On Sounds inaudible ly certain Ears. 1 89 



relax the effort of inspiration with sufficient suddenness to close 

 the Eustachian tube, and thus maintain the exhaustion ; neither 

 is it very easy to refrain long together from swallowing the saliva, 

 which instantly puts an end to the experiment. 



I may liere remark, that this state of excessive tension of the 

 tympanum is sometimes produced by sudden increase of exter- 

 nal pressure, as well as by decrease of that within, as is often 

 felt in the diving-bell as soon as it touches the water; the pres- 

 sure of which upon the included air closes the Eustachian tube, 

 and, in proportion to the descent, occasions a degree of tension 

 on the tympanum, that becomes distressing to persons who 

 have not learned to obviate this inconvenience. Those who are 

 accustomed to descend, probably acquire the art of opening the 

 Eustachian tube by swallowing, or incipient yawning, as soon as 

 the diving-bell touches the water. 



It seems highly probable that, in the state of artificial tension 

 thus produced, a corresponding deafness to low tones is occa- 

 sioned ; but, as I never have been in that situation, I have not 

 had an opportunity of ascertaining this point by direct experi- 

 ment. 



In the natural healthy state of the human ear, there does not 

 seem to be any strict limit to our power of discerning low sounds. 

 In listening to those pulsatory vibrations of the air of which 

 sounds consists, if they become less and less frequent, we may 

 doubt at what point tones suited to produce any musical ef- 

 fect terminate ; yet all persons but those whose organs are pal- 

 pably defective continue sensible of vibratory motion, until it 

 becomes a mere tremor, which may be felt and even almost 

 counted. 



On the contrary, if we turn our attention to the opposite ex- 

 tremity of the scale of audible sounds, and with a series of pipes 

 exceeding each other in sharpness, if we examine the effects of 

 them successively upon the ears of any considerable number of 

 persons, we shall find (even within the range of those tones 

 which are produced for their musical effects) a very distinct and 

 striking difference between the powers of different individuals, 

 whose organs of hearing are in other respects perfect, and shall 

 liave reason to infer, that human hearing in general is more 

 confined than ha? been supposed with regard to its perception of 

 very acute sound's, and has probably, in every instance, some de- 

 finite limit, at no great distance beyond the sounds ordinarily 

 heard. 



It is now some years since I first had occasion to notice this 

 "pecies of partial deafness, which I at that time supposed to be 

 peculiar to the individual in whom 1 oliservcd it. While I was 

 iMidcavouring to estimate the pitch of certain sharp sounds,! 



remarked 



