ncluded a large portion of the light emitted by
the light-source in the case of direct-current arcs. If this angle is
increased for a mirror of a given diameter by decreasing its focal
length, the divergence of the beam is increased and the beam-intensity
is diminished. This is due to the fact that the light-source now becomes
apparently larger; that is, being of a given size it now subtends a
larger angle at the reflector and departs more from the theoretical
point.

When the recent war began the search-lights available were intended
generally for fixed installations. These were "barrel" lights with
reflectors several feet in diameter, the whole output sometimes weighing
as much as several tons. Shortly after the entrance of this country
into the war, a mobile "barrel" search-light five feet in diameter was
produced, which, complete with carriage, weighed only 1800 pounds. Later
there were further improvements. An example of the impetus which the
stress of war 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-l