EFFECT OF WINDS ON THE FLIGHT OF BIRDS. 47 



forwards and upwards as its desire may be. If the flight of a 

 Rook be closely followed by the eye, at a convenient height 

 from the earth, it will be seen to be slightly modulatory," 

 not a perfectly level forward movement as might be thought 

 from cursory observation. This is due to the downward stroke 

 of the wings. 



That gravitation, even aided by a contrary wind of the 

 requisite nature, can alone furnish a motive power, is difficult 

 of belief, though certain species, such as the Herring-Gull, 

 seem to have the power of sustaining themselves in the air for 

 lengthened periods, without visible movements of the wings. 

 Herr Gatke's remarks in his work on " Heligoland," referring 

 to this subject, are very interesting. According to his observa- 

 tions, the above species soars and glides about in any direction 

 it chooses, and in almost all states of the weather. The only 

 times on these occasions when it is seen to use its wings is 

 when an individual darts down to the water to pick up food ; 

 two or three rapid strokes seem then to be necessary to enable 

 it to ascend to its former height. How a flight of this nature 

 is accomplished is most puzzling, and observations to be of 

 any value must be constantly carried on. After years of study 

 Herr Gatke has come to the conclusion that birds are 

 possessed of some motive power the nature of which we are 

 as yet quite ignorant. 



Perhaps the flight of wounded birds may throw some light 

 on the question of gravitation being the motive power of a 

 bird's flight. It often happens that a Partridge or other 

 species, when shot, has still sufficient strength to keep its 

 wings expanded for some considerable time ; it may also be 

 travelling at a high rate of speed when hit, but this is soon 

 lost, and it falls to the earth as soon as its impetus is 

 exhausted. Now in such cases, if gravitation supplied the 

 motive power of flight, and the wings being spread, this speed 

 ought not only to be maintained for some time but the bird 

 ought also to be able to maintain itself in the air. We should 

 expect as a natural consequence a tendency on the part of a 

 wounded bird to fall, through the shock, in spite of its desire to 

 escape, but the very act of falling should increase its speed, 

 and the expanded wings should still keep the bird afloat. As 



