exhibited by the sides of a strong jet of air, steam, or water. This explanation is not the one usually given ; but it is much less complicated, and, it is believed, will be found completely sufficient to explain the phenomenon.

To this department of science belongs the examination of the laws regulating the pressure of steam at various temperatures; and the important improvements which have been made in the application of steam power, are referable to a close examination of all these apparently abstract phenomena. Some of the conditions of steam will be noticed under the section devoted to the consideration of the laws of Heat, and beyond these it is thought that it would be unwise to introduce these high-class examinations, aided, as they must of necessity be, by mathematics, in the present work.

THE SONOROUS MOVEMENT OF BODIES.

ACOUSTICS (from aκovw, to hear). The production of Sound in general.-The Ear forms one of the most important channels of connection between the mind and the external world;—without this organ we should have no perception of sound, and the world would lose a large amount of those pleasures which we enjoy through this medium of sensation.

As we appreciate musical harmony and every variety of sonorous undulation by the ear, a brief description of the structure of that organ appears a proper introduction to the Physics of Sound. The external ear consists of a funnelshaped cartilaginous mouth (Fig. 120), called the auricula, or pinna, and sometimes the ala, or wing; the pendent part being named the lobe. The auricula, receiving many vessels

and nerves, is exceedingly sensible, and is reddened by even slight mental emotion. The auditory passage within the external ear, called the meatus auditorius, leads to the internal organs, extending to the membrane of the tympanum, or drum, which covers the cavity of the tympanum, a small opening in the hardest part of the skull. This communicates with

the mouth by the Eustachian tube, through which it is filled with air, the membrana tympani preventing, unless it is injured, the admission of any air through the ex

Fig. 120.

ternal ear. The cavities of the internal ear, or labyrinth, are hollowed out of the temporal bone, and contain a very thin and limpid fluid, in which floats the acoustic or auditory nerve, divided into two bundles of fine threads; which is the most essential part of the organ of hearing, as even the membrane of the tympanum may be injured, without a total loss of hearing.

The external ear receives the waves of sound, and transmits them along the meatus auditorius to the membrana tympani, which is set into a state of corresponding vibration. These pulsations are continued by three small bones, the malleus, the incus, and the stapes, connected with the os orbiculare; the whole forming a singularly connected chain between the foramen ovale and the membrane

N

of the tympanum. There are many parts of exceeding delicacy, and semicircular channels, which appear to enable us to distinguish the direction of sound, which have not been described, from a wish to confine attention to the principal parts of this wonderful construction. It must, however, be stated that the skull itself is admirably adapted for receiving those impulsive tremors on which sounds depend, the phenomena of which must be now described.

A clear appreciation of wave-motion is necessary to understand the philosophy of sound, and also of those theories of light and heat which involve the idea of undulations.

If we throw a stone upon water, it of course depresses the spot on which it falls; and as the particles return to their position, circular waves spread themselves in all directions from that spot as a centre. These waves consist of alternate elevations and depressions of the fluid: the water itself does not share in the advancing motion of the wave, for a floating body is moved up and down, but not onward, by the undulations. Waves depend upon the force of gravity;—an elevation or a depression is produced in the water, and the particles of the fluid are compelled, in obedience to this power, to restore the disturbed equilibrium, and produce anew the horizontal plane. These movements are termed undulations, vibrations, and oscillations; they are divisible into two kinds. Fasten a cord at one end, hold the other in the hand,

and having strained it tight, move the end you hold sharply up and down, and a

wave will be formed, which will proceed along the string. This is a progressive vibration.

Secure a piece of catgut at both ends, b b, and then strike it, or, raising it at two points, set it sudden

ly free, and we form stationary undulations.

a

Fig. 121.

The string will alternately rise and fall, the elevation becoming the depression, and the contrary; certain points of rest, a a, will be observed, which are called the nodal points, or lines of repose. We may illustrate this with a violin or an Eolian harp. If we place upon one of the strings several pieces of paper, b, c, e, d, and lightly touch the string with a bow, the undulations of the string will be shown by

the pieces of paper being thrown off, as they are situated at the point of elevation or depression of the waves. We may determine the order of vibration, and consequently the nodal points, by touching the string with the finger of one hand, a (Fig. 122), as with the other hand the string is struck with the bow. The small pieces of paper, b, c, d,