8 FIRST PRINCIPLES OF CONFORMATION. 



ately as slender as the}^ are long, and that those exhibiting 

 great strength are relatively thick and short. 



Muscles are the active and essential part of the 

 machinery used by animals in locomotion, bones being 

 merely passive agents. In fact, there are myriads of 

 the lower animals which move about with considerable 

 speed by means of their muscles, but which have no bones 

 of any kind. 



Professor Marey points out in his book, La Machine 

 Animale, that the thickness of a muscle is proportionate, 

 as a rule, to its power, and that its length is proportion- 

 ate to the extent of movement which it is able to produce. 

 He gives, as instances,, the long and narrow breast-bones 

 of birds — such as the pheasant (Fig. 12) — which can move 

 their wings through very large angles ; and the short 

 and broad ones of birds — such as the albatross (Fig. 13) 

 — which can move them only through relatively small 

 angles. The former are therefore able to work their wings 

 with great rapidity ; and the latter can overcome the 

 immense resistance of the air upon which the large 

 area of their pinions presses, only with slow, but very 

 powerful strokes. The expanse of the outspread wings 

 of birds of quick-stroke is of far less comparative size 

 than is that of those large-winged birds. The relative 

 speed with which birds can cleave the air does not, 

 of course, affect the question of the form and action of 

 their muscles. 



Endurance. — From a general point of view, endur- 

 ance (" staying power "), whether in the exhibition of 

 speed or strength, depends on the amount of force 

 (energy) which the system can supply. 



In a steam locomotive, movement is obtained by a series of 

 changes in the force which is contained in the fuel, and which is 

 the chemical affinity (chemical attraction) that the elements of the 

 fuel have for the oxygen of the air. The first change in this series 

 is the conversion of the chemical affinity into heat. For instance, 



