46 THE SCREW. 



All machines are composed of one or more of the six mc-^j 

 chanical powers which we have examined. Their force isl 

 diminished in a considerable degree by friction, by which is ] 

 meant the resistance with which bodies meet in rubbing i 

 against each other. There is no such thing as perfect smooth- i 

 ness or evenness in nature : polished metals, though they I 

 wear that appearance more than any other bodies, are far \ 

 from possessing it in reality, and through a good magnifying- ^ 

 glass their inequalities may frequently be perceived. When j 

 the surfaces of two bodies, therefore, come into contact, the \ 

 prominent parts of the one will often fall into tlie hollow ^ 

 parts of the other, and occasion more or less resistance to 1 

 motion. Friction is usually computed to destroy one third ' 

 of the power of the machine. The application of oil lessens i 

 friction, because it acts as a polish by filling up th&" cavitiesi 

 of the rubbing surfaces, and thus making them slide over \ 

 each other the more easily. There are two kinds of friction, l| 

 the one occasioned by the sliding of the flat surface of a body, '] 

 and the other by the rolling of a circular body. The re- : 

 sistance resulting from the first is much the most considera- ] 

 ble; whilst in the latter the rough parts roll over each other | 

 with comparative facility ; hence it is that wheels are often \ 

 used for the sole purpose of diminishing the resistance of < 

 friction. The power of a machine is considerably affected j 

 by the resistance of the air. ] 



In all machines what is gained in power is lost in time. If ; 

 a man can raise, fty a single fixed pulley, a beam to the top \ 

 of a house in two minutes, he will be able to raise six such ; 

 beams in twelve minutes; but with six pullies, the three | 

 lower ones being moveable, he will raise six beams with the '• 

 same ease at once ; but he will be six times as long about it, 

 that is, twelve minutes, because his hand will have six times j 

 as much space to pass over. One capital advantage in the 

 mechanical powers is, that if the six beams were in 9ne '\ 

 piece, it might be raised at once, though it would be impos- j 

 sible to move it by the unassisted strength of a single man. ! 



Questions. — 1. What is the wedge ? 2. The advantage gained ; 



by it .'' 3. In what does its great use lie ? 4. What is said of inslru-  



ments ? 5. A saw ? G. A knife ? 7. How are mill-stones obtained in | 



Derbyshire ? 7. What is the screw ? 8. What is the advantage i 



fained by it ? 9. How may the screw be conceived to be made ? 10. ^ 

 or what uses is it employed .'' 11. What is friction 1 12. What part 



of a machine's power does friction destroy.' [Note. If 60 pounds « 



