THE FOOT OF A SWIMMING BIRD. 73 



Though the loon appears to be a short-legged bird, on account 

 of its very short thigh, we see by the length of the bones 

 that it can swim with a very long stroke. Its large, webbed 

 foot presses back a great amount of water. And the little bony 

 splinter at the front of the knee is a capital device (found only 

 in the grebes and the loons) for quickening the stroke. The 

 tendons fastened to the point of this extension throw the foot 

 forward with great force and quickness, as any boy can see 

 who makes a tip-up with uneven arms, and tries to raise the 

 long arm by strings tied at different places along the short arm. 

 The farther from the central pivot the string is tied the less 

 force will be required to move the arm; and so, the farther 

 beyond the knee joint the pulling tendon is attached, the less 

 force will be required to draw the loon's leg forward into 

 position for the stroke. The short thigh is also an advantage 

 in the backward stroke. 



But the man paddling has one great advantage over the bird. 

 When he has finished his stroke he carries his paddle back 

 through the air, while the bird's foot must return through the 

 water to make its forward stroke. The paddle meets little 

 resistance on the return, but the foot will meet nearly as much 

 as it created on the stroke, unless there is some special remedy. 

 The folding of the foot, which diminishes the surface, meets 

 this difficulty. 



In all web-footed birds the toes fold close together on the 

 return so that the webs do not catch the water ; but in the loon 

 and grebe they are not only arranged to fold one behind the 

 other but are flattened besides, so that they make the least pos- 

 sible resistance. And in both the loon and grebe the tarsus is 

 compressed until it is scarcely thicker than a knife-edge at the 

 back, and cuts the water before it. The last device for speed 

 is the arrangement of the legs at the very end of the body. 



