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 1s 
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 @ maximum. 
Time does not permit me to pursue this interesting subject steer 
except to point out that exactly similar causes operate In the natura 
locomotion of other animals which move on legs. 
