GLIDING 5 



that its two surfaces must have a space of four 

 feet between them, or else they will interfere with 

 one another ; one will tumble the other's air. It 

 is true that I have seen a triplane with its " planes " 

 at a distance from one another of three feet only. 

 But whatever the minimum interspace, it must be 

 considerable. And yet when birds are flying low 

 over water they do not ruffle its surface. Pelicans 

 flying over the Nile, not more than a foot from the 

 water, so a good observer says, left the surface 

 undisturbed.* Still, I cannot help thinking that 

 the space between the birds and the water was 

 under-estimated. At any rate a pigeon's first 

 strong wing-strokes when he rises scatter straws, 

 dust, and feathers from the ground where he takes off. 

 Enough has now been said to show how effective 

 is the support that the air can give to a rapidly- 

 moving body, and how essential it is that the bird 

 or the aeroplane should be unceasingly moving 

 onward to columns of air that are fresh and 

 undisturbed. 



Lift and Drift. 



It is probable that the power of flight was first 

 attained by terrestrial birds which jumped from 

 tree to tree, their wings aiding them to float through 

 the air. If this floating through the air had been 

 their crowning achievement, they would have done 

 nothing remarkable, for mere gliding is not flight. 

 Since, however, gliding seems to have been their 

 first attainment, just as it has been with human 

 flyers, I shall begin by investigating this compara- 

 tively simple performance. At the outset there 



* Quoted by Marey, Vol des Oiseaux, p. 30. 



