96 Dr. A. M. Mayer on the powers of various Substances 



Intensity of Sounds of different Pitch/' Phil. Mag. Nov. 1872). 

 Hence the determination of the relative intensities of notes of 

 different pitch becomes very complicated, and the experimental 

 solution of the problem is encompassed with many difficulties. 

 I, however, hope to be able at some future day to present some 

 work in this direction, when I have succeeded in obtaining results 

 worthy of the appellation of measures of precision. 



2. Measurement of the powers of various substances to Transmit 

 and to Reflect sonorous vibrations*. 

 After we have succeeded in obtaining a measure of the inten- 

 sity of the vibrations of the air at a certain distance from the 



* In the Smithsonian Report for 185/ will be found an account of very 

 interesting and valuable experiments by Professor Joseph Henry, bearing 

 on "Acoustics applied to Public Buildings." In these investigations 

 Professor Henry determined the relative powers of various substances to 

 reflect, transmit, and absorb sonorous vibrations by placing on the bodies 

 the foot of a tuning-fork, and comparing the duration of its sound when 

 thus placed with that given when the fork was suspended in free air by a 

 fine cambric thread. Thus suspended the fork vibrated during 252 seconds. 

 Placed on a large thin pine board its vibrations lasted about ten seconds. 

 In this case " the shortness of duration was compensated by the greater 

 intensity of effect produced." The fork having been placed successively 

 on a marble slab, a solid brick wall, and on a wall of lath and plaster, its 

 vibrations lasted respectively 115 seconds, 88 seconds, and 18 seconds. 



Placed on a cube of india-rubber, the sound emitted by the fork was 

 scarcely greater than when it was suspended from the cambric thread, but 

 its duration was only 40 seconds. Here Henry puts the question, ''What 

 became of the impulses lost by the tuning-fork? They were neither trans- 

 mitted through the india-rubber nor given off to the air in the form of 

 sound, but were probably expended in producing a change in the matter of 

 the india-rubber, or were converted into heat, or both. Though the inquiry 

 did not fall strictly within the line of this series of investigations, yet it 

 was of so interesting a character in a physical point of view to determine 

 whether heat was actually produced, that the following experiment was 



made The point of a compound wire formed of copper andiron was 



thrust into the substance of the rubber, w r hile the other ends of the wire 

 were connected with a delicate galvanometer. The needle was suffered to 

 come to rest, the tuning-fork was then vibrated, and its impulses trans- 

 mitted to the rubber. A very perceptible increase of temperature was the 

 result ; the needle moved through an arc of from one to two and a half de- 

 grees. The experiment was varied and many times repeated; the motions 

 of the needle were always in the same direction, namely in that which w r as 

 produced when the point of the compound wire was heated by momentary 

 contact with the fingers." We have pleasure in agaiu calling attention to 

 this beautiful experiment of Professor Henry ; for he was, I believe, the first 

 to obtain the production of heat by the absorption (so to speak) of sonorous 

 vibrations ; and although several experimenters have subsequently obtained 

 the same results, they seem to be unaware of Henry's antecedent work in 

 the same direction. In 1868 I published a full account of the above expe- 

 riment in my ' Lecture-Notes on Physics,' p. 79 (Van Nostrand, New York). 



In the ssme paper Professor Henry obtained a few qualitative relations 

 on the reflecting-powers of various substances, by placing a watch between 



