ACOUSTICS APPLIED TO PUBLIC BUILDINGS. 229 



only about forty seconds. The question may here he asked_, what 

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

 transmitted 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 : 



A cylindrical piece of India rubber, about an inch and a quarter 

 in diameter was placed in a tubulated bottle, with two openings, one 

 near the bottom and the other at the top. A stuffing-box was attached 

 to the upper, through which a metallic stem, with a circular foot to 

 press upon the India rubber, was made to pass air-tight. The lower 

 tubular was closed with a cork, in a perforation of which a fine glass 

 tube was cemented. A small quantity of red ink was placed in the 

 tube to serve as an index. The whole arrangement thus formed a kind 

 of air-thermometer, which would indicate a certain amount of change 

 of temperature in the enclosed air. On the top of the stem, the 

 tuning-fork was screwed, and consequently its vibrations were trans- 

 mitted to the rubber within the bottle. The glass was surrounded 

 with several coatings of flannel to prevent the influence of external 

 temperature. The tuning-fork was then sounded, and the vibrations 

 were kept up for some time. No reliable indications of an increase 

 of temperature were observed. A more delicate method of making 

 the experiment next suggested itself. The tube containing the drop 

 of red ink, with its cork was removed, and the point of a compound 

 wire formed of copper and iron was thrust into the substance of the 

 rubber, wliile 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 transmitted 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 degrees. The 

 experiment was varied, and many times repeated ; the motions of the 

 needle Avere always in the same direction, namely, in that which was 

 produced when the point of the compound wire v/as heated by momen- 

 tary contact with the fingers. The amount of heat generated in this 

 way is, however, small, and indeed, in all cases in which it is gene- 

 rated by mechanical means, the amount envolved appears very small 

 in comparison with the labor expended in producing it. Jule has 

 shown that the mechanical energy generated in a pound weight, by 

 falling through a space of seven hundred and fifty feet, elevates the 

 temperature of a pound of water one degree. 



It is evident that an object like India rubber actually destroys a por- 

 tion of the sound, and hence, in cases in which entire non-conduction 

 is required, this substance can probably be employed with perfect suc- 

 cess. 



The tuning-fork was next pressed upon a solid brick wall, and the 

 duration of vibration from a number of trials was eighty-eight seconds. 

 Against a wall of lath and plaster the sound was louder, and con- 

 tinued only eighteen seconds. 



