frequency. For this purpose I contrived to render 

 the v'rbration itself, and the inniative sound, both audible 

 by the same ear. 



While my ear rested on the ball of tpy thumb, my elbow 

 was supported by a board lying horizontally, in which were 

 cut a number of notches of equal size, and about one-eighth 

 of an inch asunder. Then, by rubbing a pencil or other 

 round piece of wood with a regular motion along the 

 notches, I could imitate pretty correctly the tremor pro- 

 duced by the pressure of my thumb against my head ; and 

 by marks to indicaie liie number of notches passed over iri 

 five or ten seconds, observed by my watch, I found re^ 

 peated observations agree with each other as nearly as could 

 be expected ; for I could not depend upon exerting the 

 same degree of force in different trials. 



That I might not be deceived by the resemblance of tre- 

 mors, which coincided only at alternate beats, and there- 

 for^ 



