When the fighting stopped our military scientists and others cooperating with them were developing a type of ground sound-ranging apparatus which it was hoped could be utilized to give troops warning of the firing of heavy artillery shells in their general direction. Preliminary experiments show that at a distance of 4.1 miles this mechanism could record the firing of a gun some 19 seconds before the arrival of the shell. Under proper circumstances this elapsed time would enable troops properly warned to seek cover from the explosion of the projectile. This development of sound-ranging apparatus and its application to the protection of personnel was made possible by the far greater speed with which shock vibrations travel through a dense medium like the earth than through the usual sound-conveying medium, the atmosphere.
SEARCHLIGHTS.
The searchlight equipment of the United States Army prior to 1914 consisted chiefly of lights located at our coast defenses. In 1916 we began the development of mobile searchlight-and-power units for field-army work, four horse-drawn equipments, with 36-inch lights, being ordered first, and later eight other sets, with extensible towers and gasoline electric generators. When the war was approaching we ordered 85 sets of the limber-and-caisson type. The caissons of these sets carried 24-inch lights on extensible towers. In January, 1917, we ordered 50 high-intensity lights to replace as many low-intensity lamps at our seacoast fortifications. The first war order was placed in April, 1917, and consisted of 20 additional searchlights of the 60-inch dimension, the largest light ordered by the War Department.
After the entrance of America in the war the Engineer Department began studying the requirements abroad for searchlights used in defense against hostile aircraft; and in September, 1917, this investigation resulted in orders for 360 high-intensity searchlights, 693 high-intensity arc mechanisms, and 1,000 glass mirrors of standard design.
About this time we began looking to the improvement of existing searchlight equipment. The cooperation of leading scientists, manufacturers, and Government bureaus was obtained, and the product of exhaustive experiments was 18 different new kinds of searchlights either partially or wholly developed.
The first of these were produced, shipped, and were in operation with the Second Field Army in France on October 1, 1918. This was a new form of searchlight more powerful than any that had been produced before that time. It weighed one-eighth as much as lights of former design, cost only one-third as much, was about one-fourth as large in bulk, and threw a light 10 per cent stronger than any other portable projector in existence.
Without going into the details of this mechanism its most striking innovation, from the standpoint of the nontechnical observer, was the absence of the front glass through which the beams of the older type lamps are sent. The absence of the glass, while reducing the weight and cost of a light, also increased the intensity of the beam of the searchlight, since any glass, no matter how conducive to rays, absorbs light to a considerable extent.
In the first part of the war we took the 36-inch lights which the Government had on hand and mounted them on motor trucks. For generating power for the lights, motor trucks were equipped with electric generators operated by the crank-shaft of the truck engine. In moving about each truck carried not only the light and power unit and accessories, but provided space for the crew and their equipment.
When we went into the war there was only one firm in the United States that could make the large searchlight mirrors, but two other concerns developed the art and the faculties during the hostilities. These mirrors were of glass and cost about $1,000 at prewar prices. The maximum output in the United States before the war was three 60-inch mirrors per week. As the result of governmental encouragement the production of the 60-inch mirrors increased until it reached the stage of 15 a week in November, 1918; and the price was reduced to about $900 per mirror, even under war-time conditions with respect to labor and material. This was equivalent to a price of about $700 per mirror under normal conditions, or a saving of 30 per cent.