138 OUR PHYSICAL WORLD 



of their arrangement remains constant. When this arrangement 

 is disturbed the elastic body tends to resume its normal condition 

 the moment the strain is removed and rebounds with as great a 

 force as was applied to produce the distortion. Similarly, gases are 

 made up of molecules much more widely spaced than those of solids 

 or liquids, and these molecules are moving in relatively wide 

 pathways with still greater speeds than those of solids. When the 

 gas is compressed or crowded into smaller space, moving mole- 

 cules repel each other more forcefully and hit the sides of the 

 container much more frequently, because there are more of them 

 moving in a given space, and so they exert upon the walls of the 

 container an ever increasing pressure. Gases exhibit elasticity 

 to perfection. 



Various engines of war from the days of the primitive bow- 

 men to the present have largely depended upon this property of 

 elasticity for their efficiency. A device in use by ancient people 

 was the catapult (Fig. 54). It consisted of a heavy, inclined 

 plank with one end fixed firmly in a framework, the other free 

 to move, and levers and pulleys so mounted that the free end 

 of the plank could be pulled back until it was bent like one end 

 of a huge bow. A great rock was then placed on this end, 

 which was suddenly released, throwing the missile at the enemy. 

 The huge plank was bent back by the labor of many men 

 working with levers or windlass for considerable time, and this 

 energy stored in the bent plank was suddenly released to act 

 upon the rock. 



These ancient engines of war were replaced by the gun when 

 powder was introduced into Europe from China. Just when it 

 was discovered there is not known, but old pictures of naval 

 engagements show the vessels obscured in clouds of smoke, 

 presumably made by the firing of guns many centuries prior to 

 the introduction of powder into Europe. This event occurred 

 in the fourteenth century. The early gun was a metal tube on 

 the end of a straight stick. The powder was touched off through 

 a small hole in the base of the tube by means of a lighted stick. 



