SCIENTIFIC READINGS. 129 



392. Describe the wedge, and the advantage obtained 

 from it. 



393. Which of the other mechanical powers does the 

 screw resemble ? 



394. What is on an average allowed for friction? 



395. What bodies produce the least friction? 



396. Required the advantage obtained from the screw. 

 397- The ancients used to form their armies into the 



shape of one of the mechanical powers, required which, 

 and why ? 



398. To which of the mechanical powers are needles, 

 knives, and spades referred ? 



399. What degree of velocity will a body acquire in 

 descending an inclined plane ? 



400. Of what use is friction in machines ? 



401. If the length of a wedge be two feet, and the 

 breadth or thickness be six inches, what will be the ad- 

 vantage obtained ? 



402. If the distance from the fulcrum to the moving 

 power in the human arm (a lever of the third kind) be 

 one inch and a half, what will be the actual weight sus- 

 tained by holding a body of Jive pounds weight in the 

 hand with the arm extended, the length of the arm, from 

 the elbow to the hand, being 18 inches ? 



403. If a man can exert the force of one hundred 

 weight on an inclined plane, what weight will he be able 

 to raise, if the length of the plane be to its height as 

 twelve to three ? 



404. If the distance between the threads of a screw be 

 one eighth of an inch, and the radius formed by the 

 lever be three feet, what power of compression may a 

 person effect, who can use the force of one hundred 

 weight, deducting one-third for friction ? 



405. In a lever twelve feet long, if the distance be- 

 tween the fulcrum and the weight to be raised be one 

 foot, what power will be sufficient to raise three quar- 

 ters of a ton ? 



406. If a man has to raise a weight of 10 tons one 

 foot, with what length of inclined plane will he be able 

 to effect it by using a force of 160 Ibs. ? 



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