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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

beyond this, little. For though it may enlarge the luminous circle of our knowledge, it must also widen the dark boundaries of the Unknown. A little light but guides us to deeper mysteries; and even if we should find out all the secrets of molecular movement and structure, all the intersection of the working parts of matter, we are still confronted with the vast enigma, What is Matter? what is the stuff out of which Nature is made? That is a riddle which we can never solve.

Of the still more awful problem, Who made it? Science has also no solution, and can have none. It is neither experiment nor reason which can fully satisfy us here.

Revelation alone affords the true answer. It takes up the wondrous story where science is constrained by its natural limitations to stop, and finds the solution in Him "who is the image of the invisible God, the first-born of all creation, for in Him were all things created, in the heavens and upon the earth." The mystery of creation and the mystery of salvation alike centre in Christ. He it was "who His own self bare our sins in His body on the tree," and He it is who was in the beginning with God. All things were made by Him, and without Him was not any thing made that hath been made."-ED. R. T. S.

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CHAPTER VIII.

THE DYNAMO-ELECTRIC MACHINE.

THE electric light was first discovered by Sir Humphrey Davy in the year 1800, and for the last fifty years or more electricians have tried to make it of practical service to the world. Their efforts, however, were not very successful, because the source of electricity they employed was the voltaic battery. The current from such a source was not steady enough, and, moreover, a very large and cumbrous battery was required. Recent improvements of the magnetoelectric generator of electricity have, however, enabled them to procure a constant current of electricity by means of mechanical power, and led to that development of the electric light which is the topic of the day.

All magneto-electric generators, whatever their peculiar forms may be, are based upon the great discovery of Faraday, that if a closed wire or conducting ring is moved across a magnetic space a current of electricity is generated in the wire. A magnetoelectric generator is simply the best apparatus that can be devised for applying this principle to the production of an electric current. A magnetic space is provided between the poles of two or more powerful magnets,

and ccils of wire are caused to traverse this magnetic space in such a way as to excite a current in them. The stronger the magnetism of the space, the longer the wire, and the quicker it is moved, the stronger will be the current excited. Therefore, the aim of inventors is to construct their machines with powerful magnets and coils of wire having many turns, and to rapidly rotate these coils through the magnetic "field" by mounting them on an axle driven at a rapid rate by means of a pulley and a running belt from some steam-engine or other motor. Now as each coil or bobbin of wire passes between the poles of the magnet, a transient current is generated in it; but as there are a number of bobbins rapidly following each other, each with its transient current, the joint effect of the whole is a practically continuous current. By means of a collecting device termed a "commutator, these parcel currents are gathered up one after another and led away as one steady current.

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In the magneto-electric generator the magnets are of steel permanently magnetised; but a more powerful magnetic field can be obtained by employing electro-magnets, or cores of soft iron encircled with coils of insulated wire. To excite these magnets, however, a current of electricity is required to circulate in the coils surrounding the cores. This current may either be obtained from a supplementary generator, or it may be derived from the coils of the machine itself, for there is always enough "residual" or left magnetism in the core of the electro-magnets to form a weak magnetic field, which will excite feeble currents in the coils as they are driven through it. These feeble currents, diverted into the electro-magnet, help to

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