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opposite the proximal telephone; a fine glass rod or filament, 3 inches in length, was cemented to the centre of the disk of the distal telephone; to the other end of this rod two common microscopical covering glasses, having between them a drop of blood, were also cemented so as to be in a horizontal position. The drop of blood thus fixed to the telephone disk was then brought under a Hartnack's th inch, magnifying about 300 diameters, and the coloured blood corpuscles were brought into accurate focus. A key was placed near the distal telephone, by which the current might be transmitted or interrupted at pleasure. On opening the key, the circular-coloured corpuscles at once assumed an oval form, and were put somewhat out of focus, plainly the result of lateral movement. Thus, by an application of Lissajoux's method of observing vibrations optically, the movements of the disk could be seen. I found that the amplitude of the movements of the vibrating fork (moving 60 vibrations per second), near the proximal telephone, was about th of an inch; the movements of the corpuscles were about half their diameter, or about th of an inch. On stopping the current by means of the key, the movements almost immediately ceased. Again, on taking away the glass rod from the disk, and applying the ear to the disk, the low booming sound of the fork could be heard. It is, therefore, evident that these sounds were produced by movements of the disk, the amplitudes of which were about the th of an inch, an interesting example of the sensitiveness of the ear. The minimum limit of excitation of the ear has been thus stated :-The faintest sound perceptible is that caused by a ball of pith, 1 milligramme in weight, falling 1 millimetre in height upon a glass plate, may be heard at a distance of 91 millimetres from the ear (Schafhäutl).

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The telephone appears to be an instrument which illustrates the extreme sensitiveness of the ear, even better than the method of Schafhäut just alluded to. The length of the hair cells in the cochlea (which are stated to be tolerably stiff and rigid) is about theth of an inch. One of them could therefore perform an excursion of the 10th of an inch. If, as Helmholtz states, the mechanical arrangements of the bones of the ear are such as to diminish to one-third the excursion of the membrana tympani, and if no further reduction took place in the internal

ear itself, the oscillations of the plate I observed, and which were capable of causing a sensation of sound, would only amount, when they reached the hair cells, to about the of an inch, and thus cause a movement of the auditory hair through about th of its possible amplitude of excursion.

2d Method. The movements may also be observed by throwing obliquely a beam of light on a reflecting surface on the disk, and catching the reflected ray on a screen. I did not find this method so satisfactory as the one just described.

3d Method. I cemented the disk of the distal telephone to the thin membrane of one of Koenig's manometric capsules, then there was no difficulty in observing the oscillations of the flame in a rapidly revolving mirror, when a strongly vibrating tuning-fork was placed opposite the proximal telephone. I failed, however, in

seeing any movements produced by speech. The method, however, is capable of more refined application than the means at present at my disposal will allow.

4th Method. I have attempted to record the movements of the disk graphically by attaching to it a lever bent at right angles at one end, bringing the point on a rapidly moving surface. Νο oscillations could be detected. Such oscillations were, however, recorded when, instead of a disk, I used forks at the proximal and distal ends, as already described. The movements of the distal fork were then recorded, but it was observed that the sound of the fork was audible long after all oscillations could be recorded, showing that the movements of the fork were too delicate to be recorded by this method.

5. A Mode of Intensifying the Sound of the Telephone.—In study. ing the transmission of sounds of various kinds, I had occasion to use a rapidly revolving wheel opposite the proximal telephone. The wheel in my possession, which moves with greatest velocity is one about 4 inches in diameter, having placed transversely on its circumference twenty-four rectangular bars of iron. The wheel is driven by two electro-magnets, and the current employed was obtained from twenty of Sir William Thomson's tray cells. With this power it performs about eighty revolutions per second. When the proximal telephone was brought near the circumference of the wheel, the

distal telephone sounded so loudly that a rattling sound like that

room.

caused by the wheel could be heard quite distinctly all over the Evidently the effect of bringing the horizontal iron bars on the circumference of the wheel rapidly in front of the core of the proximal telephone was to intensify the currents of the telephones. The currents were so strong that a disk held in front of the core of the distal telephone could be felt vibrating by the hand.

The following Gentlemen were duly elected Fellows of the Society:

JAMES ALFRED EWING, 22 India Street.

Rev. JOHN WILSON, M. A., Bannockburn Academy.
ROBERT MACFIE THORBURN, Uddevalla, Sweden.
ANDREW PEEBLES AITKEN, Sc. D., 16 Gillespie Crescent.
JOHN MILNE, Mechanician, Trinity Grove, Edinburgh.

Monday, 18th February 1878.

Sir WILLIAM THOMSON, President, in the Chair.

The following communications were read:

1. The application of the Graphic Method to the determination of the efficiency of a direct-acting Steam-Engine. By Professor Fleeming Jenkin.

2. On the Disruptive Discharge of Electricity. By Alexander Macfarlane, M.A., B.Sc. Communicated by Professor Tait.

(Abstract.)

Last summer session, with the assistance of Messrs Salvesen, Connor, and Stewart, I applied the method of measuring great differences of potential, described in a paper by Mr Paton and myself (Proc. Feb. 19, 1877), to investigate the laws of passage of the electric spark. The method essentially consists in connecting the prime conductor of the Holtz machine, not with the electrometer directly, but with an insulated spherical ball. This ball acts

inductively upon another insulated spherical ball which is in connection with the electrometer.

Before proceeding with the investigation proper, I tested the accuracy of the method by applying it to determine how the induced potential of the ball in connection with the electrometer depends on the distance between the centres of the balls. I found that the equation

V=6081 - 42.26,

where V denotes the induced potential, and r the distance, between the centres of the balls, satisfies all the observed values of V for values of r greater than twenty-four centimetres, but for smaller values of r the function requires to be corrected by being multiplied by

ƒ (r) = ·524+·02 r.

Our method, when applied to measure the difference of potential required to pass a spark through air at the atmospheric pressure between parallel metal plates at different distances, gave a result agreeing well with that which Sir William Thomson discovered to be true for small distances. The function for V, the difference of potential in terms of s, the length of the spark is

V=66 9482+205 8}

the equation of an hyperbole, whose semi-transverse axis is 1025 centimetres, and semi-conjugate axis 6.8623 centimetre-grammesecond units. We observed, for lengths of spark, up to 1-2 centi

metre.

From the above equation we infer that—

R-66941+205}

8

where R denotes the electrostatic force; from which it is evident that as s becomes smaller, R becomes greater. But when the discs were heated well, immediately before the taking of the observations, the curve obtained satisfies the equation

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This result, in my opinion, establishes the truth of Clark-Maxwell's hypothesis, that the greater electromotive force required at the smaller distances is due to the existence of condensed gas on the surface of the discs. Precisely similar results were obtained when hydrogen was substituted for air.

The method, when applied to measure the difference of potential required to produce a 5 centimetre spark at different pressures of the air, shows that for the range between the atmospheric pressure and twenty mm.,

V=0458 {p2. +203 p}

where p denotes the pressure in millimetres of mercury.

The electric strengths of several gases were determined by comparing the differences of potential required to pass a 5 centimetre spark through the gas at 746 mm. pressure.

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Several series of observations of the difference of potential required to produce a spark between spherical surfaces for distances up to 15 centimetres confirm the result published in the paper already referred to, that the difference of potential is proportional to the square root of the length of the spark.

3. On the Compressibility of Water, Sea-Water, a four per cent. Chloride of Sodium Solution, Mercury, and Glass. By J. Y. Buchanan, M.A., F.R.S.E.

4. On the Action of Heat on some Salts of Trimethyl-Sulphine. By Professor Crum Brown and J. Adrian Blaikie, Esq.

(Abstract.)

The authors describe the preparation, properties, and action of heat upon the following compounds of trimethyl-sulphine :

4 F

VOL. IX.

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