AERONAUTICS. 



231 



the head of the bird. Now this is just the opposite of what 

 ought to be. Instead of the two wings forming one cone, 

 the base of which is directed forwards, each wing of itself 

 forms two cones, the bases of which are directed backwards 

 and outwards, as shown at fig. 116. 



Fig. 116. 



In this figure the action of the wing is compared to the 

 scuUing of an oar, to which it bears a considerable resem- 

 blance.^ The one cone, viz., that with its base directed out- 

 wards, is represented at xbd. This cone corresponds to the 

 area mapped out by the tip of the wing in the process of elevat- 

 ing. The second cone, viz., that with its base directed back- 

 wards, is represented at qp n. This cone corresponds to the area 

 mapped out by the posterior margin of the wing in the process 

 of propelling. The two cones are produced in virtue of the 

 wing rotating on its root and along its anterior margin as it 

 ascends and descends (fig. 80, p. 149 ; fig. 83, p. 158). The 

 present figure (116) shows the- double twisting action of the 

 wing, the tip describing the figure-of-8 indicated at hefghd 

 ijhl; the posterior margin describing the figure-of-8 indi- 

 cated at p r n. It is in this manner the cross pulsation or wave 

 referred to at p. 148 is produced. To represent the action of 

 the wing tlie sculling oar {a b,xs,c d) must have a small scull 

 (m % q op) working at right angles to it. This follows because 



1 In sculling strictly speaking, it is the upper surface of the oar which is 

 most effective ; whereas in flying it is the under. 



