244 Experimental Philosophy. [Lecture 16. 



body, and consider that the momentum of a 

 hammer consists of its weight multiplied by the 

 velocity with which it moves (which is consi- 

 derable), and then the effect will appear less ex- 

 traordinary. It is by means of the momentum 

 of the hammer striking with considerable ve- 

 locity, that the wedge is driven in ; and then its 

 friction keeps it from slipping out again. 



VI. The screw (fig. 102) may properly be con- 

 sidered as an inclined plane wrapt round a cy- 

 linder. The power of the screw is therefore as 

 the length of each spiral or thread is to its height, 

 or, in other words, as the circumference of the 

 threads to their distance from one another. The 

 screw, however, can only be wrought by means 

 of a handle or winch, which is, in fact, a lever, 

 and it may, therefore, be regarded as a com- 

 pound machine. To estimate its force, then, 

 let us suppose that I desire to screw down the 

 press G upon B ; every turn I make once round 

 with both handles, I shall drive the press only 

 one spiral nearer to B ; so that if there are eleven 

 spirals, I must make eleven turns of the handles, 

 FL, before I come to the bottom. In pressing 

 down the screw, therefore, I act with a force as 

 much superior to the resistance of the body I de- 

 sire to press, as the circumference of the circle, 

 which my hands describe in turning the machine, 

 exceeds the distance between two little spirals of 

 the screw. For instance, suppose the distance 

 between the two spirals to be half an inch, and 



