86 WELLS'S NATURAL PHILOSOPHY. 



11. 'When TWO PEB80N8 BTHiKE their BEADS together, one being In motion and the other 

 «t BEST, why are both equally hurt ? 



Because, when bodies strike each other, action and reaction are equal ; the 

 head that is at rest returns the blow with equal force to the head that 



strikes. 



12. When an elastic baxi. la thrown against the side of a house with a oebtaim foboi!, 

 why does it rebound t 



Because the side of the house resists the ball with the same force, and the 

 ball being elastic, rebounds. 



13. When the same ball is thrown against a pans of glass with the same force, it goes 

 through, breaking the glass ; why does it not rebound as before ? 



Because the glass has not sufficient power to resist the full force of the ball: 

 it destroys a part of the force of the ball, but the remainder continuing to act, 

 the ball goes through, shattering the glass. 



14. Why did not the man succeed who undertook to make a faib wind for his pleas- 

 CEE-BOAT, by erecting an immense bellows in the stebn, and blowing against the bails f 



Because the action of the stream of wind and the reaction of the sails were 

 exactly equal, and, consequently, the boat remained at rest. 



15. If he had blown in a contsaby dibection from the sails, instead of against them, 

 would the boat have moved ? 



It would, with the same force that the air issued from the bellows-pipe. 



16. Why can not a man raise himself over a fence by pulling upon the stuapb of his 



BOOTS? 



Because the action of the force exerted by the muscles of his arms is coun- 

 teracted by the reaction of the force, or, in other words, the resistance of his 

 whole body, which tends to keep him down. 



17. Why do watee-dogb give a bemi-eotaby movement to free themselvcB from 

 water f 



Because in this way a centrifugal force is generated, which causes the drops 

 of water adherent to them to fly off. 



18. Why is the coDBSS of rivers rarely BTB.vionT, but bebpenttne and windino? 

 "When, from any obstruction, the river is obliged to bend, the centrifugal 



force tends to throio aivay the tuater from the center of the curvature, so that 

 when a bend has once commenced, it increases, and is soon followed by others. 

 Thus, for instance, the water being thrown by any cause to the left side, it 

 wears that part into a curve, or elbow, and, by its centrifugal force, acts con- 

 etantly on the outside of the bend, until the rock, or lugher land, resists ita 

 gradual progress ; from this limit, being thrown back again, it wears a simUat 

 bend to the right band, and after that another to the left, and so on. 



19. A locomotive passes over a railroad, 200 miles in length, in 5 hours ; what is ita 

 velocity per hour ? 



20. If a bird, in flying, passes over a distance of 45 miles in an hour, what is its ve- 

 locity per minute ? 



21. The flash of a cannon three railos off w.is seen, and in 14 seconds afterward the 

 sound was heard. How many feet did the sound travel in one second f 



