In Fig. 1, let S represent a steamship going eastward at the rate of twenty miles per hour; W the wind blowing westward at the rate of twenty miles per hour; A a gull near the water’s surface, with momentum which for the instant gives him an eastward velocity of twenty miles per hour. While the bird’s momentum lasts it holds him firmly against the wind. At the point A the bird inclines his wings so that the wind strikes them on the under side, and he is lifted and lifted until, at the point B, his momentum is so reduced that he must tack; then he gives to the wind the thin edge of his wings and slides down to the point C, and then, with velocity regained, he repeats the manœuvre. Altitude sacrificed becomes velocity or momentum, and momentum sacrificed becomes altitude. In this description of the gull’s soaring to windward, the movement is reduced to its simplest elements, and it leaves out of account the graceful sinuosity of the bird’s airy travels, just as the teacher of dancing leaves grace out of account when she teaches the beginner the elements of the steps. [Illustration: _FIG. 1_] * * * * * What has here been said about the storage of energy in weights, and concerning the elements of flight, is all intended to lead up to the important subject of sliding freight downhill upon aeroplanes. It may be asked, How about a calm? There is no calm for the aeroplane. Give it altitude and it can gain velocity, and velocity gives the _wind of flight_. The plan for the transportation of freight is simply this: at each shipping-point a power-house (D, Fig. 2) may be established to operate captive balloons. These should be cellular, and should be made to hold gas with little waste. In its action the apparatus would be what might be called an inverted elevator; that is, the steam or water-motor in the power-house would not hoist the freight, but, instead, would pull the balloon down after _it_ had hoisted the freight and discharged it by means of a soaring machine, which will presently be described. [Illustration: _FIG. 2._] In Fig. 2 A represents a captive balloon at a height of one thousand feet. B and C represent the courses which would be taken by dirigible aeroplanes or soaring machines bearing loads of freight. Perhaps this seems fanciful. Then let it be remembered that the feat of safely sliding down a long and gentle incline upon an aeroplane has already been performed by Otto Lilienthal, of Steglitz, Prussia. His experiments were illustrated and described in the Berlin Illustrirte Zeitung of Oct. 7, 1893, and one of the drawings--all of which were correctly made from instantaneous photographs--is here reproduced on the first page of cover. An improvement upon Lilienthal’s device may be made by adding a pendulum.[1] [1] See U. S. Letters Pat. No. 376937.