1856] WRITINGS OF JOSEPH HENRY. 409 



would be propagated instantaneously ; the weaker the repul- 

 sion between the atoms the greater will be the time required 

 to transmit the motion from one to the other ; and the 

 heavier the atoms the greater will be the time required for 

 the action of a given force to produce in them a given 

 amount of motion. Sound also, in meeting an object, is 

 reflected in accordance with the law of light, making the 

 angle of incidence equal to the angle of reflection. The ten- 

 dency however to divergency in a single beam of sound 

 appears to be much greater than in the case of light. The 

 law nevertheless appears to be definitely followed in the 

 case of all beams that are reflected in a direction near the 

 perpendicular. It is on the law of propagation and reflection 

 of sound that the philosophy of an 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 1125 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 universal experience 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 distin- 

 guish the difference between similar sounds, as for example, 

 that from the original impulse and its reflection if they fol- 

 low each other at less than a given interval, which can only 

 be determined by actual experiment, and as this is an im- 

 portant element in the construction of buildings the attempt 

 was made to determine it with some considerable degree of 

 accuracy. For this purpose the observer was placed imme- 

 diately in front of the wall of the west end of the Smithsonian 

 building at the distance of 100 feet; the hands were then 

 clapped together. A distinct echo was perceived ; the differ- 

 ence 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 en- 

 tirely distinct impressions. The observer then gradually 

 approached the building until no echo or perceptible pro- 



