



1:21. T. If we remove the cylinder 

 from i!. ::tl only leave the helical 



line, is there any mechanical advantage in 

 the !: 



P. Yes, it is still a screw, as in the 



: the common corkscrew (Fig. 35); 



ami when i: i into a substance, 



.in inclined plane 1st, by the point 



helical line being smaller than the 



upper p trt ; 'Jnd, by the line assuming a 



as it ascends; 3rd, by the 



:.t of the thread il 



/' How does the screw act ? 



the solid screw is inserted 



into the hollow screw, and is made to act 



.ly to raise a weight? it will be found 



that when the weight has been r.t 



one turn of the screw over one thread, or 



to i-, in /'>. .39, that it has really 



ropellcd up the inclined plane me; 



i of the screw, 



independent of friction, will be as the 

 length of the plane (m e\ is to the height 

 (ft). For example, if / e is a quarter of 

 an inc.. three inches. 



then a power equal to one pound, . 

 by the screw, would balance a weight of 

 twelve pounds suspended from the nut, 



T. Can the screw be applied so 

 as to act by itself? 



/' -N i: is never used alone, but the 

 power in always applied by means of a 

 . or passing through. the 

 head of the screw [see Fig. 41], on 

 t . icrew i i 

 40 J. 



l-Vf. . 



T. If a v 



pfcot of wood, does it not act alone, On M ? 

 P. od oorre*! 



screw, and the screw 





l'2o. T. How is the t> fleet of th< 

 estimated ? 



P. By the proportion between the space 

 described by the power, and the space 

 between any two of its threads, in one 

 revolution of the screw. Therefore, the 

 power applied to a long lever inert u 

 effect of the machine, and hence it follows 

 that, in order to increase the power of the 

 machine, we must lengthen the lever, and 

 decrease the distance between the threads 

 of the screw. 



I'Jti. T. Is there no other way of in- 

 creasing the power of this machine, 

 because it frequently happens that it is 

 inconvenient to have a very long lever, and 

 if the threads of the screw are too thin, 

 they are liable to break. 



P. There is a compound screw, invented 

 by John Hunter, the great anatomist, 

 which enables us to increase the efficacy 

 of the machine, without diminishing its 



This i?, accomplished \ 

 pi<>>ing a large screw (a), which turns in a 

 hollow screw placed in the beam (b). The 

 ;y of the large screw contain* 

 a hollow scn-w, which is 

 a smaller screw (r). and, therefore, while 

 the larger screw passes forward, the smaller 

 drawn back, and as both screws 

 revolve together, each time .1 in- 

 takes place, the beam (t) is prettied 



rough a sp.ve r.,1! .1 to : 

 of the distats :he thread- 



:" 



the thread* of the upprr xrrcw are |th of an 

 inch apart, ami the. r screw J^th 



of an inch, th-n the *anM effect v 

 d JIH if th' 

 he threads of which were only ^lh 



