Heddle thinks it probable that this fact will aid in the tracing out of the individual beds-far from an easy matter in that troubled district.

The conclusion of the chapter is devoted to a speculation upon the metamorphism of these limestones.

2. On the Strength of the Currents required to Work a Telephone. By Professor Tait.

(Deferred from January 7th.)

Perhaps the most singular fact connected with the telephone is the excessive feebleness of the currents which suffice to work it. I have had no opportunity of testing any but rough arrangements set up by present or former students of my own, so that I cannot judge how far my results may apply to the instrument as sold.

1. A striking illustration of the feebleness of the currents required is furnished by using a Holtz machine driven very slowly, without condenser, and with its terminals so close that the discharge is barely audible, and certainly invisible except in the dark. When insulated wires were led from these terminals to the telephone (placed in a distant room) the effect was very curious. The instrument gave a hissing sound, quite comparable in intensity with that which was produced directly when the terminals of the machine were widely separated, one connected with the ground and the other with a large conductor discharging by brushes into the air, the machine being turned rapidly. The telephone continued to give audible sounds with slow turning, even when the terminals of the machine (somewhat tarnished) were pressed into contact.

2. To measure roughly the intensity of the current, I placed one prong of an unmagnetised tuning-fork about half an inch in front of the sending telephone, and measured by a microscope and scale the extent of its vibrations when the note just ceased to be audible to a listener at the receiving telephone. Next I substituted for the receiving telephone an exceedingly delicate astatic galvanometer, with very small moment of inertia, and measured the swing produced by one definitely assigned motion of the prong of the tuning-fork. By means of a known thermo-electric couple, I determined the strength of the current corresponding to the observed

swing. The result is, of course, only a very rough approximation. It is that a single Grove's cell would produce, in a circuit of somewhere about a billion B.A. units resistance, a current sufficient, if reversed 500 times per second, to produce an audible sound in the telephone I employed.

3. Several attempts at explanation of the action of the telephone have been given here and elsewhere, and others are promised for to-night. For my own part, I think there are at least three separate causes at work in the telephones I have used.

There can be no doubt that the inventor's own explanation is, at least to a certain extent, correct. For we can easily dispense with the magnet in the receiving telephone, using merely a thin iron dise in front of a coil. And Mr Blyth has, I believe, found that we may make the disc, even in this case, of copper, and yet have transmission (though very feeble) of intelligible sounds.

But this cannot be the full explanation. For it does not attempt to account for the peculiar nasality of the transmitted speech. Without going more closely into the matter, the difference of quality between an open and a closed pipe suggests a certain amount of constraint as the cause. And we know that the sounds in the original telephone of Reiss were produced by molecular motions due to magnetism in soft iron. Mr Blyth has shown conclusively that molecular motion in the magnet itself has a large share in the results, because he has successfully substituted other metals than iron, and even non-conductors, for the disc, and in certain cases finds that he can dispense with the disc altogether.

Besides this, however, it seems to me that there is a third cause, which in certain cases is more effective than either of the others. This is suggested by the fact that (at least with the instruments I have tried) high notes, even of comparatively small intensity, are much more clearly transmitted than low notes,-indicating that the rapidity of the molecular change has a great deal to do with the result. In fact, in this respect, the telephone is really a variety of the so-called curb-key, giving very sudden reversals.

These considerations have led me to fancy that rapid change of form in matter, whether paramagnetic or not, may probably be capable of detection by the telephone, for the associated electric currents may be in certain cases powerful enough to produce audible

sounds. I am at present engaged in a series of preliminary experi

ments on this subject.

3. Experiments with the Telephone. By James Blyth. Communicated by Professor Tait.

In the telephones used in these experiments the permanent magnets were of the ordinary horse-shoe form, about 4 inches long. No cores of soft iron were attached to the poles, the insulated wire, No. 26, being wound directly round both, in such a way that a current circulating through it followed the direction of Ampère's currents. The vibrating disc was the bottom of a shallow can of thin tinned iron, 2 inches in diameter, supported directly above the poles of the magnet, and almost touching them. The receiving instrument was so arranged that any kind of disc could be easily substituted for the ordinary vibrating plate, and tested by sound from the same transmitting instrument, so as to allow of a comparison being made between discs of various materials. Having first ascertained that no sound was audible when no vibrating plate was used, I tried discs of the following substances, and have arranged them approximately in the order of distinctness with which the sound was heard :

With the view of testing what effect would be produced by varying the position of the wire coil on the leg of the magnet, I constructed a telephone, so that the coil could be easily slipped up and down. the leg, while the sound was being sent from the transmitting instrument. Very little difference in the sound was observable till the wire coil was brought near the neutral point of the magnet. It

then became very faint. I next slipped the wire coil entirely off the magnet, and simply held it touching the pole side-ways. The sounds were still quite audible, and continued to be so when the coil was removed an inch or more away from the magnet. This led me to try the effect of dispensing altogether with the steel magnet, and merely holding the helix of insulated wire opposite the centre of the vibrating plate. With an iron disc a very faint sound was heard, but it became more distinct when a disc of copper was used. From this it would seem, that the vibrations producing the sound are caused by the attraction between the currents in the helix and the induced currents in the copper disc.

Still using the same transmitting instrument, I next employed a receiver, in which a soft iron core, carrying the insulated wire, was rigidly attached to the vibrating plate. This was accomplished by rivetting the head end of a screw-nail into its centre, and winding the wire round it. The pole of a steel magnet was then fixed, just clear of actual contact, opposite the head of the nail, and, with this arrangement, sounds were distinctly audible, though not so loud as when the wire coil surrounded the magnet.

With the view of rendering the telephone an instrument for detecting the existence of very feeble currents, I constructed a pair with magnets similar to those already described, but with ferrotype discs. These were joined together by a strong semicircular spring in such a way that they could be put on, after the manner of spectacles, and stick close to the sides of the head, inclosing both ears. India-rubber rings were provided for the double purpose of excluding all extraneous sound, and avoiding any disagreeable pressure on the ear.

With this arrangement, I proceeded to test for the existence of thermo-electric currents. A copper and iron junction was inserted in the circuit of the telephone, and attached to a spring in such a way that it could be made to vibrate rapidly out and into a gas flame. A distinct grating noise was heard in the telephone. A very peculiar rasping sound was produced when the ends of a copper and iron wire were rubbed together while hot, and also when one of the wires was attached to a file, and the hot end of the other drawn rapidly along it.

4. On the Theory of the Telephone. By

Prof. George Forbes.

The Telephone, invented by Mr Graham Bell, is an instrument by means of which any sounds, musical notes or spoken sentences, sounded into an instrument at the sending end of a telegraphic wire may be reproduced at the receiving end. The theory of the Telephone is two-fold. First, the mechanical theory of the nature of sounds and of speech; and secondly, the theory of the action of the instrument. The first part is well known. All sounds consist of a succession of waves propagated through the air, the rate and intensity of their succession determining the nature of the sound, in pitch, loudness, and tone. If the same succession of waves as a speaker makes in using his voice can be reproduced in the air in contact with the ear of any other person, by any means whatever, the latter person will hear a fac-simile of the sound uttered by the former. A theory of the action of the Telephone is distinctly given by the inventor in the specification of his patent. The receiving and sending instruments are identical. Round the end of a bar magnet a coil of fine wire is wound, connected with a telegraph wire on the one hand and the earth on the other. In front of these a thin iron plate is caused to vibrate by the sounds uttered in its neighbourhood. Consequently, it approaches and recedes from the pole of the magnet. This alters the magnetism in the neighbourhood. In Faraday's language, the lines of force are altered in position, so as to go across the coil of wire.' In consequence of this, by the well-known laws of electromagnetic action, a current of electricity is sent in one direction with each approach of the vibrating plate, and in the opposite direction with each recession. This succession of currents reaches the receiving end with a strength proportioned to the extent of vibration of the iron plate. Here they circulate through the coil, and so intensify or diminish the magnetism of the bar. Thus, the vibrating plate at the receiving end is more or less attracted, according to the direction of the current. Hence, the plate at the receiving end vibrates to and fro in vibrations which are synchronus and proportionate to those at the sending end; and so the sounds are reproduced.