A somewhat different form of sound-ranging is used for the detection of aircraft at night. The apparatus for this aerial sound-ranging consists of large sound-gathering instruments which are used to direct search-lights in the location of approaching airplanes. When a bombing-plane approaches at night the hum of the motor can be heard at a distance of from one to three miles or more, depending upon the direction of the sound and the atmospheric conditions. The direction of sound, however, particularly when it originates in the sky, is illusive to the naked ear and search-lights were obliged to sweep the heavens in the general direction from which the airplane was believed to be approaching, in an endeavor to locate it. By the use of these detectors, however, the sound of an airplane can be detected at a considerably greater distance than by the naked ear, and, what is even more important, its direction can be determined within a very small angle—less than five degrees. In this way the area over which the search-light has to sweep is greatly reduced, and the chances of locating the aerial marauder are enormously increased. Extensive experiments have been conducted in this country by the Engineer Corps in the development of these aerial sound-detectors. One form consists of four horns, two in a vertical and two in a horizontal plant, with listening-tubes leading from the small ends to the receivers of the observer’s head-set. These horns are mounted so as to permit rotation on a horizontal shaft and turning on a plane-table, the whole being supported on a sort of steel tower which, owing to its height and the fact that it cannot easily be moved, affords a rather conspicuous target for the enemy. The obvious disadvantage of this type is recompensed in a measure, however, by its accuracy and by the fact that it will so magnify a sound that the operators can hear the tick of a watch a hundred and fifty yards away. This apparatus is, however, large and cumbersome, and though excellent for seacoast and fortress defense, is not adapted for use in the field, where extreme mobility is required. For this latter purpose paraboloid sound-reflectors have been developed. These paraboloids are about nine feet in diameter, made in sectors of material similar to beaver board, and look like enormous editions of kettles used for boiling soap. They can be taken down and packed into small space for transportation, and are easily set up; being mounted on Ford chassis, they can go anywhere that a “flivver” can go. The paraboloids, like the horns, are directed by balancing the sound so that it is equally audible in both ears. These instruments have a sensitiveness double that of the unaided ear and by means of them a sound can be located to within three degrees.