102 OUR PHYSICAL WORLD 



attaches in a similar position 6 inches from the hind end of the tri- 

 angular frame. This plane takes two g-inch propellers. If the pull 

 of the tightly twisted rubber bands tends to bend the long sides 

 of the triangle, run fine wires one from the rear of each side to 

 the apex of the triangle over a 2-inch upright of light stuff set 

 on the middle of each side and bound in place. Skids may be 

 provided as in the other plane, but they are not as necessary, for 

 this plane is started off from the hands, each hand holding one 

 propeller and letting go as the plane is launched by a shove out 



FIG. 44. Front view of a biplane built by seventh-grade pupils 



from the shoulders as the person launching it stands upright. 

 (See also Fig. 44.) 



A very simple aeroplane propelled from a sling shot instead 

 of by a propeller is made thus: Split a J-inch square wood strip, 

 10 inches long, at one end. Insert a light card if by 3 inches 

 so that the ends of the card stick out equally on either side of 

 the stick and its rear edge is if inches from the end of the stick. 

 Bind it in place. Tack another card on the stick, the same size as 

 this, its surface at right angles to the first, its rear edge at the 

 end of the stick, its ends projecting equally from the sides of the 



