90 



MECHANICS. 



[Lesson X. 



LESSON X. 



WHEN an inclined plane is coiled round a cylinder, we have a machine which has 

 received the name of the screw. The screw consists of two parts, a solid cylinder with 

 the inclined plane coiled round it, called the male screw, and a hollow cylinder with an 

 inclined plane coiled round the inside (so as to receive the former one), called & female 

 tcrew. The male screw consists of a projecting ridge winding 

 round the cylinder, which is termed the thread of the .-irreic. and 

 is said to be spiral, but is really helical, being like a cork- 

 screw, which is the helical thread of the screw without the 

 cylinder. The difference between a spiral line and a helical 

 line may be seen by the annexed diagrams, Fig. 35 representing 

 the helical, and Fig. 36 the spiral line. 



The hollow screw has a helical thread winding within it, 

 corresponding to the spaces between the turns of the thread of 



the solid screw. By means of this arrangement, either screw may be made to revolve 

 while the other is kept steady ; and, therefore, any amount of pressure may be produced. 



QUESTIONS. 



Fig. 35. 



Fig. 36. 



Fif.37. 



117. T. What is the screw ? 



P. A solid cylinder with a projecting 

 ridge, winding round it 

 in the form of an in- 

 clined plane, which 

 ridge or thread may pre- 

 sent a thin sharp edge 

 as in Fig. 37, or a square 

 edge, as in Fig. 38, 

 without affecting the 

 principle of the ma- 

 c hine. 



118. T. Will you illustrate how the 

 inclined plane, being coiled round a 

 cylinder, forms a screw ? 



P. Certainly ; cut a piece of paper of 

 the same shape as I p, in Fig. 31, and let 

 the line B C be eighteen inches Jong, and 

 A C six inches high, then black the edge 

 A B. Wind this paper found a ruler, 

 commencing at the part A C, and when it 

 is all coiled round the ruler, you will 

 observe a black line like the thread of a 

 screw. 



119. T. It appears, then, that the 

 screw is like a series of inclined planes 

 coiled round a cylindrical axis. Can you 

 explain this ? 



P. If we examine Fig. 39, we observe 

 that A B c D represents a cylinder divided 



into seven equal parts (a, b, c, d, e,f, B) ; 

 if we draw lines from the perpendicular 

 line A B, each equal to the circumference 

 of the base, in 

 a plane with the 

 c hori/on and pa- 

 A rallel to each 



*' otner > 



d J "'~ 



then 



-L join A g and a h, 

 b i, c k, d I, e 

 m, we shall form 

 as many inclined 

 planes as there 

 are parts in the 

 line A B. Now, 

 if we roll these planes round the cylin- 

 der, so that the point g shall meet the 

 point a, h with b, i with c, and so on, the 

 longest lines of the inclined planes (A g, 

 a h, b i, &c.) will form a continued helical 

 line upon the cylinder, the same as the 

 thread of a screw. 



120. T. Why is the thread of the screw 

 called a helical line ? 



P. The name is derived from the Greek 

 word elix ("e\t|), a whorl ; and helical, 

 therefore, signifies winding. One turn of 

 this line is termed a helix, and several 

 turns, or more than one, helices. No two 

 points of a helix are in the same plane. 

 [See Fig. 35.] 



