Dr. Wollaston on sounds inaudible by certain ears . 307 
ence, and, even without reflecting upon the motives which 
guide them, acquire a habit of speaking to deaf persons in a 
shriller tone of voice, as a method by which they succeed in 
making them hear more effectually than by merely speak- 
ing louder. 
In elucidation of this state of hearing, which casually occurs 
as a malady, I have observed, that other ears may for a 
time be reduced to the same condition of insensibility to low 
sounds. I was originally led to this observation, in endea- 
vouring to investigate the cause of deafness in a friend, by 
trial of different modes of closing, or otherwise lessening the 
sensibility of my own ears. I remarked that, when the 
mouth and nose are shut, the tympanum may be so ex- 
hausted by forcible attempt to take breath by expansion of 
the chest, that the pressure of the external air is strongly 
felt upon the membrana tympani, and that, in this state of 
tension from external pressure, the ear becomes insensible to 
grave tones, without losing in any degree the perception of 
sharper sounds. 
The state to which the ear is thus reduced by exhaustion, 
may even be preserved for a certain time without the con- 
tinued effort of inspiration, and without even stopping the 
breath, since by sudden cessation of the effort, the internal 
passage to the ear becomes closed by the flexibility of the 
Eustachian tube, which acts as a valve, and prevents the 
return of air into the tympanum. As the defect thus occa- 
sioned 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 instant be removed by the 
act of swallowing, which opens the tube, and by allowing 
