538 



EXPERIMENTS AND INQUIRIES 



does not at all immediately depend on the 

 diameter of the pipe. It must be confessed, 

 that much lemailis to be done in explaining 

 the precise manner in which the vibration of 

 the air in an organ pipe is generated. M. 

 Daniel Bernoulli has solved several difficult 

 problems relating to the subject: yet some 

 of his assumptions are not only gratuitous, 

 bwt contrary to matter of fact; 



VI. Of the Divergence of Sound. 



It has been generally asserted, chiefly on 

 the authority of Newton, that if any sound 

 be admitted through an aperture into a 

 chamber, it will diverge from that aperture 

 equally in ail directions. The chief argu- 

 ments in favour of this opinion are deducetl 

 from considering the phenomena of the pres- 

 sure of fluids, and the motion of waves ex- 

 cited in a pool of water. But the inference 

 seems to be too hastily drawn : there is a 

 very material difference between impulse and 

 pressure ; and, in the case of waves of water, 

 the moving force at each point is the power 

 of gravity, which, acting primarily in a per- 

 pendicular direction, is only secondarily 

 converted into a horizontal force, in the di- 

 rection of the progress of the waves, being at 

 each step disposed in some measure to spread 

 in every direction : but the impulse, transmit- 

 ted by an elastic fluid, acts primarily in the 

 direction of its progress. It is well known, 

 that if a person calls to another with a 

 speaking trumpet, he points it towards the 

 place where his hearer stands. 1 am assured 

 \>y a very respectable Member of the Royal 

 Society, and it was indeed long ago observed 

 by Grimaldi, that the report of a cannon 

 appears many times louder to a person to- 

 wards whom it is fired, than to one placed 



m a contrary direction. It must have oc- 

 curred to every one's observation, that a 

 sound, such as that of a mill, or a fall of 

 water, has appeared much louder after turn- 

 ing a corner, when the house or other ob- 

 stacle no longer intervened ; and it has been 

 already remarked by Euler, on this head, 

 that we are not acquainted with any sub- 

 stance perfectly impervious to sound. 

 Many solid bodies even appear to conduct 

 sound better than the air : as in the well 

 known experiment of scratching a long 

 beam with a pin; and in discovering the ap- 

 proach of cavalry, by applying the ear to the 

 ground. Indeed, as Mr. Lambert has very 

 truly asserted, the whole theory of the speak- 

 ing trumpet, supported as it is by practical 

 experience, would fall to the ground, if it 

 were demonstrable that sound spreads equally 

 in every direction. In windy weather it may 

 often be observed, that the sound of a dis- 

 tant bell varies almost instantaneously in its 

 strength, so as to appear at least twice as re- 

 mote at one time as at another ; an observa- 

 tion which has also occurred to another gen- 

 tleman, who is uncommonly accurate in ex- 

 amining the phenomena of nature. Now, 

 if sound diverged equally in all directions, 

 the variation produced by the wind could 

 never exceed one tenth of the apparent dis- 

 tance ; but, on the supposition of a motion 

 nearly rectilinear, it may easily happen that 

 a slight change, in the direction of the wind, 

 may convey a beam of sound, either directly 

 or after reflection, in very different degrees 

 of strength, to the same spot. From the ex- 

 periments on the motion of a current of air, 

 already related, it would be expected that a 

 sound, admitted at a considerable distance 

 from its origin through ao aperture, would 

 proceed, with an almost imperceptible in- 



