to the carbon points, the positive ones being collected on the axis of the machine; while a concentric, but insulated cylinder is used for the negative ones.

This machine is large and cumbrous, being 5 feet 3 inches long, 4 feet 4 inches wide, and 5 feet high; it weighs about 2 tons, and cost 4507. Its illuminating power, when driven at a speed of 400 revolutions per minute, from a steam engine of 8 H.P., with an expenditure of somewhat over 3 indicated horse-power, is about that of 2000 standard sperm candles per hour. Two complete apparatus and engine, as just described, are provided at Cape La Hève; one for the northern and one for the southern lighthouse.

The use of the large, old-fashioned, and comparatively weak machines of this Holmes-Alliance stage may be said to have passed away, being only retained in the lighthouses above mentioned. Their only recent application, and that was but of short duration, was to provide the alternate-direction currents necessary for the Jablochkoff candles when first introduced; and until M. Gramme had devised a suitable machine for their proper supply.

The De Meritens Machine.-Quite lately, however, a modified form of machine, much reduced in size and weight, has been designed by M. de Meritens; in order to compete with the small and compact dynamo-electric machines of the present day. It consists (Plate I., Fig. 1) of eight compound horse-shoe magnets, arranged lengthwise around, and resting across two circular frames. Before the ends of these magnets rotates a wheel of light construction, having a series of sixteen flat-shaped coils arranged upon its periphery; the cores of each coil being composed of no less than fifty thin wrought-iron plates. The compound shaped cores of these coils, when placed

T

end to end, though insulated from each other, but with a copper pin between each pair, and arranged round the periphery of the wheel frame, really form themselves into one continuous annular core, like that of the coil in the Gramme machine. Owing, however, to the comparative shortness of each De Meritens coil, and to its being insulated from its neighbours, currents of opposite nature (occasioned by the reversals, when passing severally in front of the different poles) are not, to any extent, set up in the cores of the coils; as is said to be the case in the continuous annular core of the Gramme coil.

There remains, also, with them the great advantage, that the coiling, instead of being effected upon a solid and continuous ring (which is found in practice to entail considerable inconvenience), is done upon the small compound plate-core belonging to that particular segment; which can easily be removed from the general ring for this purpose, as also for any other repairs. No commutator or collector is required with the machine.

The De Meritens machine claims to unite in itself, in its action, not merely the alternately reversing currents of the magneto Alliance machines, but also those induced by the arrangement of the Gramme coil. The result of this combination reduces the necessary motive power to nearly onefifth of what. is required in either the magneto machines of the Alliance Company, and to much less than in most of the dynamo machines; likewise very little heating occurs, though the machine is driven at a very considerable speed.

DYNAMO-ELECTRIC MACHINES.-The very important discovery in 1867 of the principle of reaction between electro-magnets, and the gradual augmentation in the intensity of the electric current by these repeated passes,

soon led to abandonment of the large, costly, permanent steel magnets which had been used as a storehouse whence to draw the electric current; their place being taken by small, compact, and comparatively inexpensive electro-magnets. These, though very weak in their initial action, were soon by reacting on one another worked up to a much higher electrical efficiency than would have been produced from permanent magnets of a much larger size. The natural result was an electrical machine much smaller, more compact, and more economical.

Though more powerful, bulk for bulk, these "dynamo machines are subject to certain irregularities in the output of the electric current; and also, at times, to considerable heating of the coils, due probably to the very great internal resistances developed in them, and which render this class of machines liable to interruptions in working. "Magneto" machines are considered by some to work with more regularity and steadiness in the current generated, and with much less chance of a cessation in their working; a quality which may, in some cases, more than compensate for their extra size and costliness. Against them, however, it may be urged, that the permanent magnets gradually deteriorate and lose a portion of their magnetic power. Where the machine is used regularly, night after night, as in lighthouses, this does not amount to much; but when the illumination is liable to suspension for a lengthy period, as during the summer months, the loss during this period may become a serious inconvenience. This loss, however, is stoutly denied by some; as the deterioration, they say, can be rectified by half an hour's working of the machine.

Of electric-light machines of "dynamo" construction, the following are among some of the best known and the most extensively used, in this country at least.

The Siemens Machine (Plate I., Fig. 3).—It consists of an induction coil, with the convolutions of the copper wire wound lengthwise with the cylinder, in the form known as the modified Siemens armature. The coil is made to revolve by mechanical means between curved iron bars, which are the prolongation of the cores of large flat electro-magnets placed on either side of the induction coil; the north pole of the system being midway between the two upper electro-magnets and directly over the axis of the coil, and the south pole in a similar position below the axis upon the bar between the lower magnets.

The portion of the coil which during its revolution is travelling downwards, has (with the above arrangement) positive currents induced in it; while the ascending half of the coil is subjected to negative currents, but both in the same direction as regards circuit. The arrange

ment of the poles may, however, be exactly the reverse of the above.

The machine represented in the diagram is the type "B," of 6000 standard candles nominal power.

The light at the Lizard Lighthouse is produced by a Siemens machine and lamp. Many other applications of this system for industrial and military purposes exist in this country and on the Continent.

The Gramme Machine.-This machine in its earliest development was composed of round electro-magnet bars placed vertically, with double induction coils, one of which served to react upon the magnets; as previously referred to in Chapter I. Its very existence is nearly forgotten; and it is referred to here merely to mention that this is the form of Gramme which in the Trinity House experiments was pitted against the Holmes, the Alliance, and the Siemens just described.

The type of Gramme here about to be described is the second form, with a single induction coil, and supplying a continuous-direction current to a single light; a machine very similar in construction and principle to the Siemens one just described. It is the machine referred to by Mr. Douglass, in the concluding paragraphs of his Report upon the Trinity House experiments at the South Foreland, as having been the object of his special visit to Paris.

It is composed (Plate II., A, Fig. 2) of an induction coil having for its core a ring or cylinder of soft iron, which is made to revolve mechanically between two round bars, each forming the core of a pair of electro-magnets. These bars are placed one directly over, the other beneath the axis of the coil and lengthwise with it. Each bar extends from one cast end-frame piece to the other side of the machine; while its middle is clipped by a sort of ring of cast iron, which serves to divide the coils wound upon the half bar on either side of it, and thus forming them into two electro-magnets. These are so arranged, that the pair on each bar have like poles next to each other at the centre next to the cast ring, which serves as a double pole to convey the combined electricity to the revolving induction coil, by means of its jaw-like projections which are splayed