PARADOX OF WINDS

Waipa Post, Volume 45, Issue 3211, 2 August 1932, Page 2

PARADOX OF WINDS

BIRDS (IN FREE FLIGHT RIDE THE GALE IN A DEAD CALM.

Alorfe of all creatures birds iim free flight and insects on the wing do not feel a breath of wind that is blowing, no matter liow fierce or tempestuous the wind may be. Birds, insects, and aircraft experience nothing but a, dead calm. A bird or a mosquito feels a draught from right ahead exactly equal in strength to its own proper flight speed. Thus, for example, a seagull with a flight speed of 40 miles an hour feels a draught on its beak and between its eyes of 40 miles an hour, whether the air is still or moving with gale force at 60 miles an hour from ahead, behind, or across the direction of the bird’s flight. In short, air-borne bodies, wether birds, insects, or machines, fly always in a dead calm, but the dead calm is a moving calm. Commander Bernard Acworth, D.5.0., R.N., who has made a study of this phenomenon, writes popular language apropos of the violent gales in England in an average September, when there was a great loss of bird life. , “ Picture the saloon of an ocean liner travelling over the sea at 20 miles an hour, with windows closed, so that the enclosed air is still. Now suppose a fly to leave its perch at one end and to fly at 10 miles an hour toward the other end. What is the fly achieving, and what are its sensations?

“ Taking- sensations first, it will be appreciated that the fly feels a draught of 10 miles an hour from right ahead, and nothing else. It cleax-ly makes no difference to the sensations of the fly whether the enclosed air of the saloon is stationary, as when the liner is secured to a jetty, or moving at 20 miles an hour in any direction when the liner is under way at that speed. “ But what is really happening to the fly when the liner is under way? If the liner is steaming from Liverpool to New York at 20 knots and the fly is flying at 10 knots in the salon and toward the bow, it is making for New York at 30 knots, the liner’s speed of 20 knots, plus the fly’s speed of 10 knots. If, however, the fly continues its flight as before, but the liner alters its course toward Liverpool, the fly still heading at 10 knots for New York, is accomplishng 10 knots backwards to Liverpool (fly’s speed 10 knots minus liner’s speed 20 knots). The fly’s sensations undergo, however, no change whatsoever, for it feels throughout its little aerial voyage a draught of 10 miles an hour between its eyes.

“ Now a fly flying in the enclosed ,

calm of the saloon of a moving liner is the same phenomenon as the flight of any air-borne nody, animate or inanimate in wind. If anyone is disposed to challenge the statement the writer would very respectfully’ invite such a reader to expand the liner’s saloon in imagination into a vast transparent dome 1000 miles in diameter and suspended over the sea or land.

While the dome is stationary—and therefore the air which it encloses—let us release all manner of flying creatures and machines in its depth. Supposing the dome to be stationary, a person standing beneath the dome is thus in still air. Now imagine a giant Aeolus to stalk away with the dome in his hand, as we might carry a birdcage, at a speed of 40 miles an hour. Instantly the person on the fixed land or sea beneath the fabulous dome will feel a wind of 40 m.p.h. from the direction in which the dome is moving. This will be readily admitted.

“ But the birds and machines flying enclosed in the moving dome are as physically unconscious of the movement of the air in whch they are flying and as free from any wind pressure as are passengers in an express train walking through the train from the rear to the front. The enclosed air in our fabulous dome is identical with what we call wind.

“This simple fact of absence B of wind pressure on air-borne bodies sheds a flood of light on three matters of grealt human interest—bird x life, including migration, aviation, and the theory of evoluton—too large for discussion here.’'