INTRODUCTION. 



9 



inclined planes or kites. When an insect, a bat, or a bird 

 is launched in space, its weight (from the tendency of all 

 bodies to fall vertically downwards) presses upon the inclined 

 planes or kites formed by the wings in such a manner as 

 to become converted directly into a propelling, and indirectly 

 into a hmying or supporting power. This can be proved by 

 experiment, as I shall show subsequently. But for the share 

 which the weight or mass of the flying creature takes in flight, 

 the protracted journeys of birds of passage would be impos- 

 sible. Some authorities are of opinion that birds even sleep 

 upon the wing. Certain it is that the albatross, that prince 

 of the feathered tribe, can sail about for a whole hour without 

 once flapping his pinions. This can only be done in virtue 

 of the weight of the bird acting upon the inclined planes or 

 kites formed by the wings as stated. 



The weight of the body plays an important part in walking 

 and swimming, as well as in flying. A biped which advances 

 by steps and not by leaps may be said to roll over its extre- 

 mities,^ the foot of the extremity which happens to be upon 

 the ground for the time forming the centre of a circle, the 

 radius of which is described by the trunk in forward motion. 

 In like manner the foot which is off the ground and swinging 

 forward pendulum fashion in space, may be said to roll or 

 rotate upon the trunk, the head of the femur forming the 

 centre of a circle the radius of which is described by the ad- 

 vancing foot. A double rolling movement is thus established, 

 the body rolling on the extremity the one instant, the extre- 

 mity rolling on the trunk the next. During these movements 

 the body rises and falls. The double rolling movement is 

 necessary not only to the progression of bipeds, but also to 

 that of quadrupeds. As the body cannot advance without 

 the extremities, so the extremities cannot advance without 

 the body. The double rolling movement is necessary to con- 

 tinuity of motion. If there was only one movement there 

 would be dead points or halts in walking and running, similar 

 to what occur in leaping. The continuity of movement neces- 

 sary to progression in some bipeds (man for instance) is fur- 



1 This is also true of quadrupeds. It is the posterior part of the feet 

 which is set down first. 



2 



