90 OUTLINES OF NATURAL PHILOSOPHY. 



same angle. The spiral may be either on the 

 convex or concave surface of the cylinder, and the 

 screw is called accordingly, either the exterior or 

 the interior screw. 



The screw is properly referred to the species of wedge 

 called the Inclined Plane, and its principle, like that 

 of the wedge, is easily reduced to the composition of 

 forces. 



151. If the power be applied parallel to the base 

 of the screw, and perpendicular to the radius of 

 the cylinder ; and if the weight press perpendicu- 

 larly on the axis, an equilibrium is produced 

 when the power is to the resistance, as the dis- 

 tance between two threads of the screw to the cir- 

 cumference described by the point to which the 

 power is applied. 



MUSCHENBROEK, 482. BoSSUT, 284. SXrRAVE- 

 SANDE, 281. 



a. The screw is of great use for compressing bodies. 

 A kind of percussion is sometimes added to it, as in 

 the apparatus for coining. The great attrition or 

 friction which takes place in the screw, is useful by 

 retaining it in the state to which it has been once 

 brought, and continuing the effect after the power is 

 removed. The screw is also applied, with much ad- 

 vantage, to raise great weights to a small height, and 

 support them in that situation. 



b. The use of the screw to raise water, in the man- 

 ner invented by ARCHIMEDES, is a very remarkable 



application 



