Of all the inventions which have sprung from the war, none is more amazing, to my way of thinking, than the radio telephone. Think of standing on the ground and holding a conversation in a normal tone of voice with an aviator so high in the sky that you cannot see his airplane with the naked eye. Think of it! Before we entered the war, any one save a handful of enthusiastic scientists would have ridiculed such a suggestion, yet to-day, at any one of a score of flying-fields, you can sit at an office desk and converse with aviators in the clouds as easily as though you were sitting opposite them at a dinner-table. [Illustration: RADIO TELEPHONE APPARATUS IN OPERATION ON AN AIRPLANE. The pilot and observer are able to talk to each other through the same instrument by means of which they communicate with the ground. _Photograph by Signal Corps, U. S. A._] [Illustration: PRESIDENT WILSON TALKING WITH AN AVIATOR IN THE CLOUDS BY MEANS OF THE RADIO TELEPHONE. _Photograph by Signal Corps, U. S. A._] [Illustration: A RANGE-FINDER FOR ASCERTAINING THE ALTITUDE AND SPEED OF AIRPLANES. One of the most remarkable inventions of the war. This instrument not only ascertains the altitude and position of an airplane but by means of an electric connection automatically sets the sights on the antiaircraft gun.] The enormous advantage which such an invention would give to the army possessing it was early recognized by certain electrical engineers and a few scientifically minded officers of the Signal Corps, and, as a result of their enthusiasm, before the first contingent sailed for France work had been begun on the development of a radiotelephone set for airplanes. There is no necessity of recounting the innumerable experiments and heart-breaking failures before the first real successes were obtained. So far as the radio part of the problem was concerned, a solution was had in a comparatively short time. But working this apparatus in a swift-moving and terrifically noisy airplane was quite a different matter, it was quickly discovered, from working it under ordinary conditions on the ground, the roar of the engine and the rushing air making it impossible to hear one’s own voice, much less the weak signals of the receiver. One of the first problems to be solved, therefore, was to design a head-set which would exclude these noises while at the same time permitting the voice of the telephone to be heard. The answer was found in a form of aviator’s helmet fitting the head so closely as to exclude virtually all extraneous sounds save those coming through telephone-receivers inserted in the helmet so as to fit the ears. No sooner was this problem solved, however, than another one demanded solution. A means had been devised for protecting the receivers from outside noises—but how about the _transmitter_? Every one knows how sensitive the ordinary telephone-transmitter is to extraneous sounds, so it does not require