23i O.K. Wead— Intensity of Sound. 



is to "be measured by the energy or the momentum of the 

 sounding body. Here unfortunately he falls into the old error, 

 and tacitly assumes that the movement of the air corresponds 

 exactly with that of the fork. 



On this perplexing subject of the measure of a sound-sensa- 

 tion, most methods of experimenting are very faulty, as the 

 following considerations will show : 



First. When the experiment is to determine either the 

 minimum audible or the equality of two sounds, if the sensa- 

 tions be the same, the causes must be the same ; these causes 

 are the movements of the particles of air near the ear, and the 

 movements therefore being the same, their measure, whether 

 as energy or momentum, must be the same. No discrimination 

 then between rival theories can be made by the method of 

 equal sounds. 



Second. It is tacitly assumed in most of the experiments 

 that I know of, whether b}^ dropping balls or hammers, or by 

 telephonic arrangements of various kinds, that the efficiency of 

 the apparatus is substantially the same under all conditions, 

 and in none has it been determined. Any one familiar with 

 modern mechanical measures knows how much attention is 

 given to determining under all the varying practical conditions 

 the efficiency of electric, thermal and other machinery. But 

 such laborious work as that of Vierordt and his followers 

 seems much like trying to determine the laws of vision by first 

 producing equality of illumination between a tiny incandescent 

 lamp supplied with current from a toy boiler, engine and 

 dynamo, and a distant powerful arc lamp with its boiler, engine 

 and dynamo supplying it with many horse power, and then 

 dividing the mechanical equivalent of the fuel used in each 

 apparatus by the square of the respective distance from the 

 eye or screen. Such a method might give the ratio of the 

 efficiencies, but not anything of much value regarding the laws 

 of vision. Every one knows that a ball falling on a metal plate 

 will cause a louder sound than when falling on cloth ; that is, 

 the efficiency of the first as a sound-producing mechanism is 

 much greater than the second ; presumably if any other con- 

 dition were changed, height of fall, weight of ball, material, 

 etc., the efficiency would be changed ; but experiments have 

 been heaped up on the tacit but improbable assumption that 

 it is not changed. 



In conclusion, I desire to point out that my determinations 

 for S, the energy per sq. cm. per sec. at the limit of hearing, 

 must be taken as outside values ; they must be multiplied by 

 the efficiency of the apparatus as a sound-producer ; certain 

 experiments have led me to infer that for the Ut 3 fork the 

 efficiency was about 7 per cent, as stated in the original paper ; 



