curately with theoretical considerations than the results of Zöllner's investigations of the same point. The principal results of Müller's observations were expressed by him in formulæ or curves which showed that for a given length of wire of a given thickness and for a given strength of battery there exists a maximum intensity of the glow; and, again, if the wire of given length must be made to glow, the thickness of the wire must lie between two well-defined limits.

Von Waltenhofen shows, in addition to this, that, at least for the more intense stages of glow, we are always justified in considering the specific resistance of the glowing wire as nearly constant.-Sitzb. k. Akad. der Wissens., 1874, 79.

CURIOUS ELECTRICAL LIGHT.

Dr. James Blake states that on the evening of the 30th of September, 1874, at Placerville, El Dorado County, California, while watching the lightning continuously flashing in the southwest horizon, he observed, at a quarter past seven, a luminous appearance apparently proceeding from the crest of the range of mountains about six miles distant to the southwest, which was visible through an arc horizontally of about 15°, and extended about 8° above the horizon. The light lasted for about a quarter of an hour, was undoubtedly electrical, appeared like a fine display of the aurora, and may have been due to the silent escape of electricity from the crest of the ridge. The storm in question was the greatest electrical disturbance recorded since the settlement of the country.-Proceed. Cal. Acad. of Science, V., 406.

PHENOMENA ATTENDING ELECTRIC DISCHARGES.

Schneebeli, in a short note on the electric discharge, accompanied with diagrams showing the zigzag and other peculiarities of the discharge, states that the method which he pursues for fixing these figures consists in employing a paper wet upon one side with a solution of gum lac in alcohol, which permeates the paper, and fixes without changing its outlines the blackened trace on the other side of the paper formed by the electric discharge itself. That part of the figure made by the discharge which is near the positive pole is traversed by very fine lines, and the isolated branches shoot from the main stem in all directions. That part of the figure of the

discharge formed at the negative pole shows these fine lines only in the immediate neighborhood of the pole, and has no isolated branches. It will be remembered that the figures called Lichtenberg figures show also a very remarkable dif ference between the two electricities. The most instructive peculiarity of Schneebeli's figures is found in a very wellmarked point near the centre of the figure, and separating the positive from the negative discharge. This central point has every appearance of being that in which the layers of air meet, after moving in opposite directions from either pole toward the centre. When very strong electric discharges are employed, this peculiar meeting-point is obscured, but becomes very distinctly marked when feebler discharges are made.Bulletin Soc. des Sciences Naturelles, Neuchatel, May, 1875.

DUPLEX TELEGRAPHY.

In some historical notes on the development of duplex telegraphy, Mr. Cracknell, the superintendent of electric telegraphs in New South Wales, states that this art, which consists essentially in working in two opposite directions on one wire simultaneously, was first tried as far back as in 1853, by Dr. Gintel, the director-general of telegraphs in Austria, on the line from Vienna to Prague. In his arrangement the compensating currents of electricity could not well be controlled, and the system was not found to be a practical success, although experimentally it worked beautifully. In 1854 Frecher, Siemens, and Halske devised a modification of the duplex method by adopting a somewhat complicated system of resistance coils. These were the first Morse instruments used on telegraph lines in New South Wales, in January, 1858. They could not be made to work satisfactorily, and were reduced to single-acting Miller's recorders. Siemens and Halske's arrangement was worked on what is called the differential principle. Stearns' duplex system, lately perfected in America, and now attracting the attention of telegraph engineers in most parts of the world, is on a principle which is dependent on producing an equality of tensions, which are sometimes called potentials. With this system the relay coils are wound with double wires. It has proved a great success, partly by improvements on old principles, but more particularly in consequence of the intro

duction of more perfect appliances, since Gintel's and Siemens and Halske's inventions.-Transactions Roy. Soc. of New South Wales, 1874, 71.

THE DOUBLE-BALANCE METHOD IN DUPLEX TELEGRAPHY.

Louis Schwendler, electrician in charge of the telegraph lines of India, has communicated to the Asiatic Society of Bengal a memoir on the general theory of duplex telegraphy, and has investigated with great detail the conditions necessary to the successful use of the bridge method under two suppositions: first, that the line is in perfect insulation, and, second, that the line has appreciable leakage; the latter being of course the actual case in practice. He first shows that, in order to obtain the best results in both cases, the resistances and the currents must have certain relations to each other, such that there should be two balances, viz., a balance in one branch for the receiving current, and a balance in another branch for the outgoing current. This method elaborated by him, and which he calls the doublebalance method, fulfills the following conditions, which he states are necessary, and are sufficient to place duplex telegraphy on a par with single telegraphy. First, any variation in the resistance of the line has the least possible disturbing effect on the receiving instrument. Secondly, any disturbance can be eliminated by a single adjustment without disturbing the balance in a distant station. Thirdly, the maximum magnetic moment of the receiving instrument is obtained and employed. Fourthly, a maximum working current is available. This double-balance method was introduced by him first in June, 1874, between Bombay and Calcutta, and has since then been working so satisfactorily even during the worst time of the year, viz., the southwest monsoon, that Colonel Robinson, director-general of the telegraphs in India, has decided to introduce this duplex method also on the other long main lines of India.—Journal Asiatic Society of Bengal, 1874, 218.

SCHWENDLER'S METHOD IN DUPLEX TELEGRAPHY.

In the annual address of the President of the Asiatic Society of Bengal, it is said that the double-balance method in duplex telegraphy, as invented and perfected by Mr. Schwend

H

ler, of Calcutta, was introduced, on the 28th of January, 1875, on one of the main lines from Calcutta to Bombay, 1600 miles; since which time, and in the worst season-the southwest monsoon-the method has worked with great regularity and speed. In a few days another main line from Bombay to Madras will be worked; and it is probable that before the lapse of the current year (1875) the whole of the main traffic in India, from Calcutta to Rangoon included, will be carried on by this system. The method is also practicable on cables, as shown by the experiments with the cable from Bombay to Aden, 1800 miles long. It has been mathematically demonstrated and confirmed by practice that Schwendler's double-balance method involves all the necessary conditions to bring duplex telegraphy on a par with single telegraphy.-Proceedings Asiatic Society of Bengal, Feb., 1875.

NEW ELECTRIC APPARATUS FOR RAILROADS.

Preece's single-wire block signaling instruments are in extensive use in England. They possess one special and important advantage which no other single-wire system has: that the consentaneous action of the two signal-men at the station in advance and in the rear is necessary in order to lower the semaphore to the point which indicates "all clear," thus avoiding the production of a false safety signal from any accidental current. A part of the same apparatus consists in the single repeater which repeats back electrically the movements of a signal which may be out of sight of the man who works it.-Forty-second Ann. Rep. of the Royal Cornwall Polytechnic Society, 80.

EAST INDIAN TELEGRAPHY.

The reports from the Indo-European Telegraph Company say that the average time in transit between London and India, via Teheran, of all outward messages to India, including the messages for Penang, Singapore, China, Japan, Java, and Australia, for the week ending October 8th, was one hour and sixteen minutes.-Engineer, XL., No. 1034, 281.

ATMOSPHERIC ELECTRICITY.

Professor Van der Mensbrugghe, in a memoir on the ap plication of thermo-dynamics to the study of the variations

of the potential of liquid surfaces, is led to a conclusion of capital importance to meteorology. His equations, namely, seem to prove rigorously that any change in the surface of the liquid gives rise to a change of temperature, and, if the circuit is closed, to a thermo-electric current. His experiments show that, on the one hand, the water of the ocean under the action of the sun, being submitted to a continual evaporation, must affect the calorific and the electric state of the earth, and develop constant thermo-electric currents. On the other hand, the enormous quantity of vapor which is lifted into the atmosphere is there subjected to incessant variations in respect to the surfaces of contact with the air of its vesicles, raindrops, etc. Its reduction from a state of extreme and almost molecular tenuity to a state where, by sudden condensation it forms larger raindrops, enables it to actually produce enormous quantities of electricity, until the drops fall in turn upon the surface of the globe from which they were elevated. Thus we have at once, on the one hand, the existence of a constant source of thermo-electricity circulating around the earth, and on the other hand a permanent cause of atmospheric electricity. Bulletin Acad. Royal des Sciences de Belgique, Bruxelles, 1876, 782.

LIGHTNING FROM A CLEAR SKY.

The rare phenomenon of a stroke of lightning from a clear sky was observed on the 3d of July, at 4 P.M., at Senftenburg, Saxony. At that time a thunder-storm was observed so far distant that some thirteen to fifteen seconds elapsed between the flashes and the thunder, when suddenly in the clear sky, in the immediate neighborhood of the observer, Dr. Roch, a blinding flash and fearful thunder occurred. Especially noteworthy was it that this abnormal flash put an end to all the distant lightning and thunder; and the storm clouds very quickly disappeared from the heavens, which retained during the night their original purity.-Sitzb. Naturw. Gesells. "Isis," Dresden, December, 1875, 118.

PROTECTION AGAINST LIGHTNING STROKES.

In his annual address as President of the Meteorological Society, Dr. R. J. Mann states that Professor Melsens, of Brussels, has been for some time engaged in an interesting