BIRDS 



151 



impelled forwards ; by the second (/) it is raised upwards, and hence 

 prevented from falling. (Experiment : Construct according to the lower 

 figure an artificial wing 

 by means of a strong 

 rod, E ; a thread, P ; and 

 a piece of cloth, Z. If 

 this apparatus is thrown 

 with its surface verti- 

 cally downwards, it will 

 be found invariably to 

 glide off forwards.) 



(d) How does a bird 

 steer its course through 

 the air ? — The tail, with 

 its long tail-feathers 

 (rectrices) forms (in ad- 

 dition to the wings) the 

 rudder of our living 

 air- ship. Inasmuch as 

 the centre of gravity of 

 a bird's body lies some- 

 what behind the shoul- 

 der-joint, a flying bird, by the mere use of its wings, would always assume 

 a slanting position in the air, the posterior portion of the body being 

 somewhat inclined downwards. By depressing the tail-feathers slightly 

 downwards, however, they are forced upwards by the current of air 

 impinging upon them in consequence of the flying movement, so that the 

 body comes to assume a horizontal position. By depressing the rectrices 

 further downwards the posterior portion of the body is still further 

 elevated, the anterior portion being depressed, so that the bird now 

 comes to fly obliquely downivards. 



In ascending, the muscles of the tail relax their hold upon the tail- 

 feathers, which thus come to lie in continuation with the longitudinal 

 axis of the body. This causes the posterior portion of the body to sink 

 downwards, whereby the bird is made to rise obliquely ujiwards. (Experi- 

 ment : Take a child's paper-arrow and bend the .end of it downwards. 

 Now throw it horizontally, and it will at once fall to the ground in an 

 oblique direction.) 



When a bird flying in a direct line forwards wishes to wheel about to 

 the right, it draws up the right wings to some extent, and strikes the air 

 more forcibly with the left wing. This causes the left side of the body 

 to be propelled further than the right, and the whole body accord- 



