AIRSHIPS AND AIRPLANES 313 



zontal planes are attached to the rear. To give stability 

 in a lateral direction, several devices are used. In the Wright 

 biplanes, flaps hang down from the main planes, which can 

 be raised or lowered. Another method often found on 

 monoplanes is a warping of the tips of the planes. Ex- 

 periments have been made with automatic devices to keep 

 the plane stable, such as a pendulum or gyroscope; but so 

 far with little success. 



Power. Gasoline engines are used to furnish the power 

 to propel airplanes, because they are light and powerful. 

 Engines of great horse power are being used. The pro- 

 pellers are commonly two bladed and from five to ten feet 

 in diameter. They may be placed either in front or behind. 

 They may be single or double. 



Starting the airplane. In order that an airplane shall 

 glide through the air, it must have an initial motion given 

 it before it is launched. The Wright brothers in their first 

 experiments started their machine by means of a heavy fall- 

 ing weight, which drew the machine along a track. Modern 

 machines are mounted on wheels, and the motion is started 

 by the propellers, which force the machine over the ground. 

 The same wheels that serve for starting also serve for alight- 

 ing. Sometimes these are provided with brakes, and some- 

 times the machine has skids, on -which it alights and stops 

 in a short distance. Hydroplanes are now made which 

 can start from the water and alight on it. The wheels are 

 replaced with floats, which keep the machine on the surface 

 of the water. 



Running the airplane. For steering there must be two 

 sets of rudders, one to guide the plane up and down, and one 

 to guide it to the right and left. The horizontal plane used 

 for the first purpose may be placed either in front or at the 

 rear. The vertical plane for the second purpose is placed 

 at the rear. These planes are both controlled by levers 

 within easy reach of the aviator. 



