NO. 8 SAMUEL PIERPONT LANGLEY ABBOT 49 



its launching appliances until some assurance could be had of the 

 final success of the engine 



" It was recognized from the very beginning that it would be desir- 

 able in a large machine to use ' superposed ' sustaining surfaces 

 (that is, with one wing above another) on account of their supe- 

 riority so far as the relation of strength to weight is concerned, 

 and from their independence of guy wiring ; and two sets of super- 

 posed sustaining surfaces of dififerent patterns were built and experi- 

 mented with in the early tests. These surfaces proved, on the whole, 

 inferior in lifting power, though among compensating advantages 

 are the strength of a bridge construction which dispenses with guy 

 wires coming up from below, which, in fact, later were the cause of 

 disaster in the launching. 



" It was finally decided to follow what experiment had shown to be 

 successful, and to construct the sustaining surfaces for the large 

 machine after the ' single-tier ' plan. This proved to be no easy task, 

 since in the construction of the surfaces for the small machines the 

 main and cross ribs of the framework had been made solid, and, 

 after steaming, bent and dried to the proper curvature, while it was 

 obvious that this plan could not be followed in the large surfaces on 

 account of the necessity, already alluded to, of making them rela- 

 tively lighter than the small ones, which were already very light. 

 After the most painstaking construction, and tests of various sizes 

 and thicknesses of hollow square, hollow round, I-beam, channel, and 

 many other types of ribs, I finally devised a type which consisted of 

 a hollow box form, having its sides of tapering thickness, with the 

 thickest part at the point midway between contiguous sides and with 

 small partitions placed inside every few inches in somewhat the same 

 way that nature places them in the bamboo. These various parts 

 of the rib (corresponding to the quill in a wing) were then glued and 

 clamped together, and after drying were reduced to the proper dimen- 

 sions and the ribs covered with several coats of a special marine 

 varnish, which it had been found protected the glued joints from 

 softening, even when they were immersed in water for twenty-four 

 hours. 



" Comparative measurements were made between these large cross 

 ribs, II feet long, and a large quill from the wing of a harpy eagle, 

 which is probably one of the greatest wonders that nature has pro- 

 duced in the way of strength for weight. These measurements 

 showed that the large, ii-foot ribs (' quills ') for the sustaining sur- 

 faces of the large machine were equally as strong, weight for weight. 



