ACOUSTICS APPLIED TO PUBLIC BUILDINGS. 225 



heavier the atoms, the greater will he the time required for the action 

 of a given force to produce in them a given amount of motion. 

 Sound also, in meeting an ohject, is reflected in accordance with the 

 law of light, making the angle of incidence equal to the angle of re- 

 flection. The tendency, however, to divergency in a single heam of 

 sound appears to he much greater than in the case of liglit. The law, 

 nevertheless, appears to be definitely followed in the case of all 

 heams that are reflected in a direction near the perpendicular. It is 

 on the law of propagation and reflection of sound that the philosophy 

 of the echo depends. Knowing the velocity of sound, it is an easy matter 

 to calculate the interval of time which must elapse between the original 

 impulse and the return of the echo. Sound moves at the rate of 1,125 

 feet in a second, at the temperature of 60°.* 



If, therefore, we stand at half this distance before a wall, the echo 

 will return to us in one second. It is, however, a fact known from 

 general exjierience, that no echo is perceptible from a near wall, 

 though in all cases one must be sent back to the ear. The reason of 

 this is, that the ear cannot distinguish the difference between similar 

 sounds, as, for example, that from the original impulse and its reflec- 

 tion, if they follow each other at less than a given interval, which can 

 only be determined by actual experiment ; and as this is an important 

 element in the construction of buildings, the attempt was made to de- 

 termine it with some considerable degree of accuracy. For this pur- 

 pose the observer was placed immediately in front of the wall of the 

 west end of the Smithsonian building, at a distance of 100 feet ; the 

 hands were then clapped together. A distinct echo was perceived ; 

 the difierence between the time of the passage of the impulse from the 

 hand to the ear, and that from the hand to the wall and back to the 

 ear, was sufficiently great to produce two entirely distinct impressions. 

 The observer then gradually appraached the buildings until no echo or 

 perceptible prolongation of the sound was observed. By accurately 

 measuring this distance, and doubling it, we find the interval of space 

 within which two sounds may follow each other without appearing 

 separately. But if two rays of sound reach the ear after having passed 

 through distances the difference between which is greater than this^ 

 they produce the effect of separate sounds. This distance we have 

 called the limit of percei^tibility in terms of space. If we convert this 

 distance into the velocity of sounds we ascertain the limit of percepti- 

 bility in time. 



In the experiment first made with the wall, a source of error was 

 discovered in the fact that a portion of the sound returned was reflected 

 from the cornice under the eaves, and as this was at a greater distance 

 than the part of the wall immediately perpendicular to the observer, 

 the moment of the cessation of the echo was less distinct. In subse- 

 quent experiments with a louder noise, the reflection was observed 

 from a perpendicular surface of about 12 feet square^ and from this 

 more definite results were obtained. Tlie limit of the distance in this 

 case was about 30 i'eet, varying slightly, perhaps, with the intensity 



* From the av<u-age of all the experiments, acci^rdiiig to Sir Joiin Herscliel, tiie velocity 

 of sound is 1,090 feet at the temperature of 32°, and this is increased 1.14 feet for every 

 degree of temperature of Fahrenheit's scale. 



15 s 



