308 
Dr. Wollaston on sounds 
the air to enter, restores the equilibrium of pressure neces- 
sary to the due performance of the functions of the ear. 
In my endeavours to ascertain the extent to which this kind 
of deafness may be carried, some doubt has arisen, from the 
difficulty of finding sounds sufficiently pure for the purpose. 
The sound of stringed instruments are in this respect defec- 
tive ; for unless the notes produced are free from any inter- 
mixture of their sharper chords, some degree of deception is 
very liable to occur in the estimate of the lowest note really 
heard. I can, nevertheless, with considerable confidence, 
say, that my own ears may be rendered insensible to all 
sounds below F marked by the base cliff. But as I have 
been in the habit of making the experiment frequently, it is 
probable that other persons who may be inclined to repeat 
it, will not with equal facility effect so high a degree of 
exhaustion as I have done. To a moderate extent the ex- 
periment is not difficult, and well worth making. The effect 
is singularly striking, and may aptly be compared to the 
mechanical separation of larger and smaller bodies by a sieve. 
If I strike the table before me with the end of my finger, 
the whole board sounds with a deep dull note. If I strike 
it with my nail, there is also at the same time a sharp sound 
produced by quicker vibrations of parts around the point of 
contact. When the ear is exhausted it hears only the latter 
sound, without perceiving in any degree the deeper note of 
the whole table. In the same manner, in listening to the 
sound of a carriage, the deeper rumbling noise of the body 
is no longer heard by an exhausted ear ; but the rattle of a 
chain or loose screw remains at least as audible as before 
exhaustion. 
