FORM OF THE BIRD. 



217 



under the conditions of stable equilibrium wliich we have 

 just described. Mons. J. Pline has carefully studied the 

 different ki»nds of sliding movement which may take place ; he 

 has even represented them by means of small pieces of appa- 

 ratus which imitate the insect or the bird when they fly 

 without moving their wings. 



If we take a piece of paper of a square form, and fold it in 



7 



Fig. 90. — Representing, on the left, a contrivance intended to imitate the 

 hovering of birds; it is placed in equilibrium by two equal weights 

 attached to the extremities of a wire which is fixed in the lower part of 

 the angle formed by the folded paper. This piece of apparatus falls verti- 

 cally, as shown by the successive positions of the wire when attached to 

 the two weights. On the right is seen the same contrivance connected 

 with one w^eight only. Its fall is parabolic, as shown by the dotted 

 trajectory. 



the middle, so as to form a very obtuse angle (fig. 90) ; then, 

 at the bottom of this angle, let us fix with a little wax a 

 piece of wire attached to two masses of the same weight ; we 

 shall have a system which will maintain stable equilibrium 

 in the air. If the centre of gravity pass exactly through the 

 centre of the figure, we shall see it descend vertically when 

 we let it go, the convexity of its angle being directed down- 

 wards. 



If we take away one of the weights, so as to alter the 

 position of the centre of gravity, the apparatus, instead of 

 descending vertically, will follow an oblique trajectory, and 

 will glide through the air with an accelerated motion (fig. 90, 

 to the right). 



