66 



ON THE AUGMENTATION OF SOUNDS. 



ear, in thofe intervals of filence which were not difturbed by 

 foreign founds, at the diftance of 5\ yards; but when I re- 

 moved half a yard farther, the ftrokes of the balance were no 

 longer heard. A circular plate of rolled iron, one foot in 

 diameter, was then brought into contact with the part of the 

 watch fartheft from me, the pofition of it being fuch as to 

 prefent the plane of the circle to my ear. This circumftance 

 increafed the range of the found; for every ftroke of the in- 

 ltrument was diftincily perceived at the dittance of four yards, 

 when the filence of the place remained undifturbed by other 

 caufes. The refult of this experiment is too obvious to need 

 a comment : and I have only to add, that when the trial is 

 made within doors, the range of the clicking is greatly in- 

 creafed ; becaufe the primary found receives an acceflion of 

 force from a number of its own pulfes, which are reflected to 

 the ear by the furniture and walls. 

 Exp. 2. The Experiment II. — I proceeded, in the next place, to examine 



P fi/ ,V u * * ne e ^ e( ^ s produced on the auditory organs by the vibrations 

 found itfelf. communicated to an elaftic furface by a watch; the immedi- 

 ate found of which was fmothered by art. For this purpofe, 

 a watch was placed upon a cufbion, under an inverted por- 

 celain cup, which was wrapped externally in feveral folds 

 of flannel. The inirrument was heard with difficulty at the 

 diftance of one foot in this fort of confinement; but when I 

 placed (he muffler with the watch under it, upon a fquare 

 mahogany table four feet broad, the clicking noife imparted 

 by it to the top of the table, reached the ear very diftincily at 

 the diftance of four yards. The fame watch, covered in like 

 manner upon the iron plate mentioned above, reached my ear 

 ttt the diftance of 22 feet. The apparatus flood in this trial 

 upon a round oak table, 24 inches in diameter. 

 The enlargement The preceding fact proves indifputably, that the fecondary 



of the furiace vibrations of an elaftic furface actually affect the ear, in thole 



augments the r , . . , r , , r , 



found. cafes wherein the pulles never reach the perlon ot the hearers, 



which proceed from the part that is exclufively confidered as 



the feat of found ; in other words it proves, that the caufe of 



a found is not confined to the obftacle receiving the primary 



impulfe, but that it is propagated from the point of impact 



through the contiguous bodies. The firft experiment alfo 



fhews, that the enlargement of the vibrating furface encreafes 



the force, which a given ftroke exerts upon an ear placed at 



4 a given 



