248 ANNUAL OF SCIENTIFIC DISCOVERY. 



I 



proportion to the amplitude of the vibratory motion and to the square of the 

 intensity of the sound furnished. A deaf man may consequently ascertain 

 whether the sound exists, or whether it increases, or whether it dies away, 

 and follow the variations of the figure described by the reflected ray. Nor 

 is this all. Does the experiment-maker desire to know if this tuning-fork is 

 in accord with another tuning-fork, he places a second mirror upon this new 

 tuning-fork, and it is placed upon the angle of reflection of the first, taking 

 care that both planes of vibration shall be perpendicular to each other ; the 

 reflected ray will again be thrown upon the screen. If each of the tuning- 

 forks be vibrated singly, the luminous image will be lengthened ; if the first 

 tuning-fork produces a vertical extension, the second will produce a horizon- 

 tal extension, and when both of them vibrate together, the experiment- 

 maker will have at every instant the figure which results from the combina- 

 tion, or from two rectangular motions. He demonstrates that the figure 

 under these circumstances must be either a circle or a straight line, or some 

 one of the intermediate ellipses. The characteristic by which it may be dis- 

 covered whether the two tuning-forks vibrate in unison, is that the figure, 

 whatever it may be, remains permanent and like itself, although gradually 

 diminishing by the progressive weakness of the initial movement. If on the 

 contrary there is some difference between the two velocities of vibration, 

 warning will be given of their existence by the deformation of the optical 

 figure. In this way the eye will perceive differences which the most sensitive 

 ear could not detect. If, instead of being in unison, the tuning-forks are in 

 the octave, the optical figure becomes a sort of 8, which may change into a 

 figure like the summit of a parabola; and here, too, the constancy of the 

 change of the figure indicates that the octave is more or less just. All the 

 musical intervals represented by the commensurable relations of the number 

 of vibrations have their curves in which the two terms of the fraction are 

 joined, written in geometrical language. Mirrors are not absolutely neces- 

 sary to the application of this method, which consists in amplifying by op- 

 tical means and composing together the vibratory motions of two bodies 

 which are to be compared, so as to obtain without appealing to the ear a 

 degree of precision which has no limit, except the irregularities of the me- 

 chanical phenomenon, or its too rapid duration. 



ON THE EFFECT OF AYIND ON THE INTENSITY OF SOUND. 



The following is an abstract of a paper on the above subject, presented at 

 the British Association, Dublin, by Prof. Stokes : 



The remarkable diminution in the intensity of sound, which is produced 

 when a strong wind blows in a direction from the observer towards the source 

 of sound, is familiar to everybody, but has not hitherto been explained, so 

 far as the author is aware. At first sight we might be disposed to attribute 

 it merely to the increase in the radius of the sound-wave which reaches the 

 observer. The whole mass of air being supposed to be earned uniformly 

 along, the time which the sound would take to reach the observer, and con- 

 sequently the radius of the sound-wave would be increased by the wind in 



