The pilot slackens his speed when flying by throttling down his motor, and so reducing the pull or thrust of his propeller; or he sets his engine at full power, and so attains his maximum speed. For military work such variation is, as has been said, of great importance. An air-scout needs to reach a given point quickly, and return to headquarters at high speed. Every moment’s delay in bringing the Commander-in-Chief some vital piece of news, may spell the difference between victory or defeat. It is here, in fact, that cavalry scouts have failed. They may obtain important tidings, but before they can make their way to headquarters and report, the aspect of affairs has changed, perhaps, and their news--which would have been valuable had it been received at once--has lost its significance through delay. An air-scout who can vary his speed will fly fast till he needs to make an observation; then he will slow down and study what lies below him; and, his survey completed, he will return at top speed to headquarters. Wireless telegraphy, for communicating between an aeroplane in flight and its headquarters, is now adopted for urgent reports--a cypher being used to prevent messages being “tapped” by an enemy. At first poor results were obtained, signals being audible on the ground, when sent from a machine flying low, at a distance of only about a mile. This was due to the difficulty of producing a sufficiently light apparatus; but now, with transmitting mechanism weighing from 70 to 80 lbs., messages may be dispatched from an aeroplane to a land station 100 miles away, although problems still remain to be solved in regard to the tapping and jamming of signals. The dynamo generating the electric current is driven by a small petrol motor, which develops about 2-3/4 horse-power; and the antenna or aerial wire, by means of which the message is sent forth--a wave length of from 200 to 500 metres being adopted--may be arranged between the planes and tail, or unwound from a reel below the landing chassis when a machine is in flight, and allowed to trail earthward. Apparatus weighing 20 lbs., is also being tested, by means of which--using a magnifying receiver--an operator in an aeroplane may receive a message as well as send one. This allows communication, not only between a machine and an earth-station, but also between two craft when in flight.