128 Statics. 



power tends to describe, is to the distance between two adjacent 

 threads. 



217. Therefore, upon the same screw, the effect of the power 

 q will be so much the more considerable according as it is ap- 

 plied at a greater distance from the axis ; and upon different 

 screws, the power being applied at the same distance from the 

 axis, the effect will be so much the more considerable according 

 as the distance between the threads is less. 



218. The screw, therefore, is a compound machine partaking 

 of the inclined plane and the lever ; it is advantageously employ- 

 ed in compressing bodies and for several other purposes. Fric- 

 tion, however, greatly modifies the effect which this machine 

 ought to produce according to the ratio above established. 



219. In order that the nut may pass through the distance 

 between two adjacent threads, it is necessary, as we have seen, 

 that the power should make an entire revolution. This condn 

 tion is unalterable, and there are many occasions on which an 

 important use may be made of it. When it is proposed, for ex- 



Pig.120. ample, to measure the different parts of a very small space AB, 

 it may be done by causing this space to be described by the 

 extremity E, of a screw DE, the threads of which are accurately 

 formed at the same distance from each other throughout. If this 

 screw be made to carry, at its other extremity, a dial GIH, the di- 

 visions of which, as the screw turns, pass under the fixed index FI- y 

 having ascertained what number of turns the screw must make in 

 order that the point E shall describe the known extent AB, we shall 

 be able, by the number of revolutions and parts of a revolution 

 performed in causing the point E to pass over any part of AB, 

 to determine the length of this part, however small it may be. 

 If for instance the distance of the threads asunder be one tenth 

 of an inch, and the circumference of the dial GIH five inches, 

 any point of the circumference will move through five inches 

 while the point E advances one tenth of an inch ; consequently^ 

 the circumference being divided into tenths of an inch, while one 

 division passes under the index, the point E would be carried 

 forward only T V of T V? or ^^ of an inch. This is called a mi- 

 crometer screw. 



