and constitute the "secondary" circuit of the balance. The interruptor I may be either manipulated by hand or automatically, so as to give a continuous action. Whenever the primary circuit is closed by its means, a current traverses the primary coils P pl and induces a corresponding current in the secondary coils s s1. This current is of course audible in the telephone T; but by reversing one of the secondary coils, say s1, the current induced by the primary coil p1 in the coil s1 is made to oppose the current induced by the other primary coil p in the other secondary coils, so that it is possible to cause these two induced currents to annul one another and produce silence in the telephone. In other words it is possible to effect a practically perfect balance between the two induced currents.

This is done by making the two primary coils and also the two secondary coils alike in all respects, and placing the secondary s at the same distance from P, that s is from P1. The final adjustment to produce silence in the telephone can be made by altering the distance between a secondary coil and its primary, say, the distance of s from P, or it can be made by means of a small piece of metal adjusted near one pair of coils, as was originally shown by Professor Hughes. To employ this arrangement for detecting unseen metal masses it is only necessary to obtain a sufficiently good balance in this way, and explore the field where the metal is supposed to lie by moving about the pair of coils s1 p1. Then, if these coils come near a piece of metal the inductive disturbance which its presence creates will upset the existing balance, and the telephone, before silent or nearly so, will give out dis

tinctly audible sounds, owing to the predominance of the induced currents in the secondary s1 over those in the secondary s. The nature of the metal or its size and shape cannot of course be told by this plan, unless trial or experience should suggest some clue to a rough estimate, and thus a fragment of lead might be mistaken for gold, or a rusty anchor for a torpedo. But it will be admitted that it is an important gain to be able to detect the presence of metals at all by such means, and it is easy to see how the device may prove invaluable not only to the military and naval officer, but to the surgeon and the mining engineer. probing coils s1 p', for example, let down into a borehole, will indicate the presence of metalliferous strata ; they may reveal the nuggets in a surface digging, and they may roughly test the richness of a piece of ore brought near them.

The

When this arrangement is used for bullet-finding, the coils s1 p1 are moved over the body of the patient, and when brought nearest to the hidden metal the sound in the telephone is loudest. By exploring the region where the bullet resides, Professor Bell has becn able, with a specially adjusted balance, to locate bullets which have been lodged in the body for many years. In the case of President Garfield the spot indicated by the balance was found afterwards not to be the true position of the missile: but this is explained by the fact that a metal spring under the President's bed deceived the instrument. Professor Bell has now devised a form of balance expressly for this purpose.

The happy notion of applying the balance to the detection of submerged torpedoes has been skilfully worked out by Captain McEvoy, who by dint of patient

thought, experiment, and labour, has succeeded in reducing it to a thoroughly practical form. This actual apparatus consists of a portable case containing the adjustable coils P S, and the interruptor 1; B, a voltaic battery of two cells, which may be replaced by a small magneto-electric machine giving alternating currents; T the telephone in the secondary circuit is an insulated cable conveying the wires connecting up the two pairs of coils; and a detecting or exploring case, containing the two secondary coils s' p1. The coils P s inside the box are separated by a layer of soft india-rubber, and an ivory screw passes through both coils and the rubber, washer between. An ebonite head to the crew is adjusted by hand so as to press the coils together or let them further apart by regulating the pressure between them and the india-rubber. This simple and ingenious device adjusts the balance of induction and reduces the telephone to silence, or as Captain McEvoy prefers it, to all but silence. When the ear has a slight sound to guide it the notable increase of loudness in that sound produced by the approach of the detecting case to a metal body is perhaps more readily observed. Absolute silence is, therefore, not desired by Captain McEvoy in the use of his detector. The interruptor is another special device which is worthy of remark. It consists of a small iron reed or tongue kept in vibration by a small double-poled electro-magnet, and thereby interrupting the current a certain number of times per second, so as to give out a definite note which is easily recognisable in the telephone and cannot be mistaken. A switch at the end of the box turns the current from the battery on and off the interruptor at a moment's

notice. The battery consists of two Leclanché elements in a portable case; but it is probable that chloride of silver cells, being much smaller in size, will be found more suitable for field operations. A substitute for the battery and interruptor is also provided in the shape of a small magnéto-electric machine such as are used in the medical applications of electricity. This little instrument gives rapidly alternating currents which, when sent through the primary circuit, do not operate the vibrating interruptor, but produce a slightly musical tone of low pitch which is easily caught and followed by the ear. It forms a useful auxiliary to the battery, and indeed may be employed by some in preference to it. The telephone is the ordinary speaking receiver of Bell, and it, as well as the magneto-machine, are packed inside the box. A sufficiently small battery could also be enclosed therein for transport.

The cable is insulated with Henly's patent core, consisting of india-rubber having its pores filled up with ozokerit or black earth wax forced in under pressure and when in a hot fluid state. It is further protected with an outer braided sheathing, and is fitted to the box by an ingenious socket which in an instant establishes connexion between the corresponding primaries and secondaries, and locks them together. No confusion or imperfection in the contacts is possible through haste or nervousness on the part of those using the apparatus. The detecting case is made of wood soaked with paraffin wax, and its peculiar powder-flask shape, as well as its material, were only arrived at after many trials. It is water-tight, and contains the two exploring coils s P. When it is

lowered into the water by the cable and moved about, or dragged over the bottom, the instant it comes against a piece of metal such as a torpedo case, a chain, or a submarine cable, it disturbs the balance, and the note heard in the telephone very faintly until now becomes unmistakably loud and clear. It is indeed somewhat surprising to find the effect so loud.

As an instrument of search into the physical structure of bodies and the constitution of Nature, the induction balance promises to be very useful. It is an impalpable probe by which we can as it were sound the molecular depths, and picture the invisible changes which are for ever going on there. The subtle inductive influence penetrates through and through a mass without breaking it up or destroying it; but the right clue to its effects can only be found by patient reasoning and endless trial. Recent investigation by its means has led Professor Hughes to the conclusion that a magnet is made up of innumerable molecules, each molecule a separate magnet with its north and south pole. In the magnet all the molecules are aligned, so that their north poles face one way, and their south poles the other. When, however, the magnet is demagnetised by heating or vibration, the molecules are no longer ranged in lines, but "fall out" and pair off. This is already some glimmering into the mystery of magnetism, and it may lead to further insight.

But when we know what magnetism is, what electricity is, what heat, chemical union, gravity, and other forces are, how much after all shall we have learned? A great deal that may be useful to us in bettering the material conditions of our life. But