without the slightest warning, out of the horn of the loud-speaker came the words: “_Hello, ground-station! This is Plane Number One speaking. Do you get me all right?_” The bored expressions on the faces of the officials changed to expressions of amazement tinged with awe. Instead of the confusing dash-dot-dash which they associated with wireless, here was a human voice coming out of space clear and distinct—yet the speaker was two miles in the air. Soon the same signal came from the other plane and the exhibition was on. Under command from the ground the planes were manœuvred all over that part of the country. They climbed and volplaned and circled. They were sent on scouting expeditions and reported what they saw as they travelled through the air. Continuous conversation was carried on, even when the planes were out of sight, and finally, upon command, they came tearing down the skies like two huge homing pigeons and landed where directed. From that moment the radio telephone was sold to the government. It was no longer a question as to whether it would work, but how soon and in what quantity its manufacture could be started. The primary object of the airplane telephone is to make it possible for the commander of an air-squadron to control the movement of his men in the air just as a drill-sergeant directs the evolutions of a platoon on the ground. For this purpose extra-long range is not required or, indeed, desired, the distance over which they can talk being purposely limited to two or three miles, so that the enemy cannot overhear except when actually engaged in combat. Then it does not matter. Neither my space nor my knowledge of electrical engineering are sufficient to permit of explaining in detail the working of the radio telephone. It is enough to say that a wind-driven generator supplies electric current to a couple of vacuum tubes mounted in a box filled with coils and condensers. These tubes transform the dynamo current into a high-frequency alternating current which is fed out into space through the antenna. This antenna consists of a copper wire about 200 feet long, which with a lead weight on the end trails out behind the airplane when it is in flight. Normally this wire is wound up on a reel, being let out and wound in as occasion demands. With the special form of telephone-transmitter already described, the words of the aviator are impressed on this wire, the electric waves thus set in motion radiating out into space, where they are picked up by similar antennæ either on other planes or on masts on the ground. The receiving process is the exact reverse of that used in sending, other vacuum tubes taking the high-frequency current from the antenna and transforming it so that it can be heard in the form of speech in the telephone fitted in the aviator’s helmet or in the loud-speaking horn on the ground. That is about as near as I can come to explaining the radio telephone without writing a book.