10 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. '27 



aewed at intervals without any satisfactory result, tnough it became clear from 

 repeated failures, that the motive power at command would not suffice, even for 

 a few seconds' flight for models of sufficient size to enable a real study to be made 

 of the conditions necessary for successful flight. 



In these earliest experiments everything had to be learned about the relative 

 position of the center of gravity, and what I have called the center of pressure. 

 In regard to the latter term, it might at first seem that since the upward pres- 

 sure of the air is treated as concentrated at one point of the supporting surface, 

 as the weight is at the center of gravity, this point should be always in the same 

 position for the same supporting surface. This relation, however, is never con 

 stant. How paradoxical seems the statement that, if ab be such a supporting 

 surface in the form of a plane of uniform thickness and weight, suspended at c 

 (ac being somewhat greater than cb) and subjected to the pressure of a wind in 

 the direction of the arrow, the pressure on the lesser arm cb will overpower that 



3 



PlO. 1. Diagram of suspended plane showing position of C. P. 



on the greater arm act We now know, however, that this must be so. and why, 

 but as it was not known to the writer till determined by experiments published 

 later in " Experiments in Aerodynamics," all this was worked out by trial in 

 the models. 



It was also early seen that the surface of support could be advantageously 

 divided into two, with one behind the other, or one over the other, and this was 

 often, thougb not always, done in the models. 



At the very beginning another difficulty was met which has proved a con- 

 stant and ever-increasing one with larger models — the difficulty of launching 

 them in the air. It is frequently proposed by Ihose unfamiliar with this difficulty, 

 in launch the aerodrome by placing it upon a platform car or upon the deck of 

 a steamer, and running the car or boat at an increasing speed until the aerodrome, 



which is free to rise, is lifted by the wind of advance. But this is quite imprac- 

 ticable without means to prevent premature displacement, for the large surface 

 and slight weight renders any model of considerable size unmanageable in the 

 least wind, such as is always present in the open air. It is, therefore, necessary 

 in any launching apparatus thai the aerodrome be held rigidly until the very mo 

 menl of release, and thai instant and simultaneous release from the apparatus 

 be made at all the sustaining points at the proper moment, 



