34 EXPERIMENTS IN AERODYNAMICS. 



the plane is of perhaps equal importance, and its action has been present to my 

 mind throughout these experiments, although for the purpose of concise exposition 

 only the former is here referred to. By analogous reasoning in the case of a 

 heavy body immersed in any continuous fluid, even gaseous, while the mass 

 of air or gas whose inertia is called into action is small and affords a slight 

 sustaining power when the body is at rest, it becomes greatly multii^lied with 

 lateral motion, and the more rapid this lateral motion, the greater will be the 

 sustaining action of the fluid. So, then, in the case of any heavy body which 

 will fall rapidly in the air if it fall from rest, the velocity of fall will be more 

 and more slow if the body be given successively increasing velocities of lateral 

 translation and caused to run (so to speak) upon fresh masses of air, resting but 

 a moment upon each. 



The above analogy, in spite of its insufficiency as regards the effect of elas- 

 ticity, is useful, and may be further extended to illustrate the relative results 

 obtained Avith the difterently shaped planes and with the same plane under 

 different " aspects ; " thus the action on the air of a plane whose advancing edge 

 is twice its lateral edge — e. g., the 12x6 inch plane, with 12-inch side foremost — 

 may be compared to that of two skaters side by side, each advancing over his own 

 lines of undisturbed ice ; but the same plane with the 6-inch side foremost, to the 

 same skaters, when one is behind the other, so that the second is passing over ice 

 which has already yielded to the first and is partly sinking. 



The second series of experiments, made on the same dates as the first, was to 

 cover the third object of experiment — that is, to determine for different angles of 

 inclination what speed is necessary in order to derive an upward thrust just 

 sufficient for sustaining the planes. 



The results of these two series of experiments furnish all that is needed to 

 completely elucidate the proposition that I first illustrated by the suspended 

 plane, namely, that the effort requii'ed to support a bird or flying machine in the 

 air is greatest when it is at rest relatively to the air, and diminishes with the 

 horizontal speed which it attains, and to demonstrate and illustrate the truth of 

 the important statement that in actual horizontal flight it costs absolutely less 

 power to maintain a high velocity than a low one. It has already been explained 

 that when the planes have such an angle of elevation and such a horizontal 

 velocity that they first rise from their support and are then with a slightly 

 diminished velocity just sustained without falling, they are said to "soar," and 

 the corresponding horizontal velocity is called "soaring speed." Attention has 

 already been called to the importance thus attachable to the word " horizontal " 

 as qualifying flight, and implying its most economic conditions, when no useless 

 work is expended. 



