tween light and the other form of force has not yet been established, and it may not be going too far to conjecture that thermodynamics may possibly in the future have to appeal to the action of light upon a photographic plate. In the mean time we look forward to the promised continuation of Dr. Vogel's researches with no little hope.—Nature.
| THE ELECTRIC LIGHT FOR STEAMSHIPS. |
By JOHN TROWBRIDGE,
ASSISTANT PROFESSOR OF PHYSICS AT HARVARD UNIVERSITY.
THE employment of the electric light for general purposes of illumination has not, hitherto, been successful. The difficulty of maintaining it constant, and the expense attending its use, have prevented its employment. In the old method of producing the light by a great number of cells, the chief difficulties arose in keeping the strength of the current constant, and in regulating the distance of the carbon-points between which the light was produced. Certain forms of the Daniell cell, notably that constructed by Sir William Thomson ("Jenkins's Electricity," p. 223), give a sensibly constant current for an indefinite period, if watched with great care. The solutions of the cells, however, need to be carefully removed from time to time. The distance of the carbon-points also can be regulated by various contrivances, which do the work required of them in an admirable manner. Still, chemical action cannot be looked to as an economical and constant source of the electric light.
The remarkable improvements in magneto-electric engines have led to another source of the electric light, and seem to afford a better solution of the problem of its economical use. The principle which underlies all magneto-electric engines can be briefly stated thus: The movement of an electro-magnet in the neighborhood of a stationary magnet, which may also be an electro-magnet, is sufficient to induce a current in the coil of the first electro-magnet, and this current can be exalted in strength almost indefinitely by its proper direction, and by the rapidity of the mechanical movement. The most noted engines are those of Siemen and Hulske, Wild, Ladd, and the Gramme machine. Some idea of the power of these engines can be gained from the following statement in regard to a Wild machine, of a size intended to be used for the production of the electric light for light-houses: "When worked with a power of three horses, it will consume carbon-sticks three-eighths of an inch square, and evolve a light of surpassing brilliancy. With a machine that consumes carbons half an inch square, a light of such intensity is got, that, when put on a lofty building, it casts shadows from the flames of the street-lamps a quar-
