MECHANICS. 9J 



application of it. This screw is usually formed by 

 a spiral tube winding round a cylinder, and it may 

 be applied to raise any body that can pass within 

 the tube as well as a fluid. If a screw thus form- 

 ed be placed obliquely, so as to make, with the ver- 

 tical, an angle equal to that which the spiral makes 

 with the lines parallel to the axis, there will be in 

 each turn of the spiral a part parallel to the hori- 

 zon, where, if a body were placed, it would be at 

 rest. Jf, then, the screw be turned, the body will 

 ascend, because the part of the screw behind it be- 

 comes more inclined than the part before it, so that 

 the body is urged forward, and consequently as- 

 cends. 



c. If the screw used in this manner, be turned with 

 great rapidity, the body may acquire a centrifugal 

 force so great as to overcome its gravity ; in which 

 case it will descend. 



On the subject of ARCHIMEDES'^ Screw, see BERNOU- 

 ILLI Hydrodynamics Sect. ix. 26. Also EULER, 

 Nov. Com. Petrop. v. p. 259, &c. HENNERT, sur 

 la Vis cF ARCHIMEDE. 



d. The screw is also employed in the division of ma- 

 thematical instruments, and in reading off from them. 

 The contrivance known by the name of the Micro- 

 meter Screw, is used for measuring angles with great 

 exactness. In RAMSDEN'S Dividing Instrument, the 

 screw is applied for the same purpose. 



Of 



