108 



WELLS'S NATURAL PHILOSOPHY. 



What are fa- 

 miliar exam- 

 ples of tliL' use 

 or applicatioa 

 of the Wedge 

 in the arts ? 



Pig. 90. 



What is the 

 Thread of a 

 Screw ? 



blows upon its back. The tremor produced when the wedge is struck with 

 a violent blow, caus3S it to insinuate itself much more rapidly than it other- 

 wise would. 



239. The edges of all cutting and piercing instruments, 



such as knives, razors, chisels, nails, pins, etc., are wedges. 



The angle of the wedge in all these cases is more or 



less acute, according to the purpose to which it is applied. 



Chisels intended to cut wood have their edges at an angle of 

 about 30° ; for cutting iron from 50° to 60°, and for bra.ss about 80° to 90°. 

 In general, tools which are urged by pressure admit of being sharper than 

 those which are driven by percussion. The softer, or more yielding the sub- 

 stance to be divided is, the more acute the wedge may be constructed. 



Avhat is the 240. The Screw is an inclined plane wind- 



screwf ^jjg round a cylinder. 



This may be illustrated by cutting a strip 

 of paper in such a way as to represent an in- 

 clined plane, and then winding it round a 

 cylinder, or common lead pencil, as is repre- 

 sented in Fig. 90. 



241. The edge of the 



inclined plane winding 



about the cylinder, or 

 the coil of the spiral line which 

 it describes upon the cylinder, con- 

 stitutes the Thread of the screw, 

 and the distance between the suc- 

 cessive coils is called the distance 



between THE THREADS. 



The screw, surrounded by its spiral line is represented in Fig. 91. 



The screw is not applied directly to the resistance to be FiG. 91. 

 overcome, as in the case of the inclined plane and wedge, but 

 the power is transmitted by means of what is called the NuT. 



What is the 242. The Nut of a screw is a 



Nutofascrewf ijjock, with a Cylindrical cavity, 

 having a spiral groove cut round upon the 

 Burface of this cavity corresponding with the 

 thread of the screw. 



In this groove the thread of the screw will move by causing 

 the screw to rotate. Each turn of the screw in the nut will cause it to advanco 

 or recede a distance just equal to the interval between the threads. 

 Is the Screw, Generally, the nut is stationary and the screw movable, but 



movaWe?^"'' *^® ^^^ ™^^ ^® movable, and the screw stationary. 



