The Power of Flight Possessed by Birds 85 



wings and then forcibly biinging down the spread wings 

 — must be incessantly repeated during the flight of every 

 bird, in the same manner as a. swimmer, by pressing the 

 water downwards with his spread hands, keeps himself 

 afloat, and by directing the motion obliquely backwards is 

 thereby pushed forward. It may also be remarked that 

 the swimmer raises his hands before renewing the 

 stroke with the fingers closed, slanting, in a similar way to 

 the bird raising its folded wings, so as to diminish the sur- 

 face opposed to the resisting medium. 



When birds fly horizontally, their motion is not in a 

 straight line, but obliquely upwards, and they allow the 

 body to come down to a lower level before a second stroke 

 is made by the wings, so that they move in a succession 

 of curves. To ascend obliquely, the wings must repeat 

 their strokes upon the air in quick succession, and in des- 

 cending obliquely these actions are proportionally slower. 



In birds of prey the form of the wings is very oblique, 

 so that they cannot rise in the air perpendicularly unless 

 they fly against the wind. They have, however, a greater 

 power of horizontal motion than other birds, because the 

 extreme parts of the wings are long, and the ends of the 

 feathers lap over each other, which opposes a uniform 

 resistance to the air, while in other birds the air passes 

 through between the feathers, which lessens the power of 

 keeping the wing oblique. To enable themselves to turn 

 to the right or left, they move one wing more rapidly than 

 the other. This is attended with difficulty when the 

 flight is rapid ; they therefore make a large sweep before 

 they can turn round. 



Wonderful Travelling. 



The most wonderful bird flight noted is the migratory 

 achievement of the Virginia plover, which leaves its 

 haunts in North America, and, taking a course down the 

 Atlantic, usually from four to five hundred miles east of 

 the Bermudes, reaches the coast of Brazil in one unbroken 

 flight of fifteen hours, covering a distance of over three 

 thousand miles at the rate of four miles a minute. 



