gives to technical accomplishments is found in the development of a particular mobile searchlight.  Two months after the War Department submitted the problems of design to certain large industrial establishments a new 60-inch search-light was placed in production.  It weighed one fifth as much as the previous standard; it had one twentieth the bulk; it was much simpler; it could be built in one fourth the time; and it cost half as much.  Remote control of the apparatus has been highly developed in order that the operator may be at a distance from the scattered light near the unit.  If he is near the search-light, this veil of diffused light very seriously interferes with his vision.

Mobile power-units were necessary and the types developed used the automobile engine as the prime mover.  In one the generator is located in front of the engine and supported beyond the automobile chassis.  In another type the generator is located between the automobile transmission and the differential.  A standard clutch and gear-shift lever is employed to connect the engine either with the generator or with the propeller shaft of the truck.  The first type included a 115-volt, 15-kilowatt generator, a 36-inch wheel barrel search-light, and 500 feet of wire cable.  The second type included a 105-volt, 20-kilowatt generator, a 60-inch open searchlight, and 600 feet of cable.  This type has been extended in magnitude to include a 50-kilowatt generator.  When these units are moved, the search-light 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.