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watches, the same result follows ; and it is the same if instead of two 

 arms of the stethophone we employ only one. This remarkable 

 separation of the components of a sound may be effected also when 

 the sounding bodies are enclosed in a box capable of transmitting 

 sound, or when separated from us by the interposition of materials 

 capable of conducting sound ; and by successive trials and comparisons 

 of intensity at different places, and by a process of exclusion of those 

 parts which fail to cause sensation, the respective positions of two 

 adjacent sounding bodies may be predicated. If, for example, we have 

 two watches, A and B, enclosed in a box, and through one cup, A, 

 we hear watch A, and with the other cup, B, we hear watch B, we 

 may conclude that cup A is nearer watch A than cup B is, and so on. 

 In the same manner we may auscultate the morbid sounds of the heart. 

 By cup A, placed at the apex, and cup B placed at the base, we hear 

 separately the morbid sounds of the two parts ; for example, a blow- 

 ing murmur at the apex in one ear, and a rasping murmur at the 

 base in the other ear. This we are enabled to do, although at any 

 intermediate point with the single ear, either with or without a 

 stethoscope, we hear the conjoined two sounds. It is obvious that 

 with the stethophone we not only succeed in separating sound, but 

 that this instrument, or some similar contrivance, affords the only 

 possible means of hearing, with two ears at once, sounds emanating 

 from the same region or surface, for the sides of the head can be 

 applied, of course, to the same sounding surface only in turn or 

 succession. With this instrument we, as it were, place our ears in 

 our hands, apply them where we choose, and listen with them both 

 at adjacent or distant points of the same surface, at one and the 

 same instant of time. 



It is not unlikely that the property which the stethophone pos- 

 sesses of pointing out with precision where sound is most intense, 

 may be very usefully employed. It seems possible that it might be 

 turned to account in discovering the points where operations in mili- 

 tary mining may be going on. 



It is, however, in the practice of medicine only that the differential 

 stethophone has been hitherto applied, and it may be here permitted 

 to me to point to some of the chief purposes for which it is adapted, 

 and for which it has been employed. 



In respect to respiration, we may compare at once, and without 



