NO. 3 LAN'GI.F.Y MEMOIB ON T MECHANICAL FLIGHT 87 



its twisting, was greatly reduced by the manner in which the frame was put 

 together, the whole construction permitting a return to the system of wire guy 

 Lng at first adopted, which had been found inapplicable to a wing having but a 

 single longitudinal rib forming its front margin. When completed, the wings 

 were strongly guyed with piano wire, both above and below, to guy-posts at- 

 tached to the midrod, and each cross-rib was separately guyed with wire chords. 

 Although these wings had cost much in time and labor, and contained many 

 points of improvement, they were eventually found to be too weak to support 

 the aerodrome, and were therefore abandoned without a trial in the field. 



For the plane horizontal tail hitherto used a pair of curved wings was sub- 

 stituted, similar in all respects to those just described, but having only half 

 their area, and these were later replaced by a pair equal in size and in every 

 way the counterpart of the front wings. The tail as hitherto used accordingly 

 disappeared, and gave place to another having a wholly different function to 

 perform; for while the old tail, like the rear pair of wings which superseded 

 it, was intended to bear a definite part of the weight of the aerodrome, the new 

 tail which was now added behind the rear pair of wings was not supposed to 

 bear any part whatever of the weight, but to act solely as a guide, and this new 

 feature, first introduced in October, 1895, was continued to the end. 



This arrangement of the surfaces is quite different from that adopted by 

 Penaud in 1872, in which the tail became automatic in its action through its 

 small angle of elevation as compared with that of the wings, while still acting 

 as a supporting surface, whereas in the present arrangement the function of 

 the tail was solely one of guidance. This, I believe, was one of the important 

 changes which perhaps as much as any other led to final success. 



During the fall of 1895 a large number of experiments were made both in 

 free flight with gliding models, and in constrained flight with the whirling-table, 

 to determine the relative lifting power of the front and rear wings per unit of 

 area, and from these the following new rules were deduced for finding the cen- 

 ter of pressure: 



If a following wing is the size of the leader, assume that its efficiency is 66 

 per cent per unit of surface. 



If it is half the size of the leader, assume that its efficiency is 50 per cent 

 per unit of surface. 



If it is half as large again as the leader, assume that its efficiency is 80 per 

 cent per unit of surface. 



For intermediate sizes of surface, proportionate values per unit of surface 

 may be assumed. 



If we consider the area of the front wing to be unity, and that of the rear 

 wing to be n, and if m be the efficiency of the rear wing per unit of surface, 



