THE SWIFT'S FOOT. 259 



one direction only, and that is the direction against 

 which the weight of the bird pulls. The tarsal, and 

 also the tibial joints are bent as far as they will bend, 

 when these birds hold on upon the upright surface of 

 a rock or wall, and the articulation of the toes is, at 

 the same time, in contact with that surface. There- 

 fore, if there is the least hold for the points of the 

 claws, the strain upon them is always such as to make 

 them retain that hold the more firmly the greater the 

 strain. 



Upon carefully examining these feet, of which the 

 most perfect is that of the swift, a figure of which will 

 be found in the sequel, it will be perceived that, 

 while the spread of the toes gives great stability to the 

 individual foot, the two are, when the bird hangs, or 

 adheres by means of them, so placed as that the 

 greatest resistance of each is in the direction of the 

 centre of gravity of the whole bird, so that the weight 

 actually draws them into closer contact with the surface 

 on which they have hold. If in common mechanics, 

 it were required to find an instrument consisting of 

 hooks which should be the best for holding on against 

 a strain, parallel or nearly parallel, to the surface on 

 which it held on, the very best model would be the 

 foot of the swift. The pull upon the individual toes, 

 being toward the point of their general articulation, 

 draws them together so that they clutch the surface 

 upon which they have a hold ; and then the strain of 

 the bird's weight pulling upon each foot toward the 

 centre of gravity of the whole body, acting in two 

 lines, which, produced, pass a little outwards of the 

 average hold of the feet, makes the whole bird hold 

 on with the feet, as if it were grasping or embracing a 

 substance between them. 



The goat-suckers, being more woodland birds, have 

 s2 



