2-28 ACOUSTICS APPLIED TO PUBLIC BUILDINGS. 



of precisely the same size and form, would reproduce the same 

 vibrations. 



It is a lact well established by observation, that when two bodies are 

 in perfect unison, and separated fit)m each other by a space filled with 

 air, vibrations of the one will be transmitted to the other. From this 

 consideration it is probable that relatively the same effect ought to be 

 produced in transmitting immediately the vibration of a tuning-fork 

 to a reflecting body, as to duration and intensity, as in the case of 

 transmission through air. This conclusion is strengthened by float- 

 ing a flat piece of wood on water in a vessel standing upon a sounding- 

 board ; placing a tuning-fork on the wood, the vibrations will be 

 transmitted to the board through the water, and sounds will be pro- 

 duced of the same character as those emitted when the tuning-fork is 

 placed directly upon the board. 



A tuning-fork suspended from a fine cambric thread, and vibrated 

 in air, was found, from the mean of a number of experiments, to con- 

 tinue in motion 252 seconds. In this experiment, had the tuning- 

 fork been in a perfect vacuum, suspended without the use of a string, 

 and, further, had there been no ethereal medium, the agitation of 

 which would give rise to light, heat, electricity, or some other form of 

 ethereal motion, the fork would have continued its vibration forever. 



The fork was next placed upon a large, thin pine board, the top 

 of a table. A loud sound in this case was produced, which continued 

 less than ten seconds. The whole table as a system was thrown into 

 motion, and the sound produced was as loud on the under side as on 

 the upper side. Had the tuning-fork been placed against a partition 

 of this material, a loud sound would have been heard in the adjoining 

 room ; and this was proved by sounding the tuning-fork against a door 

 leading into a closed closet. The sound within was apparently as 

 loud as that without. 



The rapid decay of sound in this case was produced by so great an 

 amount of the motive power of the fork being communicated to a large 

 mass of wood. The increased sound was due to the increased surface. 

 In other words, the shortness of duration was compensated for by the 

 greater intensity of effect produced. 



The tuning-fork was next placed upon a circular slab of marble, 

 about three feet in diameter and three quarters of an inch thick. The 

 sound emitted was feeble, and the undulations continued one hundred 

 and jifteen seconds, as deduced from the mean of six experiments. 



In all these experiments, except the one in a vacuum, the time of the 

 cessation of the motion of the tuning-fork was determined by bringing 

 the mouth of a resounding cavity near the end of the fork; this cavity, 

 having previously been adjusted to unison with the vibrations of the 

 fork, gave an audible sound when none could be heard by the unaided 



ear. . n 



The tuning-fork was next placed upon a cube of India rubber, and 

 this upon the marble slab. The sound emitted by this arrangement 

 was scarcely greater than in the case of the tuning-fork suspended 

 from the cambric thread, and from the analogy of the previous ex- 

 periments we might at first thought suppose the time of duration 

 would be great ; but this was not the case. The vibrations continued 



