Chap. 7-] Male and female Screw's. 81 



or cylinder A B (fig. 5.) upon the circumference of 

 \vhich is cut a fpiral groove or gorge CFG. The 

 partition C F between the rounds of the gorge is 

 called the thread of the fcrew ; and the diftance C G> 

 which there is between one thread and another, is 

 called the ftep or pace of the fcrew. 



The thre-ad and the gorge are fitted fometimes into 

 a cylindrical cavity made in a piece of metal or wood* 

 which is fometimes called a focket, but more fre- 

 quently a femak fcrew, while the other is named the 

 male, or principal fcrew. 



It is eafy to fee that the thread of a fcrew is an in- 

 clined plane, at the bafe of the cylinder AB (fig. 5.) 

 The height of this plane is the pace or fpiral of .the 

 fcrew, or, which is the fame thing, the diftance of one 

 thread from another : its bafe is the circumference of 

 the fcrew, and its length is eftimated by this circum- 

 ference and the height of the pace ; for if one of the 

 threads a b is developed, it will form with its pace b c 9 

 and its bafe, or the circumference a c of the fcrew, a 

 triangle a b c , and a rectangle at c, of which it is eafy 

 to find the fide a b, fince the two others are known, as 

 well as the angle at c : hence by a fcrew turning in its 

 focket they conftitute two inclined planes Hiding the 

 one upon the other. 



The threads of fcrews afiume different forms ac- 

 cording to the materials of which they are made, or 

 thofe into which they are to enter, or according to" the 

 efforts they are defigned to make. In wooden fcrews 

 the thread* C, G, F, are generally angular, which add 

 greatly to their ftrength ; for by this form they have a 

 larger bafe upon the cylinder which fupports them. 

 This form is alfo given to the threads of thofe fmall 

 iron fcrews which are conical, ending almoft in a 

 point, and which are defigned to enter into wood, irt 

 , G 



