Though these observations, we conceive, sufficient ly establish the opinion, that vibration is the cause of sound, it may he satisfactory to deduce the same doctrine from the two following experiments.
Let AFB be a string, stretched between the two pins A and B, one of which, B, is moveable round a hinge at C, and has attached to it an arm CD, from which the weight E is suspended. With the point of a quill in flect the string into the position AGB, and let the string then slip from the point. It will immediately fly towards AFB, on each side of which it will vibrate, and at the same time send forth sound. Here it is evident, that neither the quill, nor the tension of the string, acts in exciting sound, farther than as they arc favourable to the production of motion in the string ; for their action produces no sound till, by the removal of the quill, the tension is enabled actually to produce motion. Instead of forcibly removing the string, as in the former experi ment, raise the arm CD, so that the string may hang loose in the form AHB, and then suddenly let go the arm I) ; in this case also the string will sound, and give exactly the same tone as in the former experiment These two modes of exciting sound are similar in one respect only. In both the string is at freedom, in a position from which it is moved by its tension ; and dur ing this motion it sounds. We conceive it would be superfluous to adduce more proofs that motion is the cause of sound, and that generally this motion is vibra tory ; we shall therebwe proceed to consider the pro duction of sounds indued with peculiar qualities.
THE differences in quality by which sounds are dis tinguished from each other, do not immediately depend on corresponding differences in the nature of the bodies from which the sounds issue ; fur, by various artifices, we can procure, from bodies consisting of the same sub stance, sounds which shall differ in almost any respect we choose. It is then by examining those artifices, and investigating in what respects their mode of operation can differ, or correspond, that the immediate causes of the different qualities of sound can alone be discovered. The circumstances which affect the sounds of bodies are, their form, their magnitude, their density, the mode by which they are excited, and the comparative force of the power by which they vibrate. As these must
evidently affect the vibration of the sonorous bodies, the above-mentioned investigation necessarily supposes, that we trace in what manner the vibration of a body is chang ed by a variation in these circumstances.
This research, however, requiring a considerable ac quaintance with the higher analysis, we shall at present take for granted the results to which it leads ; referring those readers who are versant in mathematical inquiries to the word VIBRATION, and to the authors quoted at the end of this article.
Those philosophers who have investigated the vi brations of bodies, have arrived at the following con clusion, that whatever affects the vibration of a body produces a corresponding effect on the qualities of the sounds which it emits ; and, conversely, that those bo dies whose sounds are similar, have some thing in com mon in their mode of vibration ;—consequently the dif ferent qualities of sound are caused by something pe culiar in the vibration of the bodies which the sounds are produced.
All continued sounds, which remain in any degree uniform throughout their duration, are capable of being compared with each other in their degree of acuteness. When sounds are equally acute, they are said to have the same pitch ; but when they differ in acuteness, that sound which is shriller is said to be acute, or to have a higher pitch ; and that which is less shrill, is said to be graver, and to have a lower pitch, or a deeper tone. A difference in pitch forms the chief character by which musical sounds are distinguished from each other, and is the foundation of their use in music. In unmusical sounds, it generally holds a place subordinate to their other qualities.
Musical sounds have occupied the attention of phi losophers more than any other class of sounds. The superior precision with which the ear can estimate any variation in pitch, renders these sounds more easily com flared ; and the vibration of the sonorous bodies which produce them, are, on account ol their superior simplici ty of form, more easily investigated.