DAVY ON THE MOTION OF HEAT. 97 tory motion, or a motion of the particles round their axes, or a motion of the particles round each other. It seems possible to account for all the phenomena of heat, if it be supposed that in solids the particles are in a constant state of vibratory motion, the particles of the hottest bodies moving with the greatest velocity, and through the greatest space; that in fluids and elastic fluids, besides the vibratory motion, which must be conceived greatest in the last, the particles have a motion round their own axes with different velocity, the particles of elastic fluids moving with the greatest quickness, and that in ethereal substances the particles move round their own axes, and separate from each other, penetrating in right lines through space. Temperature may be conceived to depend upon the velocity of the vibrations; increase of capacity in the motion being performed in greater space; and the diminution of temperature during the conversion of solids into fluids or gases, may be explained on the idea of the loss of vibratory motion, in consequence of the revolution of particles round their axes, at the moment when the body becomes fluid or aëriform, or from the loss of rapidity of vibration in consequence of the motion of the particles through space. 98 LECTURE IV. [February 13, 1862.] THE TREVELYAN INSTRUMENT - GORE'S REVOLVING BALLS-IN- APPENDIX: NOTE ON THE TREVELYAN INSTRUMENT PHYSICAL EFORE finally quitting the subject of expansion, I BEFORE wish to show you an experiment which illustrates in a curious and agreeable way the conversion of heat into mechanical energy. The fact which I wish to reproduce was first observed by a gentleman named Schwartz, in one of the smelting works of Saxony. A quantity of silver which had been fused in a ladle was allowed to solidify, and to hasten its cooling it was turned out upon an anvil. Some time afterwards, a strange buzzing sound was heard in the locality, and was finally traced to the hot silver, which was found quivering upon the anvil. Many years subsequent to this, Mr. Arthur Trevelyan chanced to be using a hot soldering-iron, which he laid by accident against a piece of lead. Soon afterwards, his attention was excited by a most singular sound which, after some searching, was found to proceed from the soldering-iron. Like the silver of Schwartz, the soldering iron was found VIBRATIONS OF HEATED METALS. 99 in a state of vibration. Mr. Trevelyan made his discovery the subject of a very interesting investigation. He determined the best form to be given to the rocker' as the vibrating mass is now called, and throughout Europe at present, this instrument is known as Trevelyan's Instrument.' Since that time the subject has engaged the attention of Prof. J. D. Forbes, Dr. Seebeck, Mr. Faraday, M. Sondhaus, and myself; but to Trevelyan and Seebeck we owe most. Here is such a rocker made of brass. Its length, A c (fig. 26), is five inches, the width, A B, 15 in., and the length of the handle, which terminates in the knob F, is ten inches. A groove runs at the back of the rocker, along its centre; the cross section of the rocker and its groove is given at M. I heat the rocker to a temperature somewhat higher than that of boiling water, and lay it on this block of lead, allowing its knob to rest upon the table. You hear a quick succession of forcible taps. But you cannot see the oscillations of the rocker to which the taps are due. I therefore place on it this rod of A FIG. 27. brass, A B (fig. 27), with two balls of brass at its end, the oscillations are thereby rendered much slower, and you can easily follow with the eye the pendulous motion of the Its rod and balls. This motion will continue as long as the rocker is able to communicate sufficient heat to the carrier on which it rests. Thus we render the vibrations slow, but I can also render them quick by using a rocker with a wider groove. The sides of this rocker do not overhang so much as those of the last; it is virtually a shorter pendulum, and will vibrate more quickly. Placed upon the lead, as before, it commences an unsteady and not altogether pleasant music. It is still restless, sometimes seeming to expostulate, sometimes even to objurgate, as if it disliked the treatment to which it is subjected. Now it becomes mellow, and fills the room with a clear full note. taps have become periodic and regular, and have linked themselves together to produce music. Here is a third rocker, with a still wider groove, and with it I can obtain a shriller tone. You know of course that the pitch of note augments with the number of the vibrations; this wide-grooved rocker oscillates more quickly, and therefore emits a higher note. By casting a beam of light upon the rocker I obtain a better index than the rod and balls. This index is without weight, and therefore does not retard the motion of the rocker. To the latter I have fastened, by a single screw at its centre, a small disk of polished silver; on which the beam of the electric lamp now falls, and is reflected against the screen. the rocker vibrates, the beam vibrates also, but with twice the angular velocity, and there you see the patch of light quivering upon the screen. When What is the cause of these singular vibrations and tones? They are due simply to the sudden expansion by heat of the body on which the rocker rests. Whenever the hot rocker comes into contact with its lead carrier, a nipple suddenly juts from the latter, being produced by the heat. communicated to the lead at the point of contact. The rocker is tilted up and some other point of it comes into THE TREVELYAN INSTRUMENT. 101 contact with the lead, a fresh nipple is produced, and the rocker is again tilted. Let A B (fig 28) be the surface of the lead, and R the cross section of the hot rocker; tilted to the right, the nipple is formed as at R; tilted to the left, it is formed as at L. The consequence is that until its temperature A L FIG. 28. R B falls sufficiently, the rocker is tossed to and fro, and the quick succession of its taps against the lead produces a musical sound. I have here fixed two pieces of sheet-lead in a vice; their edges are exposed, and are about half an inch asunder. I balance a long bar of heated brass across the two lead edges. It rests first on one edge, which expands at the point of contact and jerks it upwards; it then falls upon the second edge, which also rejects it; and thus it goes on oscillating, and will continue to do so as long as the bar FIG. 29. can communicate sufficient heat to the lead. This fireshovel will answer quite as well as the prepared bar. I balance the heated shovel thus upon the edges of the lead, and it oscillates exactly as the bar did (fig. 29). I may |