962 ON FORM AND MECHANICAL EFFICIENCY [ch. 



amount of arch* or "belly," slight in the rapid fliers, deeper in the 

 slow, flattened in the strong wind, bulging in the gentler breeze; 

 and how advantageous is all possible stiffening of sail or wing, and 

 why accordingly the yachtsman inserts "battens" and the Chinaman 

 bamboos in his sail. We are shewn by Lilienthal himself how a 

 powerful eddy, the so-called "ram," forms under the fore-edge, and 

 is sometimes caught in a pocket of the bird's under wing-coverts 

 and made use of as a forward drive. 



We have lately learned how the gaps or slots between the primary 

 wing-feathers of a crow, and a slight power of the wing-feathers 

 to twist, like the slats of a Venetian blind, play their necessary 



1 



Fig. 457. Ligaments in a swan's wing. 1, 2, 3, remiges; A, B, longitudinal 

 ligaments, with their oblique branches; C, small subcutaneous ligaments. 

 From Marey, after Pettigrew. 



part under certain conditions in the perfect working of the machine. 

 Nothing can be simpler than the mechanism by whi(!h all this -is 

 done. Delicate Ugaments run along the base of the wing from 

 feather to feather, and send a branch to every quill (Fig. 457); by 

 these, as the wing extends, the quills are raised into their places, 

 and kept at their due and even distances apart. Not only that, but 

 every separate ligamentous strand curls a httle way round its feather 

 where it is inserted into it; and thereby the feather is not only 

 elevated into its place, but is given the little twist which brings it 

 to its proper and precise obliquity. 



* On the curvatura veli, cf. J. Bernoulli, Acta Ertidit. Lips. 1692, p. 202. Studied 

 also by Eiffel, Resistance de Vair et V aviation, Paris, 1910. 



