82 ?be Screw. [Book I. 



which they form the fockets for themfclves. Drills 

 and gimblets may be confidered as of the fame na- 

 ture, the fpiral points of which enter the wood fo 

 much eafier in proportion as they are (harper at the 

 end. But with refpect to large metal fcrews which 

 are ufed for preffes, vices, &c. their threads are gene- 

 rally made fquare, in order that die friction may be in- 

 creafed by augmenting the furface of each thread ; for 

 it frequently happens that the principal effect of fcrews 

 arifes from the clolenefs of the friction : this form hin- 

 ders the- cheeks or chaps of the vice from fwerving 

 backwards, to which they have a natural tendency by the 

 re-action of the piece which they prefs between them. 



Screws are ufed principally for the preffing of bo* 

 dies firmly againft each other, and fometimes for raif- 

 ing weights or burdens, or for forcing backwards or 

 forwards certain mattes of a determinate quantity. For 

 this purpofe a male and female fcrew are made ufe of, 

 one or the other of which ferves as a fulcrum or prop.* 

 Sometimes the male fcrew is fixed, while the female 

 fcrew is moveable ; but in both cafes the effect of the 

 fcrew is the fame. 



When this machine is made ufe of, one of the 

 two pieces (the male or the female) is applied 

 to the refiftance which is to be overcome, and 

 the other ferves as a fulcrum or prop to the ma- 

 chine ; then by the act of turning, the focket is 

 made to move upon the fcrew, or the fcrew into 

 the focket. In fmiths' vices, for inftance, one of 

 the cheeks is preffed, by the action of the fcrew, 

 againft the other cheek ; it appears, therefore, that the 

 power muft move one complete round, in order to ad- 

 vance the refiftance one pace or fpiral of the fcrew, 

 that is, a quantity equal to the diftance of one thread 



from 



