SAFETY DEVICES IN BIRDS ' WINGS GRAHAM 287 



which they overlap. Air pressure from beneath helps these surfaces 

 to engage (the stiff down-curved front webs are not affected by the 

 suction from above), and any further forward movement takes the 

 form of spreading the whole wing, because all the primaries, and to 

 a certain extent the secondaries, are then practically locked together. 



The need for this automatic limit stop to prevent overspreading is 

 strong evidence that the final stages of the expanding of a wing, at 

 any rate a slotted one, are done by air pressure and not by muscular 

 force, except in so far as the breast muscles are preventing the wing 

 from flapping upwards, or are actually pulling it down, as in flap- 

 ping flight. 



This description of the opening process of the slots in a multislot 

 wing applies in limited degree to a single-slot one. 



VII. THE MULTIPLE WING-TIP SLOT 



One outstanding difference between the multi- and the single-slot 

 wing is that in the former the slots extend right across the wing 

 from front to rear. They must, therefore, serve some purpose addi- 

 tional to that of simply delaying the moment at which the wing 

 surface in rear of them stalls. With the notable exception of the 

 game birds, most of the bigger birds which have a high development 

 of the multislot wing, such as rooks, ravens, eagles, buzzards, etc., 

 are in the habit of soaring, or at least of gliding very slowly if they 

 do not actually soar. As any experienced airman knows, the control 

 of lateral balance becomes increasingly difficult as air speed is re- 

 duced, so one is led to suspect that there may be some connection 

 between slots and lateral control at the low air speeds used by soar- 

 ing birds. 



Think of one of these birds as it glides slowly, with wings set at 

 a comparatively large angle of incidence,^ in order that it may make 

 the best use of the low air speed. If the tips of the wings were solid 

 (i. e., unslotted), and the bird wanted to alter its lateral attitude 

 (put on "bank"), a small change of the incidence of one wing tip 

 would only have the effect of altering the lift slightly on that side 

 and of tilting the bird a little one way or the other; but if the 

 feathers in that wing tip were already lying near the angle of " no 

 lift" (as they would be if the wing were slotted), a small alteration 

 of their incidence would either double the lift they might already 

 be giving, reduce it to nothing, or actually reverse the direction of 

 force and convert it into a downward reaction. In other words, a 

 small movement of the control surfaces of a slotted wing has the 



' Sir G. T. Walker, in his paper on this subject, which appeared in the Journal of the 

 Asiatic Society of Bengal, in 1924, makes out this incidence to be in the region of 28° 

 for a soaring vulture. 



