VIBRATION

, in Mechanics, a regular reciprocal motion of a body, as, for example, a pendulum, which being freely suspended, swings or vibrates from side to side.

Mechanical authors, instead of Vibration, often use the term oscillation, especially when speaking of a body that thus swings by means of its own gravity or weight.

The Vibrations of the same pendulum are all isochronal; that is, they are performed in an equal time, at least in the same latitude; for in lower latitudes they are found to be slower than in higher ones. See PENDULUM. In our latitude, a pendulum 39 1/<*> inches long, vibrates seconds, making 60 Vibrations in a minute.

The Vibrations of a longer pendulum take up more time than those of a shorter one, and that in the subduplicate ratio of the lengths, or the ratio of the square roots of the lengths. Thus, if one pendulum be 40 inches long, and another only 10 inches long, the former will be double the time of the latter in performing a Vibration; for √40 : √10 :: √4 : √1, that is as 2 to 1. And because the number of Vibrations, made in any given time, is reciprocally as the duration of one Vibration, therefore the number of such Vibrations is in the reciprocal subduplicate ratio of the lengths of the pendulums.

M. Mouton, a priest of Lyons, wrote a treatise, expressly to shew, that by means of the number of Vibrations of a given pendulum, in a certain time, may be established an universal measure throughout the whole world; and may fix the several measures that are in use among us, in such a manner, as that they might be recovered again, if at any time they should chance to be lost, as is the case of most of the ancient measures, which we now only know by conjecture.

The Vibrations of a Stretched Chord, or String, arise from its elasticity; which power being in this case similar to gravity, as acting uniformly, the Vibrations of a chord follow the same laws as those of pendulums. Consequently the Vibrations of the same chord equally stretched, though they be of unequal lengths, are isochronal, or are performed in equal times; and the squares of the times of Vibration are to one another inversely as their tensions, or powers by which they are stretched.

The Vibrations of a spring too are proportional to the powers by which it is bent. These follow the same laws as those of the chord and pendulum; and consequently are isochronal; which is the foundation of spring watches.

Vibrations are also used in Physics, &c, and for several other regular alternate motions. Sensation, for instance, is supposed to be performed by means of the vibratory motion of the contents of the nerves, begun by external objects, and propagated to the brain.

This doctrine has been particularly illustrated by Dr. Hartley, who has extended it farther than any other writer, in establishing a new theory of our mental operations.

The same ingenious author also applies the doctrine of Vibrations to the explanation of muscular motion, which he thinks is performed in the same general manner as sensation and the perception of ideas. For a particular account of his theory, and the arguments by which it is supported, see his Observations on Man. vol. 1. |

The several sorts and rays of light, Newton conceives to make Vibrations of divers magnitudes; which, according to those magnitudes, excite sensations of several colours; much after the same manner as Vibrations of air, according to their several magnitudes, excite sensations of several sounds. See the article COLOUR.

Heat, according to the same author, is only an accident of light, occasioned by the rays putting a fine, subtile, ethereal medium, which pervades all bodies, into a vibrative motion, which gives us that sensation. See Heat.

From the Vibrations or pulses of the same medium, he accounts for the alternate fits of easy reflexion and easy transmission of the rays.

In the Philosophical Transactions it is observed, that the butterfly, into which the silk-worm is transformed, makes 130 Vibrations or motions of its wings, in one coition.