GLIDING 15 
and 5° respectively would mean 70 and 90 miles 
per hour. Even 1 horse-power, with an angle of 
5° gives a velocity of 43; in other words, 1 horse- 
power, if the plane is set at an angle of 5°, is more 
effective than 3 horse-power when the angle is 30°. 
In fact, if you have a good aeroplane and the skill 
to use it well, you require less power to fly fast than 
to fly slow. But if you reduce the angle of inclina- 
tion below 5° you find that the tables are turned 
upon you. So far from economising power, you 
would have to use it in lavish style, to overcome 
friction. 
Curve of Wings. 
We must now turn to the question of curves. 
The curved surface is undoubtedly superior to the 
flat. A toy paper glider, being a mere feather- 
weight, can dispense with curves, but without 
curved surfaces—cleverly designed ones too— 
aviation would be out of the question. An aeroplane 
presents concave surfaces to the air; its “ planes ” 
curve from front to back, and a bird’s wings have 
concavities that are probably better adapted for 
flight than anything that human ingenuity has 
designed. On this subject even an umbrella can 
tell us something. When its ample concavity is 
turned towards the wind its efficiency in catching 
and holding the air is so great that it speedily 
becomes a wreck of wires. Lilienthal, a distin- 
guished flight pioneer, whose experiments in gliding 
did much to make aviation practicable, fully ap- 
preciated the value of the curved surface. Some- 
times when he was carrying his glider to a little 
