ight and its
carriage are loaded upon the rear of the mobile generating equipment. An
idea of the intensities obtainable with the largest apparatus is gained
from illumination produced at a given distance. For example, the
15-kilowatt search-light with highly concentrated beam, produced an
illumination at 930 feet of 280 foot-candles. At this point this is the
equivalent of the illumination produced by a source having a luminous
intensity of nearly 250,000,000 candles.

Of course, the range at which search-lights are effective is the factor
of most importance, but this depends upon a number of conditions such as
the illumination produced by the beam at various distances, the
atmospheric conditions, the position of the observer, the size, pattern,
color, and reflection-factor of the object, and the color, pattern, and
reflection-factor of the background. These are too involved to be
discussed here, but it may be stated that under ordinary conditions
these powerful lights are effective at distances of several miles.
According to recent work, it appears that the range of a search-light in
revealing a given object under fixed conditions varies about as the
fourth root of its intensity.

Although the metallic parabolic reflector is used in the most powerful
search-lights, there have been many other developments adapted to
warfare. Fresnel lenses have been used above the arc for search-lights
whose beams are directed upward in search of aircraft, thus replacing
the mirror below the arc, which, owing to its position, is always in
danger of deterioration by the hot carbon particles dropping upon it.
For short ranges incandescent filament lamps have been used with
success. Oxyacetylene equipment has found application, owing to its
portability. The oxyacetylene flame is concentrated upon a small pellet
of ceria, which provides a brilliant source of small dimensions. A tank
containing about 1000 liters of dissolved acetylene and another
containing about 1100 liters of oxygen supply the fuel. A beam having an
intensity of about 1,500,000 candles is obtained with a consumption of
40 liters of each of the gases per hour. At this rate the search-light
may be operated twenty hours without replenishing.

Although the beacon-light for nocturnal airmen is a development which
will assume much importance in peaceful activities, it was developed
chiefly to meet the requirements of warfare. These do not differ
materially from those which g