502 NATURE'S TEACHINGS. 



collar is connected with the safety-valve. Thus, if the engine 

 should be working beyond its proper powers, the Governor 

 draws up the collar, and releases sufficient steam to take the 

 undue pressure off the boiler. Thus the engine may be left, 

 so to speak, to manage itself. 



NEXT are shown two examples of Centrifugal Force as 

 applied in ancient warfare, namely, the Sling, which is now 

 retained merely as a boy's toy, and the Amentum, which was 

 practically a sling attached to a spear. Both weapons have 

 been superseded by the modern firearms, but the Sling is 

 really a more formidable offensive weapon, in skilful hands, 

 than is generally suspected. 



A good slinger is as sure of his aim as a good rifleman, and 

 can send his missile to a wonderful distance. Were I to be 

 armed with the best pistol hitherto invented, I should be sorry 

 to fight an accomplished slinger, unless under cover. 



The really tremendous power of the Sling is obtained by 

 Centrifugal Force, the weapon, with its missile, being whirled 

 in the air, and then one string being loosed with a peculiar 

 knack something like the " loose " of a good archer. In 

 consequence, the centrifugal force is converted into direct 

 force, and the missile flies directly forwards. 



The Amentum is simply a cord tied to a javelin, so that the 

 thrower has the advantage of a lever, which, after all, is only 

 the conversion of centrifugal force. 



The very familiar Mop, flinging off its moisture to a con- 

 siderable distance, needs no description ; but I have introduced 

 it to show the action of centrifugal force in small as well as 

 in great things. 



THE next illustration shows how this very same power acts 

 upon the greatest as well as the least of objects, and enables them 

 to maintain positions which otherwise they must of necessity fail 

 to do. Take, for example, our own Earth, and its peculiar 

 position of being tilted on one side, so as to give us the alter- 

 native seasons as it flies on its annual course. 



This is simply due to its own rapid revolution, which, on 

 the same principle that keeps the arrow and the rifle-ball 

 straight on their course, prevents it from altering its position. 



