Screw. 125 



of four parts ABCD, CDFE, FEGH, ABGH, the exterior and Figll6 

 interior surfaces of which are circular and concentric, and the 

 same force be applied to the centre of gravity of each part, and 

 be directed toward the common centre of the whole, no separa- 

 tion can take place among the parts, however great the force 

 employed, provided the material itself be sufficiently hard. For 

 it will be seen, that the force belonging to each part may be con- 

 sidered as decomposed into two others perpendicular respectively 

 to the two plane faces of this part, and that consequently between 

 each pair of contiguous planes there will be two equal and directly 

 opposite forces ; so that the several forces will mutually destroy 

 each other, and a general equilibrium will be the result. The 

 parts ABCD, &c., are called voussoirs. In a regular arch, the 

 upper voussoir is distinguished by the name of key-stone. The 

 surfaces which separate the voussoirs are technically termed 

 joints. The interior curve of the arch is called the intrados, and 

 the exterior, or that which limits all the voussoirs, when they are 

 in equilibrium, is called the extrados ; the masses of masonry at 

 each end, that support the arch, are the abutments. The begin- 

 ning of the arch is called the spring, the middle the crown, and the 

 parts between the spring and the crown, the haunches of the arch. 

 The part of the abutment from which the arch springs, is termed 

 the impost ; and the distance between the imposts the span of the 

 arch. 



Of the Screw. 



213. The screw AB, is a solid cylinder having a protuberance Fig.i 17. 

 or thread raised upon its convex surface, and carried round 118 ' 

 obliquely, and continually with the same inclination to the axis. 



The nut is a hollow cylinder with a spiral groove cut upon 

 the concave surface, and fitted to receive the thread of the screw. 

 The former is sometimes called the external, and the latter the 

 Internal screw. 



Sometimes the nut is fixed, and the screw in turning has all 

 its threads carried successively through it ; sometimes the screw 

 is fixed, and the nut in turning passes the whole length of the 

 screw. In each case, while the power is applied at the same 

 distance from the axis of the screw, there is always the same 



