THE SLING, BOW, AND OTHER WEAPONS 139 



This gun could not be aimed with accuracy, but was merely 

 pointed in the general direction of the enemy. From this primitive 

 arm to the modern high-powered rifle or the great coast-defense 

 guns is a far cry, and yet the steps have been merely improvements 

 on the primitive weapons, not the application of new principles. 

 The force exerted on the bullet in the gun is that of the elas- 

 ticity of the gases formed when the powder is burned. Gun- 

 powder consists of a mixture of several solids; charcoal, sulphur, 

 and saltpeter have been the ones most commonly used. When 

 charcoal burns it unites with the oxygen gas in the air and forms 

 the gas known as carbon dioxide. Saltpeter is a substance 



FIG. 54. The catapult 



containing a large amount of oxygen which it readily gives up. 

 When the gunpowder is exploded the supply of oxygen, to combine 

 with the carbon, is thus obtained, not from the air, but from 

 the solid saltpeter. Sulphur and oxygen also readily unite to 

 form a gas, and they unite at a considerably lower temperature 

 than do carbon and oxygen; the sulphur, therefore, is put into 

 the gunpowder so that it may be readily touched off. When heat 

 is applied to the gunpowder the oxygen of the saltpeter combines 

 with the sulphur and the carbon to form gases that occupy a 

 large amount of space. The gases formed occupy at atmos- 

 pheric pressures from 300 to 500 times the space occupied by 

 the solid substances of the gunpowder. A small quantity of 



