II] OF GLIDING FLIGHT 49 



Langley, Lilienthal and the Wrights — all careful students of birds — 

 renewed the attempt*; and only after the Wrights had learned to 

 ghde did they seek to add power to their glider. Flight, as the 

 Wrights declared, is a matter of practice and of skill, and skill in 

 gliding has now reached a point which more than justifies all 

 Leonardo da Vinci's attempts to fly. Birds shew infinite skill and 

 instinctive knowledge in the use they make of the horizontal accelera- 

 tion of the wind, and the advantage they take of ascending currents 

 in the air. Over the hot sands of the Sahara, where every here 

 and there hot air is going up and cooler coming down, birds keep 

 as best they can to the one, or ghde quickly through the other; 

 so we may watch a big dragonfly planing slowly down a few feet 

 above the heated soil, and only every five minutes or so regaining 

 height with a vigorous stroke of his wings. The albatross uses the 

 upward current on the lee-side of a great ocean- wave ; so, on a lesser 

 scale, does the flying- fish; and the seagull flies in curves, taking 

 every advantage of the varying wind- velocities at different levels 

 over the sea. An Indian vulture flaps his way up for a few laborious 

 yards, then catching an upward current soars in easy spirals to 

 2000 feet ; here he may stay, effortless, all day long, and come down 

 at sunset. Nor is the modern sail-plane much less efiicient than a 

 soaring bird ; for a skilful pilot in the tropics should be able to roam 

 all day long at willf . 



A bird's sensitiveness to air-pressure is indicated in other ways 

 besides. Heavy birds, hke duck and partridge, fly low and ap- 

 parently take advantage of air-pressure reflected from the ground. 

 Water-hen and dipper follow the windings of the stream as they fly 

 up or down; a bee-hne would give them a shorter course, but not 

 so smooth a journey. Some small birds — wagtails, woodpeckers and 

 a few others — fly, so to speak, by leaps and bounds ; they fly briskly 



* Sir George Cayley (1774-1857), father of British aeronautics, was the first to 

 perceive the capabilities of rigid planes, and to experiment on gliding flight. He 

 anticipated all the essential principles of the modern aeroplane, and his first paper 

 "On Aerial Navigation" appeared in Nicholson's Journal for November 1809. 

 F. H. Wenham (1824-1908) studied the flight of birds and estimated the necessary 

 proportion of surface to weight and speed; he held that "the whole secret of 

 success in flight depends upon a proper concave form of the supporting surface." 

 See his paper "On Aerial Locomotion" in the Report of the Aeronautical Society 

 1866. 



t Sir Gilbert Walker, in Nature, Oct. 2, 1937. 



