IN POPULAR PHYSICS. 7 



37. Wliy are we not sensible of the rapid motion of the 

 earth ? 



Because all the objects around us are moving in the same 

 direction with the earth, and there is nothing at hand with 

 which to compare. 



38. A feather is dropped from a balloon which is im- 

 mersed in and swept along by a swift current of air. Will 

 the feather be blown away, or will it appear to a person in 

 the balloon to drop directly doivu ? 



It will seem to drop directly downward, as if in a dead 

 calm. Its fall is vertical, however, only as regards the balloon, 

 and not as regards the earth. 



(See Stewart's Physics, p. 18.) 



39. Suppose a bomb-shell, flying through the air at the 

 rate of 5OO feet per second, explodes into two parts of equal 

 weight, driving one half forward in the same direction as 

 before, but with double its former velocity. What would 

 become of the other half? 



One half will go forward with a double velocity ( 1,000 feet 

 per sec.), and the other half will be checked and will fall directly 



to the ground. 



(See Stewart's Physics, p. 37.) 



40. Wliich ivould have the greater penetrating power, 

 a small cannon-ball with a high velocity, or a large one 

 with a low velocity ? 



The former would penetrate, while the latter would have 

 the greater momentum. . - 



41. There is a story told of a man who erected a huge 

 pair of bellows in the stern of his pleasiire-boat 9 that lie 

 miglit always have a fair ^vind. On trial, the plan failed. 

 In which direction should he have turned the belloivs ? 



In the manner adopted at first, of turning the nozzle toward 

 the sails, the action of the wind against the sails and the reac- 

 tion of the bellows against the boat just balanced each other. 

 If the man had turned the nozzle backward, he could have saved 

 the reaction of the bellows to move the boat. This would, how- 

 ever, have been a costly and bungling mode of navigation. 



