Everybody is aware that there is an enormous difference in the lifting power of still air and air in motion, and that this power is dependent upon velocity. The difference between the puff of wind that barely lifts a thin sheet of paper from the table, and the tornado that uproots trees and wrecks stone buildings, is one of velocity. Obviously, then, moving air is quite a different substance from still air when it comes to dealing with aeronautics. One of the most familiar examples of the lifting power of moving air is that of the kite. An ordinary kite is many times heavier than the air and has no more tendency to rise in the air than a corresponding weight of lead under ordinary conditions. Yet this same kite, if held by a string with its surfaces inclined to the wind at a certain angle, will be lifted with a force proportionate to the velocity of the wind and the size of the surfaces. On a windy day the kite-flyer holding the string and standing still will have his kite pushed upward into the air by the current rushing beneath its surface. On a still day he may accomplish the same thing by running forward with the kite-string, thus causing the surface of the kite to "slide over" the opposing atmosphere. In short, it makes no difference whether the air or kite is moving, so long as the effect of the current rushing against the lower surface is produced. Obviously, then, if in place of the kite-flyer holding the string and running at a certain speed, some kind of a motor could be attached to the kite that would push it forward at a rate of speed corresponding to the speed of the runner, the kite would rise--in short, would be converted into a flying-machine. Looked at in another way, the action of the air in sustaining a body in motion in the air has been compared by Professor Langley to the sustaining power of thin ice, which does not break under the weight of a swiftly gliding skater, although it would sustain only a small fraction of his weight if he were stationary. Supposing, for example, the skater were to stand upon a cake of ice a foot square for a single second; he would sink, let us say, to his waist in the water. On a cake having twice the surface area, or two square feet, he would sink only to his knees; while if the area of the cake is multiplied ten times the original size, he would scarcely wet his feet in the period of a second. Now supposing the cake to be cut into ten cakes of one square foot each, placed together in a line so that the skater could glide over the entire ten feet in length in one second. It is evident that he would thus distribute his weight over the same amount of ice as if the cakes were fastened together in a solid piece.