THE October Bolletina of the Italian Geographical Society contains a short account of the Progress of the Italian African Expedition, and letters from Lieut. Bove who accompanies Prof. Nordenskjöld in his North-East Passage Expedition. In the Bulletin of the Paris Society is a translation of the Grand Duke Nicholas's paper on the Shortest Route for a Railway to Central Asia; a paper by M. L. Simonin on the Indians of the United States, with a map showing the Indian reservations; the continuation of Dr. Decugis' Account of his Journey in Morocco; a long article by the Abbé Ménager on Guinea, besides shorter papers on a Uniprojectional Atlas, and the Rio Casca of Peru.

THE Portuguese African expedition, under Serpa Pinto, which left Benguela a year ago, reached Bihé in March last, and was to enter the unknown interior in two divisions. The Lisbon Geographical Society are moving Government for a scientific expedition into Portuguese Senegambia.

ON SOME Improved METHODS OF PRODUCING AND REGULATING THE ELECTRIC

LIGHT

A MONG the manifold functions which the elementary the least important is that by which it becomes the great source of artificial illumination, whether derived from oils, coal gas, or from coke rendered incandescent by the action of powerful electric currents. Since the time when Davy first produced the voltaic arc, between two points of wood charcoal, through which was transmitted the current from the great battery of 2,000 plates belonging to the Royal Institution, many experiments have been made to determine the best kinds of carbon for developing the electric light. The carbon which, until recently, was most commonly employed for this purpose, is obtained from the sides of gas retorts, where it accumulates in the form of coke during the destructive distillation of coal. The shells of coke from the retort are sawn up into pencils from one quarter to half an inch square, and from six to nine inches in length. Although very good results are obtained from carbon of this kind, it is a difficult material to work on account of its hardness, and it sometimes contains impurities which interfere with its conductivity. It is also liable to fracture when suddenly heated by the transmission of powerful electric currents. These defects have led to the introduction in electric lighting of artificial carbon, composed of powdered coke and lampblack, formed into a paste with molasses and

substance carbon performs in organic nature, not

be regular in its action, but the distance between the carbon points must not alter, which necessitates the use of some arrangement for bringing them nearer together in proportion as they are consumed. Much ingenuity has been displayed by electricians in solving this problem, and the automatic contrivances invented by Staite, Duboscq, Foucault, Serrin, and others, leave little to be desired in regard to the steadiness of the light, when the regulators are in good order, and in the hands of intelligent operators. All automatic instruments, however, from the delicacy of their mechanism, are liable to derangement, and their action is not easily understood by persons not having a special knowledge of their construction. To obviate the objection to the use of such instruments by unskilled attendants, I devised, a few years since, a regulator for use on H.M.'s ships of war, to be actuated by hand. In this arrangement the carbons are made to approach and separate from each other by means of a right and left-handed screw connected with the carbon holders. Each of the screws, with its carbon holder, can be actuated independently of the other, for the purpose of adjusting the points of the carbons to the proper focus of the optical apparatus used in connection with it. The regulator, with its carbon points, is placed in the focus of a dioptric lens, which parallelises the divergent rays of light into a single beam of great intensity. The lens with the regulator is pivoted horizontally and vertically on the top of a short iron column, fixed on a raised platform above the deck, and the beam of light may be projected upon any distant object within its range. This special application of the electric light, however, as will be seen, requires the frequent adjustment of the carbons by the operator, but as he is always required to be in attendance to manipulate the projector, no inconvenience is experienced through the absence of the automatic arrangement. This method of regulating the electric light has now been in use in the Royal Navy for more than three years, and has proved very satisfactory.

Simultaneously with the progress of improvements in the mechanism for regulating the electric light, experiments have been made with the object of dispensing with the regulator altogether. The most recent, as well as the most successful, of these attempts has been made by M. Jablochkoff, a Russian inventor. In the specification of his letters patent of 1877 he proposes to place the carbons side by side (as had been previously proposed by Werdermann in 1874), and to separate them by an insulating substance to be consumed along with the carbon. The inventor states that the insulating substance for separating the carbons may be kaolin, glass of various kinds, alkaline earths, and silicates, which he prefers to apply in the form of powder rammed into an asbestos cartridge-case containing the carbons. A powder which the inventor found serviceable consists of one part lime, four parts sand, and two parts talc. These materials are rammed into the cartridge-case surrounding and separating two parallel sticks of carbon placed in the case, at a little distance apart. One of the carbons is made thicker than the other to allow for its more rapid waste. The lower ends of the carbons are inserted into pieces of copper tube or other good conductor, separated from one another by asbestos, and the ends of the tubes are pinched between two limbs of a screw vice, connected respectively to the conducting wires. This combina

diameter, and half a metre in length.

When the electric light is to be used for illumination, it is necessary that it should be as continuous as other modes of lighting. For this purpose not only should the current

an electric candle, which, when mounted on a stand or candle-stick, has the appearance of the Roman candle of pyrotechnists. The inventor further states that the heat produced by the electricity fuses the material between the

afforded by the fused material to the electric current permits the subdivision of the light by placing several lamps in the course of one electric circuit. It is also stated that the construction of the candle may be varied; and, among the forms described, is one in which the carbons, instead of being contained in a cartridge case, are separated by a partition of kaolin or other similar insulating material.

I have thought it well to describe, as nearly as possible in the words of the inventor, the electric candle, which is now the subject of so much attention in its application to electric lighting; so that its relation to what follows may be more clearly perceived. A remarkable peculiarity of the direct current in electric lighting is that of its consuming the positive carbon at twice the rate of the negative one, and while the negative carbon is a pointed cone, like that of a pencil, the positive pole takes the form of a hollow cavity or crater.

M. Jablochkoff's early experiments seem to have been made with the direct current, and hence his carbons are described as being of unequal thickness, in order that the positive and negative carbons of the candle might be evenly consumed. When the alternating current is used for producing electric light both carbons are of the same hickness, and are consumed at an equal rate, and both points terminate in regular cones. This property of the alternating current, besides other advantages, always maintains the luminous point in the focus of any optical apparatus used in connection with it, that is, when the carbons are placed end to end, as I had occasion to point out in a former paper read before the Society in 1873, on an electro-magnetic induction machine for producing alternating currents.

M. Jablochkoff, in the course of his experiments, would appear to have met with some difficulties in adapting the direct or continuous current to a system of lighting with his electric candles, and now uses the alternating current for this purpose. The candle has also been simplified by substituting a slip of plaster of Paris for the cartridge and partition of kaolin formerly employed.

To produce the alternating currents, however, to supply a number of lights, it was found necessary to employ powerful electro-magnetic induction machines, excited by the currents from other smaller machines, according to the principles laid down in my paper read before the Royal Society, and published in the Philosophical Transactions of 1867. From sixteen to twenty lights are produced from one of these electro-magnetic machines, each light absorbing about one-horse power.

The system of electric lighting above described would now seem to be definitely established in some places as a substitute for gas, and visitors to the French capital during the present summer will have been struck with the fine effects produced in the avenues and squares where the light is displayed.

My connection with the history of this system of lighting placed me in a position to make some experiments with the Jablochkoff candle, and led to the discovery of the following facts. One of the conditions necessary for producing a constant light from the candle, in its most recent form, was that the quantity and intensity of the alternating current should be such that the carbons consume at a rate of from four to five inches per hour. If the electric current were too powerful, the carbons became unduly heated, and presented additional resistance to the passage of the current; the points at the same time lost their regular conical form. If, on the other hand, the current were too weak, the electric arc played about the points of the carbons in an irregular manner, and the light was easily extinguished by currents of air.

In the course of these experiments I was struck with the apparently insignificant part which the insulating material played in the maintenance of the light between the carbon points; and it occurred to me to try the effect of covering each of the carbons with a thin coating of hydrate of lime, and mounting them parallel to each other in separate holders, and without any insulating material between them. The use of the lime covering was intended to prevent the light from travelling down

the contiguous sides of the carbons. On completing the electric circuit the light was maintained between the two points, and the carbons were consumed in the same regular manner as when the insulating material had been placed between them.

Two plain cylindrical rods of carbon three-sixteenths of an inch in diameter and eight inches long, were now fixed in the holders parallel to each other, and one-eighth of an inch apart. The strength of the alternating current was such that it would fuse an iron wire 0025 of an inch in diameter and eight feet in length. On establishing the electric current through the points of the carbons by means of a conducting paste composed of carbon and gum, the light was produced, and the carbons burnt steadily downwards as before.

Four pairs of naked carbons mounted in this manner were next placed in series in the circuit of a four-light machine, and the light was produced from these carbons simultaneously, as when the insulating material was used between them. The light from the naked carbons was also more regular than that from the insulated ones, as the plaster of Paris insulation did not always consume at the same rate as the carbons, and thereby obstructed the passage of the current. This was evident from the rosy tinge of the light produced by the volatilisation of the calcium simultaneously with the diminution of the brilliancy of the light from the carbons.

The only function, therefore, which the insulating material performs in the electric candle, as shown by these experiments, is that it conceals the singular and beautiful property of the alternating current to which I have directed attention.

As I have already said, the strength of the alternating current must bear a proper proportion to the diameter of the carbons used, and when a number of such lights are required to be produced in the same circuit, the quantity and property of the current will remain constant, while the tension will require to be increased with the number of lights.

This simple method of burning the carbons will, I believe, greatly further the development of the electric light, as the carbons can be used of much smaller diameter than has hitherto been possible. They may also be of any desired length, for as they are consumed they may be pushed up through the holders without interrupting the light. One of these developments will be a better method of lighting coal and other mines. In this application the alternating currents or waves from a powerful electro-magnetic induction machine may be used for generating, simultaneously, alternating secondary currents or waves in a number of small induction coils, placed in various parts of the mine. The light may be produced in the secondary circuits from pairs of small carbons inclosed in a glass vessel having a small aperture to permit the expansion of the heated air within. Diaphragms of wire gauze may be placed over the aperture to prevent the access of explosive gas. By generating secondary currents or waves without interrupting the continuity of the primary circuit, the contact-breaker is dispensed with, and the subdivision of the light may be carried to a very great extent.

A STUDY IN MAGNETISM

THE name of Faraday will go down to posterity foremost amongst the names of the scientific men of this century, for the simple comprehensiveness and original beauty of his researches in electricity and magnetism; chiefly, perhaps, for his discovery of magnetoelectricity-the kind of electricity that can be induced in conductors which are caused to pass near magnets. Those who have carefully read Faraday's works know how he was led to this discovery by the conception he had formed of magnetic force. Until his time magnetic seeking magnetism were placed on a line near one pole it would not pass over from pole to pole, but would follow the line where it swerves away to the side. We know (by experiment) that two north-seeking poles repel one another, and we see here that the lines of force of two such poles never run into one another, but turn aside mutually repellant. You will find that this is always the case when two poles repel.

Such experiments as these led Faraday to enunciate

that of Fig. 5. It is interesting to observe how in the middle of the loop there are no lines, only dots. The lines of force run through the loop, perpendicularly to its plane, and we only see them end-ways as points. It is clear that a magnetic particle such as we have imagined would be either attracted into the middle of the loop, or would be repelled out of it, according to its polarity.

Now what is the effect of carrying two parallel currents through two wires side by side? Take a piece of card or

the lines of force, by means of which we can learn from the lines what kind of action they will produce, whether attraction, or repulsion, or rotation. Firstly. - All lines of force tend to shorten themselves. If the lines running across in our first figure were replaced by actual threads of stretched elastic material, we see that any "shortening" of them would bring the poles nearer together, which, indeed, is precisely the tendency of the magnetic attraction between the poles. Secondly. - Lines of force repel each other when placed side by side. If this be the case then the lines in our second figure, which bend outwards, and run off side by side, repel one another, the two poles must be experiencing a tendency to move away from one another; and this we know is the case. Thirdly, Like magnetic lines of force, when "end on" to each other, run into each other; while, unlike magnetic lines, when end on, repel each other. Here, of course, we apply the terms "like" and "unlike" to the cases of the directions in which our supposed particle of north-seeking magnetism would move along the lines. These notions of Faraday's are full of meaning, and it is not many years since Prof. Clerk Maxwell showed how well they agreed with the most perfect mathematical expression of of the forces that operate in the medium filling the surrounding space.

Keeping these simple principles in mind, let us apply them to some further cases of magnetic action, and see if they are equally applicable. We know that the wires carrying electric currents possess certain magnetic properties, and will deflect magnetic needles; that two electric currents may attract or repel each other; and that current may make a magnet pole rotate round it. Can we explain such electrodynamic actions also by applying the principles of Faraday to the magnetic lines of force existing in these various cases?

In the first place, what are the lines of force belonging to a wire through which an electric current is passing? To ascertain this we will bore a hole through a card or a piece of glass, and pass a wire up through the hole. Then, joining the ends of the wire to the poles of a powerful battery, we will, while the current is passing, sprinkle on iron filings, and, tapping lightly, will permit them to assume their places in the lines of force. Fig. 3 was thus obtained. It shows us a series of concentric circles. If a supposed north-seeking magnetic particle were put down on one of these circles it would move round and round in one direction; supposing the current to come up through the hole, this direction would be opposite to that of the hands of a watch. If the current went down through the hole, the movement would be the other way round. But we may examine the current in another way. Lay the conducting-wire down flat, and place over it the card or piece of glass. The forms assumed by the iron filings are in this case (Fig. 4) straight lines across the wire-are edge-views, so to speak, of the systems of circles we just now obtained.

These two figures were discovered by Faraday, and are given in his researches. They are also given by Dr. F. Guthrie in his book on "Magnetism and Electricity."

If we wind up our conducting-wire into a simple knot er loop, carefully preventing the overlapping parts from

pass a couple of wires joined to two batteries, so that the two currents are either both ascending or both descending through the flat surface. The magnetic field mapped out by the iron filings will then show a series of curves, the outermost of which are rough ovals inclosing both the currents, whilst the innermost are small ovals round each wire. The lines between the inner and outer systems present a sort of hour-glass shape or lemniscate. Had the two parallel currents, however, passed in opposite directions through the plate, one ascending and the other descending, the filings in the magnetic field would have taken the form given in Fig. 7. Here we find two separate systems of distorted and flattened circles surrounding the wires, each separate system of circles having displaced the other. The outer curves do not run into each other as in the preceding case. Let us apply Faraday's principles to these two figures. In the former (Fig. 6) any "shortening" of the exterior lines would tend to draw the centres nearer together. In the latter case (Fig. 7) no such consequence need result. A tendency of the successive lines to repel each other and to maintain equal distances from each other, would in the former case tend to reduce the entire figure to a system of concentric circles, which could not be accomplished unless the two centres approached each other and coalesced. In the latter case, since the systems of lines round the two centres never join across, this tendency would have the result of driving the two centres far apart to allow of the lines becoming perfect sets of circles. Now we know from Ampère's classical researches on parallel currents, that they attract one another when they run in the same direction, but are mutually repellant when they run in opposite directions. Our application of Faraday's principle enables us to foresee this electro-dynamical action as a consequence of the distribution of magnetic force in the field. In an exactly similar manner we may reason out the action of the forces in the field which is produced by two currents crossing one another at a right-angle (Fig. 8), the conducting wires attracting one another across those quadrants in which the currents flow both towards or both

from the point of intersection.

We may apply our study further and investigate, with iron-filings, the action which currents exert on magnets. Let us conduct a current vertically through a hole in a plate, and fix near it a small magnetic needle, as in Fig. 9. The needle has been placed so as to point with one pole towards the current. The lines of force radiating from that pole run round and coalesce on one side with the circular lines of force of the current. On the other side of the pole they absolutely refuse to unite with the circles, and repel them away. Clearly, the "tendency to shorten," which Faraday predicated of the lines, would drag the pole of the magnet in one direction round the current. Looking at the other pole of the magnet we see that the tendency acts in the opposite direction, so that the total result would be a tendency to turn round the magnet about its middle point, and set it at right angles to its present position. This consequence, too, is, as we know from Oersted's famous experiments, the fact.

If, instead of laying the needle down flat, we had reared it up on end, as in our Fig. 10, where a square black