ACOUSTICS APPLIED TO PUBLIC BUILDINGS. 231 



ascends to it, leaving other points of ppace deficient in sonorous im- 

 pulses 



Water, and all liquids which offer great resistance to compression, 

 are good reflectors of sound. This may he shown hy the following 

 experiment. When water is gradually poured into an upright cylin- 

 drical vessel, over the mouth of which a tuning-fork is vihrated, until 

 it comes within a certain distance of the mouth, it will reflect an 

 echo in unison with the vibrations of the fork, and produce a loud 

 resonance. This result explains the fact, which had been observed 

 with some surprise, that the duration of the resonance of a newly plas- 

 tered room was not perceptibly less than that of one which had been 

 thoroughly dried. 



There is another principle of acoustics which has a bearing on this 

 subject. I allude to the refraction of sound. It is well known that, 

 when a ray of sound passes from one medium to another, a change in 

 velocity takes place, and consequently a change in the direction or a 

 refraction must be produced. The amount of this can readily be cal- 

 culated where the relative velocities are known. In rooms heated by 

 furnaces, and in which streams of heated air pass up between the au- 

 dience and speaker, a confusion has been supposed to be produced, and 

 distinct hearing interfered with, by this cause. Since the velocity of 

 sound in air at 32° of Fahrenheit has been found to be 1,090 feet in a 

 second, and since the velocity increases 1.14 feet for every degree of 

 Fahrenheit's scale, if we know the temperature of the room, and that 

 of the heated current, the amount of angular refraction can be ascer- 

 tained. But since the ear does not readily judge of the difference of 

 direction of two sounds emanating from the same source, and since 

 two rays do not confuse the impression which they produce upon the 

 ear, though they arrive by very different routes, provided they are 

 within the limit of perceptibility, we may therefore conclude that the 

 indistinctness produced by refraction is comparatively little. Professor 

 Bache and myself could perceive no difference in distinctness in hear- 

 ing from rays of sound passing over a chandelier of the largest size, 

 in which a large number of gas jets were in full combustion. The 

 fact of disturbance from this cause, however, if any exist, may best be 

 determined by the experiment with a parabolic mirror and the hear- 

 ing trumpet before described. 



These researches may be much extended ; they open a field of in- 

 vestigation equally interesting to the lover of abstract science and to 

 the practical builder ; and I hope, in behalf of the committee, to give 

 some further facts with regard to this subject at another meeting. 



I shall now briefly describe the lecture room, which has been con- 

 structed in accordance with the facts and principles previously stated, 

 so far at least as they could be applied. 



There was another object kept in view in the construction of this 

 room besides the accurate hearing, namely, the distinct seeing. It was 

 desirable that every person should have an opportunity of seeing the 

 experiments which might be performed^ as well as of hearing dis- 

 tinctly the explanation of them. 



'By a fortunate coincidence of principle, it happens that the arange 



