Dr Wollaston on Sounds Inaudible bij certain Eats. 159 
ears. I remarked, that when the mouth and nose are shut, the 
tympanum may be so exhausted by a forcible attempt to take 
breath by expansion of the chest, that the pressure of the exter- 
nal air is strongly felt upon the membrana tympani, and that, in 
this state of tension from external pressure, the ear becomes in- 
sensible to grave tones, without losing in any degree the percep- 
tion of sharper sounds. 
The state to which the ear is thus reduced by exhaustion, 
may even be preserved for a certain time, without the continued 
effort of inspiration, and without even stopping the breath, since, 
by sudden cessation of the effort, the internal passage to tlie ear 
becomes closed by the flexibility of the Eustachian tube, which 
acts as a valve, and prevents the return of air into the tympa- 
num. As the defect thus occasioned is voluntary, so also is the 
remedy ; for the unpleasant sensation of pressure on the drum, 
and the partial deafness which accompanies it, may at any in- 
stant be removed by the act of swallowing, which opens the tube, 
and by allowing the air to enter, restores the equilibrium of pres- 
sure necessary 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 dif- 
ficulty of finding sounds sufficiently pure for the purpose. The 
sounds of stringed instruments are in this respect defective ; for 
unless the notes produced are free from any intermixture 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 1 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 1 have done. To a moderate extent 
the experiment is not difficult, and well worth making. The ef- 
fect 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, tfie 
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 
