Page:Popular Science Monthly Volume 5.djvu/741

Rh of a mile distant upon the neighboring walls. The same light, at two feet from the reflector, darkened ordinary sensitized photographic paper as much in twenty seconds as the direct rays of the sun at noon on a clear day in March in one minute" (Ferguson's "Electricity"). With a ten-inch quantity armature, Mr. Wild succeeded in melting an iron rod fifteen inches long and one-quarter inch thick. The entire machine by which this was accomplished was under five feet in length and height, was only twenty inches wide, and weighed a ton and a half. The Ladd engine dispenses with the use of permanent steel magnets, and is a more compact form of the machine than Wild's. The Gramme machine returns again to the use of powerful steel magnets, between the poles of which revolves a ring-shaped electro-magnet. The problem of producing the best machine for the production of electricity by mechanical power is not yet solved. The machines now before the public will doubtless be very much improved. At present, however, the means to attain the sought-for ends seem to be limited. There are not many combinations which can be made. A field of magnetic force being given, the question arises. What is the most economical means of cutting the greatest number of lines of force of the greatest intensity in the unit of time? With the most improved forms of the magneto-electric machine, we are, however, in a condition to produce an electric light of a reasonable degree of constancy and cheapness. If it is a desideratum that steamships should be provided with more powerful lights than those now in use, the electric light is the one to which attention is naturally directed. The first points to be considered are in relation to its cost, its constancy, and readiness of adjustment, and its efficiency in penetrating fogs. The light-house service of Great Britain and France affords the only experience on these points.

The electric light has been tried by Great Britain at Dungeness, and by the French Government at La Hève. The source of the light in both instances was a magneto-electric machine. A force of one and a quarter horse-power was required to drive the British machine, and one and a half the French. The descriptions of these machines show that they were extremely bulky, compared with the more improved forms, like Ladd's and the Gramme machine. The observations on the lights at La Hève are especially interesting, because they afforded a means of comparison between the fog-penetrating power of the electric light and the ordinary oil-light of light-houses. There were two light-houses at La Hève, one of which was provided with an electric light, and the other with an oil-light. The electric light was equivalent to 3,500 Carcel-burners. The oil-light had an intensity of 630 Carcel-burners.

"In foggy weather, in the hundred times of observation, the electric light was seen twice as often as the oil-light, or more. When the intensity of the electric light, compared with that of the oil-lamp