the G gently enough to give the gradual transition from, let us say for example, four uniform beats per second, through the case of four beats per second with every alternate beat somewhat louder, to the case of only every second beat perceptible, or, in all, two beats per second; but it can be done, and the result is an interesting and instructive illustration of the slowing down from the quick beat of the binary harmony to half speed, or to one-third speed, or to onefifth speed, as the case may be, by the introduction of a third note. In the several cases I have foundthat I can, by making the added note faint enough, produce a succession of beats of which every second, or every third, or every fifth, as the case may be, is louder than the others, and that, as the intensity of the added note is gradually increased, the fainter beats become imperceptible, and a regular unbroken slow beat is heard distinctly alone, always in the theoretical time of the whole imperfection of the harmony. I have verified this distinctly in the cases of 1, 2, 3; 2, 3, 4; 3, 4, 5; 4, 5, 6 (as stated above); 5, 6, 7; and 6, 7, 8. I have not succeeded in hearing the beats on the approximations to the harmonies 8:9 and 9: 10. But the slow beat on the 8, 9, 10 (with vibrational frequencies 256, 288, 320), with any one of the three notes slightly flattened, is very remarkable. The sound is like that of a wheel going round with decided roughness of motion in every part of its revolution, but much rougher in one part than another, with a loudly perceptible periodic return of the roughness in the theoretical period of the approximate harmony. The beats on the harmony CEG (vibrational frequencies 256, 320, 384), with any one of the three notes slightly flattened, are very perceptible: untrained ears hear them instantly the first time without any education, and the beat is heard almost to the very end of the sound if three of Koenig's forks, one of them, the C, for example, being slightly flattened by a brass sliding piece screwed to it, be caused to sound. The sound dies beating, the beats being distinctly heard all through a large room as long as the faintest breath of the sound is perceptible. The smooth melodious periodic moaning of the beat is particularly beautiful when the beat is slow (at the rate, for instance, of one beat in two seconds or thereabouts), being, in fact, sometimes the very last sound heard when the intensities of the three notes chance at the end to be suitably proportioned. Monday, 15th April 1878. SIR WILLIAM THOMSON, President, in the Chair. The following Communications were read: 1. On Vortex Vibrations, and on Instability of Vortex Motions. By Sir William Thomson. 2. On the Theory of Vowel Sounds. By Professor 3. Preliminary Note on a Method of Detecting Fire-Damp in Coal Mines. By Professor George Forbes. The author exhibited two instruments, both founded upon the same principles, for measuring the quantity of fire-damp present in a coal mine. The first instrument consists of a tuning-fork fixed above the open end of a resonating tube, whose other end is closed by a piston whose position (read off on a scale) regulates the length of the resonating tube. The length of the tube, which resounds to the definite pitch of the tuning fork, depends upon the nature of the gas with which it is filled. The more fire-damp, the longer is the tube. Barometric pressure has no effect upon this instrument. The correction for temperature is made by reading off, not a fixed mark upon the piston, but the top of the mercury of a thermo VOL. IX. 4 M meter attached thereto, of dimensions determined by actual experiment. The only source of error to which the instrument seems liable is the counteracting influence of dense carbonic acid gas in choke-damp. But it is found that the presence of choke-damp destroys the explosive character of fire-damp; and, so far as experiments go, it seems certain that, in all cases when the presence of choke-damp prevents the instrument from indicating the presence of fire-damp, the fire-damp is denuded of its explosive character. The second instrument is a combination of a harmonium reed and an organ pipe, through which the air is driven. They are arranged so as to sound the same note when pure air is used, so that when there is a lighter gas present the organ pipe sounds a higher note, thus producing beats. So far as the experiments have gone hitherto, the first form is by far the most accurate, being capable of detecting the presence of 1 or 2 per cent. of fire-damp. 4. Note on Electrolytic Conduction. By Professor Tait. It is commonly said that there is a resistance to a current at the surface of contact of a solid conductor and an electrolyte. Some good authorities, however, say that we have as yet no proof of this, as the effects observed may be due to polarisation. It is obvious that, if the reverse electromotive force due to polarisation contain a term directly proportional to the strength of the current, the ordinary methods of measurement would not enable us to distinguish this from the surface resistance above mentioned. For, in the expression if the numerator contain a term of the form el, it may be expunged, provided e be added to the denominator. ments. To clear up this point I have recently made a number of experiThese have led me to some curious results bearing on the theory of electrolysis, which I propose to bring before the Society on a future occasion. At present I refer to them merely so far as to say that they establish fully the existence of the surface resistance above mentioned. Thus I was led to see that if a slip of platinum be inserted between the electrodes of a decomposing cell it ought, except in extreme cases, to produce almost precisely the same result as a similar and equal slip of glass or mica. This was easily verified. Here we have the singular result of a marked diminution of the current by the insertion into the electrolyte of a substance which is in itself a much superior conductor. Even when the platinum completely closes the path from one electrode to the other, so as to form two decomposing cells instead of one, a comparatively small hole made in it at once changes its function from that of common electrode to each of two decomposing cells into that of a mere obstruction in one cell. It is an interesting experimental inquiry to trace the intermediate stages between these two states, as a pinhole in the platinum is gradually enlarged. Whatever, then, be the behaviour of the particles of an electrolyte, they do not behave like little pieces of platinum. 5. Note on Thermal Conduction. By Professor Tait. Monday, 6th May 1878. SIR C. WYVILLE THOMSON, Vice-President, in the Chair. The following Communications were read :- 1. On the Indications of Molecular Action in the Telephone. By R. M. Ferguson, Ph.D. The accepted theory of the telephone represents that the vibrations of the sending plate to and from the pole of the magnet before which it is fixed is the origin of the currents generated in the pole bobbin of wire, and that these currents transmitted to the receiving telephone produce corresponding to-and-fro excursions of its plate. This theory, which is that of the inventor, may be shortly designated, in the happy words of Sir William Thomson for a kindred action, the push-and-pull theory. We have had in this session of the Society two communications of a practical nature, which seem directly confirmatory of this view. I refer to the lucid exposition of Gott's telephone experiment in the island of St Pierre, and the beautiful and successful demonstration of the action of the phonograph, both by Professor Fleeming Jenkin. In the former of these, as we learned, one end of a thread was attached to the one side of the light suspended coil of a Thomson ink recorder, and the other to the paper disc of an ordinary mechanical telephone. This was done at the two communicating stations. When the sending disc was agitated by the voice, the coil to which it was attached twisted round in the powerful and uniform magnetic field in which it was placed, and dispatched corresponding electric current waves to the receiving instrument, the coil of which was thereby moved similarly in its field, and transferred its motion to its paper disc. A more beautiful manipulation of an exquisitely designed and executed apparatus it is not easy to conceive. In the phonograph we have as it were a mechanical telephone, with the string connecting the discs cut, and nothing left of it but the two ends stiffened into pricking pins. Instead of the sending disc dealing directly with the receiving one, its energy is employed in imprinting, by means of the pricker, its vibrations on the tinfoil, and this imprint, when again vivified by the energy of the rotating drum, reproduces the vibrations which originally stamped it. After two such demonstrations, it may be held as proved that the electric telephone is equivalent to a mechanical telephone with an electro-magnetic intervening action instead of a mechanical one. It seems therefore a hopeless task to seek for indications of molecular action where mechanical action declares itself so manifestly. The mechanical action of the voice and of the membrane of the tympanum of the ear is above question, and that mechanical vibrations are dealt to the sending instrument, and emitted by the receiving one, is equally undoubted; but the intervening electric agency, how generated in the one and how transformed in the other, is a fair field for discussion. The action is novel, and it is surely a likely inquiry to investigate whether its explanation by the first principle that comes to hand, viz., the push-and-pull of the discs, fully covers the case, The question may be raised, for instance, whether the mere impact of the waves of air on the iron disc may not affect its magnetic condition by internal change or vibration,* so as to excite currents without vibrations of the push-and-pull kind, or whether in Something like this was suggested by Professor Forbes. |