NATURAL PHILOSOPHY. 153 



DETERMINATION OF THE VELOCITY OF SOUND BY THE METHOD OF 



COINCIDENCES. 



THIS method, said Professor Bache, occurred to me about the year 

 1832, when observing the marching of. the corps of cadets on the plain 

 at West Point, to martial music and at a quick step. At first the left 

 foot was brought to the ground in the cadences of the music, and, as 

 the music and marching body receded, the right foot appeared to 

 strike the ground in the cadences. There was a succession of such 

 alternations. This suggested the idea that, by providing a regular 

 series of alternating motions, any remarkable phase of which was 

 simultaneous with a sound, and observing at a suitable distance corre- 

 sponding to the loudness of the sound, and with proper appliances to 

 assist our sight, a very simple and exact mode of determining the 

 velocity of sound would be had. This method is of such easy applica- 

 tion that it might be used to investigate many points yet requiring 

 full experimental determination in regard to the effect of temperature, 

 moisture, the force and direction of the wind, &c. 



Professor Coffin remarked that by connecting the pendulum, pro- 

 posed to be used by Professor Bache, with a galvanic circuit-breaker, 

 the signals could be conveyed to any desired point and registered 

 there, to which Professor Bache assented as an interesting devel- 

 opment of his proposition. American Association, Cincinnati. 



LDIIT OF PERCEPTIBILITY OF DIRECT AND REFLECTED SOUND. 



PROFESSOR HEXRY stated, that at the meeting of the Association at 

 Cambridge, he had made a communication relative to the application 

 of the principles of acoustics to the construction of rooms intended for 

 public speaking. In that communication he had stated, as an impor- 

 tant proposition, that when two portions of the same sonorous wave 

 reach the ear of an auditor, one directly from the origin of the sound, 

 and the other indirectly, after one or more reflections, if the two do 

 not differ in the paths they travel by a difference greater than a given 

 quantity, the two sounds will enforce each other, and one louder 

 sound will be perceived ; if, however, the interval is greater than a 

 certain limit, the two sounds will appear distinct, or an echo will be 

 perceived. As an illustration, suppose a speaker to stand before a 

 wall at the distance of, say, ten feet ; in this case the audience in front 

 would hear but one sound. The direct and the reflected impulse meet 

 the ear within the limit which I have called the limit of perceptibility. 

 This limit, a knowledge of which is of considerable practical impor- 

 tance, may rather be expressed in time or in space. The simplest 

 method of obtaining its amount is that of clapping the hands while 

 standing before a perpendicular wall ; if the distance of the observer be 

 sufficient, an echo wiU be heard. If, in this case, the observer gradually 

 approach the wall, and continue to make the sound at a definite point, 

 the echo will cease to be perceived, and the two sounds will appear as 

 one. If the distance from the wall be now measured, twice the dis- 

 tance found will give the limit of perceptibility in space. If the same 



