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Our Surroundings 



AI * COfc 



AfR 



States Post Office Department has established, and is rapidly 

 extending, an air mail service which is making remarkable records 

 for efficiency and reliability. This service has more than cut in 

 two the time necessary for transmission of mail between distant 



points. 



While the balloon 

 craft depend for their 

 ability to stay aloft on 

 a gas which is lighter 

 than air, airplanes re- 

 main aloft by utilizing 



Popular Mechanics Magazine. ^ ne pfCSSUre of air it- 



WIND PRESSURE ON AN AIRPLANE WING sdf> althou g h the ma _ 



chines are far heavier than the air. The main supporting surface 

 consists of the wings, which have a very large area, with a convex 

 surface on the upper side and often a concave surface on the lower 

 side. There are mono- 

 planes, 

 having 



CROSS SECTION OF WING 

 MOVING FORWARD SWIFTDT 



or airplanes 

 one general 

 supporting plane; bi- 

 planes, having two 

 planes; and triplanes 

 with three. 



The wings are so 

 shaped that when 

 slightly tilted into the 

 wind the wind pres- 

 sure strikes into the 

 lower surface and 

 tends to force the plane upward, while the convex surface on the 

 upper side tends to throw the wind off and forms a partial 

 vacuum. The machines are kept on an even keel by the operation 

 of little hinged surfaces at the rear of the wings. When one of 

 these surfaces is tilted up it causes the air to exert great downward 

 pressure on the wing to which it is attached, and the entire plane 

 is tipped in that direction. When the surface is tilted down, the 

 plane tips in the opposite direction. They also have elevating 



Popular Mechanics Magazine. 



AIR HOLES ON A SUNNY DAY 



