TRAVELING AT HIGH SPEEDS HELE-SHAW. 631 



nil. To understand the matter we must study the nature of animal 

 locomotion. The surface of the earth is rough, sliding along it being 

 obviously out of the question; nature has made provision for animal 

 movement as follows : One part of the body first rests on the ground, 

 another part supported by this is advanced, being raised clear of the 

 ground, to rest in turn upon the ground and serve in turn as a support, 

 so that the part behind may be raised and advanced to a fresh posi- 

 tion. In man and other animals the feet form the points of support 

 for this process; but the same method of locomotion is employed by 

 creatures without feet, which have to crawl or glide, such as snakes 

 or worms. 



This process, whether with animals or reptiles, as you will see, 

 involves in the raising of the body an expenditure of work which is 

 not recovered, and further an expenditure of work in stopping and 

 starting some portion of the body in its movements. My assistant 

 now walks in front of the blackboard holding a piece of chalk level 

 with his head , and you will see the rising and falling motion. I have 

 prepared a wooden model to represent the action of his legs, and you 

 will see that these legs, being equal to his in length, produce almost 

 exactly the same curve underneath, so that you have a complete 

 explanation of this movement, viz, the rotation of the hip about the 

 ankle as a pivot. There is a third case of loss, namely, the energy 

 involved in swinging the legs. About 30 years ago the distinguished 

 French professor, Marey, actually investigated the loss involved from 

 each of these three causes, and I have on the wall a diagram in which 

 you will see all three given graphically. The number of steps per 

 minute, you will notice, increases until a pace is reached when it 

 becomes painful to walk faster, and you will also notice from the 

 diagram that at about 90 steps per minute the gait changes to a run — 

 that is to say, a springing action takes place, the hind foot leaving 

 the ground before the front is put down upon it. 



I have another diagram showing how the length of stride at first 

 increases with the pace, and afterwards begins to fall off before the 

 walking breaks into a run. The reason why a man or an animal 

 changes his pace at this point is obvious, and it is because a faster 

 speed is possible with a less effort. As the speed of running is 

 increased the total effort becomes greater, but the three elements 

 shown on the diagram are differently divided; the rise and fall ele- 

 ment is less, but the work done in swinging the legs is more, while the 

 chief element, in the muscular effort expended, is the loss of energy 

 involved in stopping and starting as each spring reaches a maximum. 

 Time does not permit me to pursue this interesting subject further 

 except to point out that exactly similar causes operate in the natural 

 locomotion of other animals which move on legs. 



