THE SCREW 



469 



USE OF THE WEDGE 



blocks of stone to great heights. It would have been next 

 to impossible for them to do this simply by using brute 

 force . Some simple machine 

 was necessary. They prob- 

 ably used the same kind of 

 machine that is used to-day 

 in rolling a barrel into a 

 wagon or in grading wagon 

 roads- or railroads .over 

 mountain passes an in- 

 clined plane. The more 

 gradual the inclination up 



which the weight travels, the smaller the power required to 

 lift the weight. Again, what is gained in 

 A power is sacrificed in distance. 



M 



The Wedge. The wedge consists 

 simply of two inclined planes placed back 

 to back. It is principally used in forcing 

 substances apart, as when wedges are 

 used to split wood and stones, or as 

 needles and pins are used in pushing 

 apart the fibers of cloth. 

 Axes and chisels and most 



cutting tools except saws act on the principle 



of the wedge. 



FIGURE 145 



The Screw. The screw is simply an in- 

 clined plane ascending around a central axis. 

 (Figure 145.) The projection of the plane 

 from the axis is called the thread. The 

 plane moves the distance between the threads in making 

 one turn around the axis. A spiral staircase is a machine 



FIGURE 146 



