THE SCREW. 



4^ 



in the proportion of the slant side to half the thickness of 

 the back ; so that if the back of a wedge be two inches 

 thick, and the side twenty inches long, any weight pressing 

 on the back will balance twenty times as much acting ok 

 the sides. But the great use of a wedge lies in its bein^- 

 urged, not by pressure, but usually by percussion, as by the 

 blow of a hammer or mallet; for the momentum of the blow 

 is greater, beyond comparison, than the application of any 

 dead weight, or pressure, such as is employed in the other 

 mechanical powers. Hence it is used in splitting wood 

 and rocks, and even a large ship may be raised to a small 

 height by driving a wedge below it. As all instruments, 

 which slope off to an edge on one side only, may be ex-' 

 plained by the principle of the inclined plane ; so those that 

 decline to an edge on both sides, may be referred to the prin- 

 ciple of the wedge. A saw is a series of wedges, on which 

 the motion is oblique to the resistance. A knife cuts best 

 when it is drawn across the substance which it is to divide ; 

 and the reason is, that the edge of a knife is In reality a very 

 fine saw, and therefore acts best when used like that instru- 

 ment. It is usual in separating large mill-stones from the 

 siliceous sand-rocks, in some parts of Derbyshire, in Eng- 

 land, to bore horizontal holes under them in a circle, and 

 fill these with wedges made of dry wood, which gradually 

 swell as they imbibe moisture, and in a day or two lift up 

 the mill-stone without breaking it. 



The last mechanical power is the screw, which is a kind 

 of perpetual inclined plane, the power of which is still farther 

 assisted by the addition of a handle or lever, where the power 

 acts ; so that the advantage gained is in proportion as the 

 circumference of the circle, made by the handle or lever, is 

 greater than the distance between thread and thread in the 

 screw. The screw may be conceived to be made by cutting 

 a piece of paper into the form of an inclined plane, and 

 then wrapping it round a cylinder. The edge of the paper 

 will form a spiral line round the cylinder, which will answer 

 to the thread of the screw. With the addition of the lever, 

 the screw forms a very powerful machine, employed either 

 for compressions, or to raise heavy weights. It is used by 

 book-binders to press the leaves of books together ; and is the 

 principal machine used for coining money ; for taking off 

 copper-plate prints ; and for printing in general. 



