520 



THE xMECHANICAL FUNCTIONS. 



would have done by resting upon both ; because 

 in the latter case, the weight of the body, being 



divided between them, does not stretch the ten- 

 dons sufficiently. In this position the bird not 

 only sleeps in perfect security, but resists the im- 

 pulse of the wind and the shaking of the bough. 



The great length of the toes of birds enables 

 them to stand steadily on one leg ; and in this 

 attitude many employ the other foot as a hand ; 

 especially Parrots, whose head is too heavy to be 

 readily brought to the ground. Some birds, which 

 frequent the banks of rivers, are in the practice of 

 holding a stone in one foot, while they rest upon 

 the other : this contributes to increase their stabi- 

 lity in two ways, first, it adds to the weight of 

 the body, which is the force that stretches the ten- 

 dons, and causes them to grasp the bough ; and, 

 secondly, it also lowers the centre of gravity. 



The Stork, and some other birds belonging to 

 the same order, which sleep standing on one foot, 

 have a curious mechanical contrivance for locking 

 the joint of the tarsus, and preserving the leg in a 



