An umbrella, held in a proper position over the head, may serve to collect the force of a distant sound by reflection, in the manner of a hearing-trumpet ; but its substance is too slight to reflect any sound perfectly, unless the sound fall on it in a very'oblique direction. The exhibition of the Invisible Girl is said to depend on the reflection of sound ; but the deception is really per formedbv conveying the sound through pipes artfully concealed, and opening opposite to the mouth of the trumpet, from which it seems to proceed.
When a portion of a pulse of a sound is separated by any means from rest of the spherical or hemispherical surface to which it belongs, and proceeds through a wide space, without being supported on either side, there a certain degree of divergence, by means of which it some times becomes audible in every part of the medium transmitting it: bitt the sottnd t h is diverging is comparatively very fai nt. Ilenee, in order that a speaking-trumpet may produce its full etfeet, it must be di rected in a right line towards the hearer ; and the sountl collected into the focus of a concave mirror is far more powerful than at a little distance from it, which could not happen, if sound, in all cases, tended to spread equally in all directions. it is said that tIte report of a cannon ap pears many tittles louder to a person to wards whom it is fired, than to one placed x contrary direction. It must, says Dr. Young, have occurred to every one's ob servation, that a sound, such as that of a mill, or a fall of water, bas appeared much louder after turning a corner, when the house or other obstacle »o longer inter vened. Indeed, the whole theory of the speaking-trumpet wouldfitll to the ground, if it were demonstrable that sound spreads equally in all directions. In windy Ixea ther, it may be often observed, that the sound of a distant bell varies almost in stantaneously in its strength, so as to ap pear twice as remote at one time as an other. Now, ifsound diverged equally in all directions, the variation produced by the wind would not exceed one-tenth of the apparent distance ; but on the suppo sition of a motion nearly rectilinear, it may easily happen that a slight change in the direction of the wind shall convey a sound, either directly or after reflection, in very different degrees, to the same spot.
The decay of sound is the natural con sequence of its distribution throughout a larger and larger quantity of matter, as it proceeds to diverge every way from its centre. The actual velocity of the parti
cles of the medium transmitting itoppears to diminish, simply, in the same proportion as the distance from the centre increases; consequently, their energy, which is to be considered as the mea.sure of the strength of sound, must vary as the square of the distance ; so that, at the distance of ter, feet from the sounding body, the velocity of the particles of the medium becomes one-tenth as great as at the distance of one foot, and their energy, or the stren,g-th of the sound, only one-littudredth as great An echo is a reflection of sound strik ing against some object, as an hnage is reflected in a glass : but it has been dis puted, what are the proper qualities in a body for thus reflecting sounds. It is in general known, that caverns, g-rottoes, mountains, rind ruined buildings, return this reflection of sound. We have heard of a very extraordinary eeho. at a ruined fortress near Louvain, in Flanders. If a person sung, he only heard his own voice, without ally repetition ; on the contrary, those who stood at some distance heard the echo, but not the voice ; but then they heard it with surprising variations, some. times louder, sometimes softer, now more near, then more distant. There is an ac count, in the 'Memoirs of the French aca demy, of a similar echo near Rouen. It has been already observed, that every point against which the pulses of sound strike becomes the centre of a new series of pulses, and sound describes equal dis tances in equal times ; therefore, when any sound is propagated from a centre, and its pulses strike against a variety of obstacles, if the sum of the right lines drawn from that point to each or the ob stacles, and from each obstacle to a second point, be equal, then will the latter be point in which an echo will be heard. Thus, let A, fig. 4, be the point from which the sound is propagated in all directions, and let the pulses strike against the ob stacles C, D, E, F, G, II, I, &c. each of these points becomes a new centre of pul ses by the lirst principles, and therefore trona each of them one series of pulses will pass through the point B. Now, if the several sums of the right lines A C C B,