3 
of Muscular Action. 
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 degree of 
pressure upon the tympanum ; for, on the contrary, the vibra- 
tory sound is most distinct when that pressure is slight, if the 
finger 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 its whole 
weight upon the ball of the thumb 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 communicated to the ear, by any inadvertence 
in the method of conducting the experiment. 
When I endeavoured to estimate the frequency of these 
vibratory alternations, they appeared to be in general between 
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. 
It 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 
was supported by a board lying horizontally, in which were 
cut a number of notches of equal size, and about £ of an inch 
asunder. Then, by rubbing a pencil or other round piece of 
B 2 
