"The beauty of a ship depends on the way it glides through the water. Watch a liner, and you can see that it is being driven by its screws, but look at a racing yacht: there is no sense of effort whatever. She seems to move like a bird, by natural means.
"Here is the secret of the beauty of the aeroplane. It seems to be completely master of the element in which it moves. It flies with no visible effort and at a little distance one could imagine it endowed with magic power, moving by natural force, like a bird.
"All the early attempts at flying were made on the theory of wing motion, and the failures resulting were doubtless due to careless study of what nature could teach. There was a great deal more to be learned from nature than from mathematics. An examination of the different types of birds testifies, among other things, to their rigid backs, and to the fact that nearly all their bones are hollow and have air cavities. An erroneous deduction had been drawn from this that the hollows were purely for the sake of lightness, and that the cavities were for hot air to make the bird light when it wanted to fly. The amount of lightness so obtained, however, was so small as not to be worth consideration. The passages are simply reservoirs for air, and they allow the bird more energy than a less freely breathing animal. The wing of the bird does a double duty: it is an aeroplane and a propeller combined. The valvular action has nothing at all to do with the flight. Some explanation of how a sparrow can rise from the gutter to the eaves may be seen by the difference in the construction of its wings from those of the swallow, which cannot rise from the ground like a sparrow, but has to get initial velocity. The swallow, however, has much more mastery over its movements in the air than the sparrow has. These, and many other things in connection with bird flight, under proper methods of scientific investigation, may show us the whole theory of aviation. I am inclined to think that scientific men will soon be able to solve the problem, and to give us better control of the coming aeroplanes, or even direct their flight by the aid of electric waves or other natural forces.
"In kite-flying, it is well to know something of the wind and its pressure, and, in this connection, the following short table will give some idea of the force exercised on objects in its path: A light air current presses 0.004 lbs. per square foot.
| Light wind has a pressure of | 0.125 | lbs. per sq. foot | |
| Light breeze | 0.246 | lbs. per sq. foot | |
| Moderate breeze | 0.406 | lbs. per sq. foot | |
| Strong breeze | 2.00 | lbs. per sq. foot | |
| Moderate gale | 2.98 | lbs. per sq. foot |
"This last should be the limit, as a kite or aeroplane of any kind will find it hard to manœuvre in a breeze stronger than a moderate gale. Of course, there are winds sometimes that have a velocity of 60 to 75 miles an hour, and a pressure of over 40 pounds to the square foot, but these would prove disastrous to any kind of a flying machine, if it was in action."
"Father," asked Fred, "how can one tell the velocity of the wind, without one of those expensive machines I see at the weather office, an anemometer, I think it is called?"
"I am glad," said the father, "that you have noticed those and other instruments for gauging and foretelling weather conditions. It is an indication that you keep your eyes open when you visit such places, and to learn by observation is almost as effectual as to obtain knowledge by experience. I have in mind a very simple contrivance you can make yourself, for measuring wind pressure from a couple of ounces to four pounds to the foot. I will make a sketch of it, which I am sure you will understand.
