FLIGHT 129 



taken within 48 hours at distances varyiiifr from 300 yards to 

 a mile; but the direction of the wind undoubtedly has a 

 great influence on the distance which they travel. Mosqnitos 

 as a rule do not fly very far, but are frequently carried many 

 miles by a strong wind. On a still day they will not cover 

 more than a few hundred yards. 



The flight of birds is of three kinds. Firstly, by the active 

 stroke of the wings. Secondly, by gliding or skimming, 

 supported on the outstretched wings which do not flap up or 

 down; to glide or skim a bird must have a certain velocity, 

 acquired either by previous strokes of the wings or by slanting 

 down from a height like a parachute or by commencing flight 

 in a wind of some velocity. Thirdly, by sailing or soaring by 

 means of extended wings. This can only take place in a high 

 wind, and is only possible ^vith certain birds. It differs from 

 gliding by the fact that the bird does not lose either in velocity 

 or in vertical position as a result of the resistance of the air 

 to the bird's passage. The muscles which move the wings 

 up or down are very bulky and average about one-sixth of 

 the weight of the entire bird ; but in strong fliers such as the 

 house-pigeon the proportion is 45 per cent, of the total body- 

 weight. Many birds fly immense distances. The swift, for 

 instance, which is but a short summer visitor in Great Britain 

 and begins to disappear early in August, passes its winter 

 in Central Africa. These birds have greatly developed salivary 

 glands, the secretion of which forms the transparent nests 

 so beloved by the Chinese for making soup. Our swallows 

 migrate and winter far beyond the equator in Africa. The 

 curious thing is that the young leave England some weeks 

 before their parents, and without experience and without 

 guidance find their way to their winter home. The Asiatic 

 swallow will get as far as India or Burma and even farther 

 south, occasionally as far as Australia, whilst American 

 species winter in Southern Brazil. The American golden 

 plover breeds in the extreme north of Canada and winters in 

 Southern Brazil and the Argentina. But these great distances 

 are equalled or almost equalled by the larva of the eel, which 

 makes its way unguidcd and untaught from somewhere 

 between Bermuda and the West Indies to the rivers of 



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