DURATION OF MUSCULAR ACTION. 2^1 



When I endeavoured to estinaate the frequency of these Frequency 6f 

 vibratory alternations, they appeared to be in general be* ^^^ ^^^'^^*''°"** 

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

 method I employed may be found defective ; and it is to 

 be hoped, that my estimate may be corrected, by some 

 means better adapted to the determination of intervals, that 

 cannot actually be measured. 



Tt was by imitation alone, that I was enabled to judge of 

 their frequency. For this purpose I contrived to render the 

 vibration itself, and the imitative sound, both audible by the 

 same ear. 



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

 was supported by a board lying horizontally, in which were measuring 

 cut a number of notches of equal size, and about ^ of au inch 

 asunder. Then, by nibbing a pencil or other round piece 

 of wood with a regular motion along the notches, I could 

 imitate pretty correctly the tremor produced by the pree** 

 sure of my thumb against my head, and by marks to indicate 

 the number of notches [wssed over in 5 or 10 seconds, ob- 

 served by my watrh, I found repeated observations agree 

 with each other as nearly as could be expected ; for I could 

 not depend vipon exerting the same degree of force in dif- 

 ferent trials. 



That I might not be deceived by the resemblance of Thi- ext^erl- 

 tremors, which coincided only at alternate beats, and *"^"^ ^^'i-^- 

 therefore might be considered as octaves in music to 

 each other, I sometiniCs employed notches at greater and 

 sometimes at less distances from each other, but the result 

 was nevertheless the same; and in order to avoid auy errour 

 that might be caused by some accidental quality of thp 

 sound arising frOm the length of the muscle eiDployed, or 

 length of the boties concerned in conveying the imitative 

 sound to uiy ear, I mude the following variation of the ex- 

 periment. My ear wa? stopped by a cushion pressssd upon 

 by the end of h notched stick that rested on my foot, and 

 thus conveyed the vibration from the nuiscle^ of my leg to 

 the ear, along with the tremor produced by friction upon 

 /the notches; and still the results were nearly the same; 

 varying in frequency between -20 and 30 in a second, ac- 

 U 2 cording 



