SOUND.

LECTURE I.

VARYING DENSITY

THE NERVES AND SENSATION-PRODUCTION AND PROPAGATION OF SONOROUS MOTION-EXPERIMENTS ON SOUNDING BODIES PLACED IN VACUO-ACTION OF HYDROGEN ON THE VOICE-PROPAGATION OF SOUND THROUGH AIR OF REFLECTION OF SOUND ECHOES REFRACTION OF SOUND-INFLECTION OF SOUND; CASE OF ERITH VILLAGE AND CHURCH -INFLUENCE OF TEMPERATURE ON VELOCITY INFLUENCE OF DENSITY AND ELASTICITY-NEWTON'S CALCULATION OF VELOCITY-THERMAL CHANGES PRODUCED BY THE SONOROUS WAVE-LAPLACE'S CORRECTION OF NEWTON'S FORMULA RATIO OF SPECIFIC HEATS AT CONSTANT PRESSURE AND AT CONSTANT VOLUME DEDUCED FROM VELOCITIES OF SOUND-MECHANICAL EQUIVALENT OF HEAT DEDUCED FROM THIS RATIO-INFERENCE THAT ATMOSPHERIC AIR POSSESSES NO SENSIBLE POWER TO RADIATE HEATVELOCITY OF SOUND IN DIFFERENT GASES VELOCITY IN LIQUIDS AND SOLIDS-INFLUENCE OF MOLECULAR STRUCTURE ON THE VELOCITY OF

SOUND.

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HE various nerves of the human body have their origin in the brain, and the brain is the seat of sensation. When you wound your finger, the nerves which run from the finger to the brain convey intelligence of the injury, and if these nerves be severed, however serious the hurt may be, no pain is experienced. We have the strongest reason for believing that what the nerves convey to the brain is in all cases motion. It is the motion excited by sugar in the nerves of taste which, transmitted to the brain, produces the sensation of sweetness, while bitterness is the result of the motion produced by aloes. It is

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the motion excited in the olfactory nerves by the effluvium of a rose, which announces itself in the brain as the odour of the rose. It is the motion imparted by the sunbeams to the optic nerve which, when it reaches the brain, awakes the consciousness of light; while a similar motion imparted to other nerves resolves itself into heat in the same wonderful organ.*

The motion here meant is not that of the nerve as a whole; it is the vibration, or tremor, of its molecules or smallest particles.

Different nerves are appropriated to the transmission of different kinds of molecular motion. The nerves of taste, for example, are not competent to transmit the tremors of light, nor is the optic nerve competent to transmit sonorous vibration. For this latter a special nerve is necessary, which passes from the brain into one of the cavities of the ear, and there spreads out in a multitude of filaments. It is the motion imparted to this, the auditory nerve, which, in the brain, is translated into sound.

We have this day to examine how sonorous motion is produced and propagated. Applying a flame to this small collodion balloon, which contains a mixture of oxygen and hydrogen, the gases explode, and every ear in this room is conscious of a shock, to which the name of sound is given. How was this shock transmitted from the balloon to your organs of hearing? Have our exploding gases shot the air-particles against the auditory nerve as a gun shoots a ball against a target? No doubt, in the neighbourhood of the balloon, there is to some extent a propulsion of particles; but air shooting through air comes speedily to rest, and no particle of air from the vicinity of the balloon reached the ear of any person here present. The

* The rapidity with which an impression is transmitted through the nerves, as first determined by Helmholtz and confirmed by Du Bois Raymond, is 93 feet a second.

PRODUCTION AND PROPAGATION OF SOUND.

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process was this:-When the flame touched the mixed gases they combined chemically, and their union was accompanied by the development of intense heat. The air at this hot focus expanded suddenly, forcing the surrounding air violently away on all sides. This motion of the air close to the balloon was rapidly imparted to that a little further off, the air first set in motion coming at the same time to rest. The air, at a little distance, passed its motion on to the air at a greater distance, and came also in its turn to rest. Thus each shell of air, if I may use the term, surrounding the balloon, took up the motion of the shell next preceding, and transmitted it to the next succeeding shell, the motion being thus propagated as a pulse or wave through the air.

In air at the freezing temperature this pulse is propagated with a speed of 1,090 feet a second.

The motion of the pulse must not be confounded with the motion of the particles which at any moment constitute the pulse. For while the wave moves forward through considerable distances, each particular particle of air makes only a small excursion to and fro.

The process may be rudely represented by the propagation of motion through a row of glass balls, such as are employed in the game of solitaire. I place these balls along a groove thus, fig. 1, each of them touching its

neighbours. Taking one of them in my hand, I urge it against the end of the row. The motion thus imparted. to the first ball is delivered up to the second, the motion of the second is delivered up to the third, the motion of the third is imparted to the fourth; each ball, after having given up its motion, returning itself to rest. The last ball only of the row flies away. Thus is sound conveyed from particle to particle through the air. The particles which fill the cavity of the ear are finally driven against the tympanic membrane, which is stretched across the passage leading to the brain. This membrane, which closes the drum' of the ear, is thrown into vibration, its motion is transmitted to the ends of the auditory nerve, and afterwards along the nerve to the brain, where the vibrations are translated into sound. How it is that the motion of the nervous matter can thus excite the consciousness of sound is a mystery which we cannot fathom.

Let me endeavour to illustrate the propagation of sound by another homely but useful illustration. I have here

five young assistants, A, B, C, D, and E, fig. 2, placed in a row, one behind the other, each boy's hands resting against the back of the boy in front of him. E is now foremost, and A finishes the row behind. I suddenly push A; A pushes