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 Orseaux, p. 30. 
