IV. "On Electrotorsion." By GEORGE GORE, F.R.S. Received November 26, 1873. (Abstract.)

This communication contains an account of a new phenomenon (of rods and wires of iron becoming twisted while under the influence of electric currents), and a full description of the conditions under which it occurs, the necessary apparatus, and the methods of using it.

The phenomenon of torsion thus produced is not a microscopic one, but may be made to exceed in some cases a twist of a quarter of a circle, the end of a suitable index moving through a space of 80 centimetres (=31 inches). It is always attended by emission of sound.

The torsions are produced by the combined influence of helical and axial electric currents, one current passing through a long copper-wire coil surrounding the bar or wire, and the other, in an axial direction, through the iron itself. The cause of them is the combined influence of magnetism in the ordinary longitudinal direction induced in the bar by the coil-current, and transverse magnetism induced in it by the axial

one.

The torsions are remarkably symmetrical, and are as definitely related in direction to electric currents as magnetism itself. The chief law of them is-A current flowing from a north to a south pole produces lefthanded torsion, and a reverse one right-handed torsion (i. e. in the direction of an ordinary screw). Although each current alone will produce its own magnetic effect, sound, and internal molecular movement, neither alone will twist the bar, unless the bar has been previously magnetized by the other. Successive coil-currents alone in opposite directions will not produce torsion, neither will successive and opposite axial ones.

The torsions are influenced by previous mechanical twist in the iron, by mechanical tension, and by terrestrial magnetic induction. The direction of them depends both upon that of the axial and of the coil-currents, but appears to be determined most by the former. A few cases occur in which the currents, instead of developing torsion, produce detorsion; but only two instances, out of many hundreds, have been met with in which torsion was produced in a direction opposite to that required by the law.

Single torsions vary in magnitude from 0.5 millim. to nearly 30 millims. of movement of the end of an index 47 centimetres long; the smaller ones occur when the two currents are transmitted alternately, and the large ones when they are passed simultaneously; the former generally leave the bar in a twisted state, the latter do not. Those produced by axial currents succeeding coil ones are nearly always much larger than those yielded by coil-currents succeeding axial ones, because the residual magnetism left by the coil-current is the strongest. The order of succession of the currents affects the torsions in all cases, altering their magnitudes, and in some few instances even their directions. In steel

all the torsional effects are modified by the mechanical and magnetic properties of that substance.

Each current leaves a residuary magnetic effect in the bar, amounting in iron to about one tenth of its original influence. The residuary magnetism of coil-currents is affected and sometimes reversed by axial ones; and that of axial currents is also removed by coil ones, and by a red heat. The condition left by an axial current is smaller in degree and less stable, in a vertical iron wire or one in the terrestrial magnetic meridian, than that left by a coil one, partly because of the influence of terrestrial magnetism; but in a position at right angles to that the effect is different.

The torsion produced by a coil-current may be used as a test, and partly as a measure, of the residuary effect of an axial one; and that produced by an axial current may be employed to detect, and to some extent measure, ordinary magnetism in the bar. As an opposite coilcurrent at once reverses the ordinary longitudinal magnetism of a bar of iron, so also an opposite axial one at once reverses its transverse magnetism.

Many instances have been met with in which the transverse and longitudinal magnetic states produced by the two currents coexisted in the same substance. The torsional influence of the excited helix is distributed equally throughout its length; so also is that of the current in the bar. All the torsions are closely related to the well-known electric sounds, and to particular positions and internal movements of the particles of the iron.

Signs of electrotorsion were obtained with a bar of nickel, but not with wires of platinum, silver, copper, lead, tin, cadmium, zinc, magnesium, aluminium, brass, or German-silver, nor with a thick rod of zine, or a cord of gutta percha.

January 15, 1874.

JOSEPH DALTON HOOKER, C.B., President, in the Chair.

The following Paper was read :

I. "Preliminary Account of an Investigation on the Transmission of Sound by the Atmosphere." By JOHN TYNDALL, D.C.L., LL.D., F.R.S. Received January 1st, 1874.

This notice embraces the scientific results of an inquiry on Fogsignals, undertaken at the instance of the Elder Brethren of the Trinity House, and communicated with their friendly concurrence to the Royal Society.

The investigation was begun on the 19th of May, 1873, and continued till the 4th of July. It was resumed on the 8th of October, and con

tinued to the end of November. It also includes observations made during the dense fog which enveloped London on December 9 and the succeeding days.

Gongs and bells were excluded from this investigation, in consequence of their proved inferiority to other instruments of signalling. The experiments were made with trumpets blown by powerfully compressed air, with steam-whistles, guns, and a steam syren, associated with a trumpet 16 feet long.

Daboll's horn, or trumpet, had been highly spoken of by writers on fog-signals. A third-order apparatus of the kind had been reported as sending its sound to a distance of from 7 to 9 miles against the wind, and to a distance of 12 to 14 miles with the wind. Holmes had improved upon Daboll; and with two instruments of Holmes, not of the third but of the first order, our experiments were made. On the 19th of May at 3 miles distance they became useless as a fog-signal; at a distance of 4 miles, with paddles stopped and all on board quiet, they were wholly unheard. At a distance of 2 miles from the Foreland the whistles tested on May 19 became useless. The 12 o'clock gun, fired with a 1 lb. charge at Drop Fort in Dover, was well heard on May 19, when the horns and whistles were inaudible. On the 20th of May the permeability of the atmosphere had somewhat increased, but the steam-whistle failed to pierce it to a depth of 3 miles. At 4 miles the horns, though aided by quietness on board, were barely heard. By careful nursing, if I may use the expression, the horn-sounds were carried to a distance of 6 miles. The superiority of the 18-pounder gun, already employed by the Trinity House as a fog-signal, over horns and whistles was on this day so decided as almost to warrant its recommendation to the exclusion of all the other signals.

Nothing occurred on the 2nd of June to exalt our hopes of the trumpets and whistles. The horns were scarcely heard at a distance of 3 miles; sometimes indeed they failed to be heard at 2 miles. By keeping every thing quiet on board, they were afterwards heard to a distance of 6 miles a result, however, mainly due to the improved condition of the atmosphere. Considering the demands as to sound-range made by writers on this subject, the demonstrated incompetence of horns and whistles of great reputed power to meet these demands was not encouraging. On the 3rd of June the atmosphere had changed surprisingly. It was loaded overhead with dark and threatening clouds; the sounds, nevertheless, were well heard beyond 9 miles. On June 10th the acoustic transparency of the air was also very fair, the distance penetrated being upwards of 83 miles. The subsidence of the sound near the boundary of the acoustic shadow on the Dover side of the Foreland, and before entering the shadow, was to-day sudden and extraordinary, affecting equally both horns and guns. We were warned on June 3 that the supremacy of the gun on one occasion by no means implied its supremacy on all occasions,

the self-same guns which on May 20th had so far transcended the horns being on this day their equals and nothing more. The 11th of June was employed in mastering still further the facts relating to the subsidence of the sound east and west of the Foreland; the cause of this subsidence being in part due to the weakening of the sonorous waves by their divergence into the sound shadow, and in part to interference.

The atmosphere on the 25th of June was again very defective acoustically. The sounds reached a maximum distance of 6 miles; but at 4 miles, on returning from the maximum distance, they were very faint. The day had, as it were, darkened acoustically. On this day the guns lost still further their preeminence, and at 54 miles were inferior to the horn. No sounds whatever reached Dover Pier on the 11th; and it was only towards the close of the day that they succeeded in reaching it on the 25th. Thus by slow degrees the caprices of the atmosphere made themselves known to us, showing us subsequently that within the limits of a single day, even within a single minute, the air, as a vehicle of sound, underwent most serious variations. The 26th of June was a far better day than its predecessor, the acoustic range being over 94 miles. The direction of the wind was less favourable to the sound on this day than on the preceding one, plainly proving that something else than the wind must play an important part in shortening the sound-range.

On the 1st of July we experimented upon a rotating horn, and heard its direct or axial blast, which proved to be the strongest, at a distance of 10 miles. The sounds were also heard at the Varne light-ship, which is 123 miles from the Foreland. The atmosphere had become decidedly clearer acoustically, but not so optically; for on this day thick haze obscured the white cliffs of the Foreland. In fact, on days of far greater optical purity, the sound had failed to reach one third of the distance attained to-day. In the light of such a fact, any attempt to make optical transparency a measure of acoustic transparency, must be seen to be delusive. On the 1st of July a 12-inch American whistle, of which we had heard a highly favourable account, was tried in place of the 12-inch English whistle; but, like its predecessor, the performance of the new instrument fell behind that of the horns. An interval of 12 hours sufficed to convert the acoustically clear atmosphere of July 1 into an opaque one; for on the 2nd of July even the horn-sounds, with paddles stopped and all noiseless on board, could not penetrate further than 4 miles.

Thus each succeeding day provided us with a virtually new atmosphere, clearly showing that conclusions founded upon one day's observations might utterly break down in the presence of the phenomena of another day. This was most impressively demonstrated on the day now to be referred to. The acoustic imperviousness of the 3rd of July was found to be still greater than that of the 2nd, while the optical purity of the day was sensibly perfect. The cliffs of the Foreland could be seen to-day at ten times the distance at which they ceased to be

visible on the 1st, while the sounds were cut off at one sixth of the distance. At 2 P.M. neither guns nor trumpets were able to pierce the transparent air to a depth of 3, hardly to a depth of 2 miles. This extraordinary opacity was proved conclusively to arise from the irregular admixture with the air of the aqueous vapour raised by a powerful sun. This vapour, though perfectly invisible, produced an acoustic cloud impervious to the sound, and from which the sound-waves were thrown back as the waves of light are from an ordinary cloud. The waves thus refused transmission produced by their reflection echoes of extraordinary strength and duration. This I may remark is the first time that audible echoes have been proved to be reflected from an optically transparent atmosphere. By the lowering of the sun the production of the vapour was checked, and the transmissive power of the atmosphere restored to such an extent that, at a distance of 2 miles from the Foreland, at 7 P.M., the intensity of the sound was at least thirty-six times its intensity at 2 P.M.

On October 8 the observations were resumed, a steam syren and a Canadian whistle of great power being added to the list of instruments. A boiler had its steam raised to a pressure of 70 lbs. to the square inch; on opening a valve this steam would issue forcibly in a continuous stream, and the sole function of the syren was to convert this stream into a series of separate strong puffs. This was done by causing a disk with 12 radial slits to rotate behind a fixed disk with the same number of slits. When the slits coincided a puff escaped; when they did not coincide the outflow of steam was interrupted. Each puff of steam at this high pressure generated a sonorous wave of great intensity; the successive waves linking themselves together to a musical sound so intense as to be best described as a continuous explosion.

During the earlier part of October 8 the optical transparency of the air was very great; its acoustic transparency, on the other hand, was very defective. Clouds blackened and broke into a rain- and hail-shower of tropical violence. The sounds, instead of being deadened were improved by this furious squall; and, after it had lightened, thus lessening the local noises, the sounds were heard at a distance of 7 miles distinctly louder than they had been heard through the preceding rainless atmosphere at a distance of 5 miles. At 5 miles distance, therefore, the intensity of the sound had been at least doubled by the rain-a result entirely opposed to all previous assertions, but an obvious consequence of the removal by condensation and precipitation of that vapour the mixture of which with the air had been proved so prejudicial to the sound. On this day a dependence was established between the pitch of a note and its penetrative power-the syren generating 480 waves being slightly inferior to the horns, while generating 400 waves a second it was distinctly superior. The maximum range on October 8 was 9 miles. On October 9 the transmissive power had diminished, the maximum range being 7 miles. On