Another ingenious system of signalling, from an aeroplane to its headquarters, when the former is flying within view, is by means of smoke signals from the exhaust of the engine. As the exhaust gases are shot forth into the air they are rendered black by the mixing with them of a powder, the admission of which to the exhaust pipes the pilot can regulate by a lever. In this way he is able to send black puffs of smoke into the air, and spell out a message in the Morse code, which an observer on the ground can read with field-glasses. A more simple method, when an observer wishes to communicate with those below, and yet avoid the delay of a descent, is to drop a written message in a little bag, to which a flag and a weight are attached. This is seen as it falls, and is retrieved and carried to headquarters. [Illustration: FIG. 63.--Sopwith Military Biplane. A. Propeller; B. Motor, partly hidden by shield; C.C. Main-planes; D. Pilot’s seat; E. Observer’s seat; F. Outlook windows in side of hull; G. Rudder; H. Elevating-plane; I. Landing gear.] A machine that has achieved success, owing to its power of varying speed, is the Sopwith military biplane, as shown in Fig. 63. Adopting a practice that has become general, its wings are fitted upon what is practically a monoplane body. Tail-planes and rudder are the same as in a monoplane. The top main-plane, as will be seen, is set slightly in advance of the lower. The system is called “staggering”; and the idea is that, by placing the upper plane ahead of the lower, the total lifting power will be increased. It has been proved a disadvantage of the biplane that, when the main-planes are placed one above another, there is a slight loss of lift owing to the fact that, acting upon the air as they do quite close to each other, a certain amount of interference occurs between them--one tending to disturb the air-stream in which the other moves. By “staggering” the two planes this interference is overcome; but some makers regard it as a small consideration, and build their planes in the ordinary way, allowing as large a gap as possible between them. In the Sopwith military machine, engine and propeller are in front of the main-planes; then come the places for pilot and observer. The pilot sits first, and the body of the machine is so high that only his head appears above it, while just in front of his face, to deflect the wind-rush from the propeller, there is a raised section of the hull which acts as a screen. Behind the pilot, sitting in a second opening in the hull, is the observer. He has a view forward, rendered the better by setting back the lower-plane; while at the point at which it joins the body of the machine, immediately below him, this plane is hollowed out, so that he can look directly upon the earth below. Small windows are also fitted upon either side of the hull. Through those in front the pilot may glance when descending from a flight, so as to judge