When the officer in charge of one of these sound-detectors hears through the receivers of his head-set the rhythmic hum which denotes an approaching airplane—and I might mention, parenthetically, that experienced observers can tell with almost absolute certainty not only the nationality of the approaching machine but even the type and power of its engines—he orders several sound-readings to be taken at definite intervals of time. With these readings as a basis for the calculation, the probable location of the airplane at the end of the next time interval is plotted and the search-light is flashed in that direction just long enough to locate the machine. Quick work is required, however, for the airplane often travels at a hundred miles or more an hour and may abruptly change its course at any moment. Then, the plane once spotted, the beam of the search-light never leaves it, and the waiting crews of the antiaircraft guns get to work. Experiments are now being conducted to enable these listening devices to be used in synchronization with search-lights, so that, when the light is flashed, the airplane will be within the beam and no indication of the presence of the search-light will be given the aviator until he finds himself illuminated as a spot-light follows the movements of a dancer on a darkened stage. In the autumn of 1917 the National Research Council, at the request of the Chief of Engineers, inaugurated an extensive series of search-light investigations, which, thanks to the enthusiastic co-operation of scientists, manufacturers, and certain government bureaus, resulted in a number of remarkable developments. Eighteen different kinds of search-lights were developed during these experiments, the first being placed in operation in France in October, 1918. This represented an entirely new form of light, more powerful than any heretofore produced by any nation. It weighs about one-eighth as much as the most powerful search-light theretofore produced, costs only about one-third as much, and has about one-quarter the cubage. Other improvements now in progress give assurance that its range will be doubled, its cost still further reduced, and its mobility greatly increased. And, what is of almost equal importance, the designs are now becoming so simplified that production need no longer be confined to highly specialized shops, but may be distributed over the country to all classes of machine manufacturers, thus making it possible to produce a large quantity in a relatively short time. With this new equipment the United States will possess a search-light having an effective range approximately twice that of the best search-light produced before the war, with four times as great a field. Two features of the latest types of lamps are particularly worthy of notice. These are, first, the “dish-pan” type of light, the chief characteristic of which is that it has no lens; and, second, the metal