110 



WELLS'S NATURAL PHILOSOPHY. 



Fig. 94. 



Describe the 

 construction 

 and advantage 

 of Hunter's 

 Screw. 



Fig. 95. 



mous energy by means of tlie momentum of two heavy balls attached to the 

 end of a long lever, or handle, as is represented in Fig. 93. A force of sev- 

 eral tons may thus be applied at one ellbrt. 



When the thread of a screw 

 ^dlesslcr^w f ^^^ks in the teetii of a wheel, as 

 is shown in Fig. 94, it constitutes 

 what is called an endless screw. Such a con- 

 trivance is oftentimes a very convenient method 

 of applying power. 



246. The efficacy of a screw 

 increases with the fineness of 

 the thread ; but a practical limit 

 is soon attained, for if the thread 

 be made too fine, it will become 

 weak, and be liable to be torn off. To obtain 



an indefinite increase of the strength of the screw 

 ■without diminishing the strength of the thread, we 

 have a contrivance known as " Hunter's screw," rep- 

 resented in Fig. 95. It consists of a screw, A, work- 

 ing in a nut. To a movable bottom-board, D, a sec- 

 ond screw, B, is affixed. This second screw works in 

 the interior of A, which is hollow, and in which fe 

 corresponding thread is cut. When, therefore, A is 

 screwed downward, the threads of B pass upward, and 

 the movable piece, D, urged forward by the screw 

 •^vhich has the greater thread, it is drawn back by that 

 •which has the less ; so that during each revolution the 

 screw instead of being advanced through a space equal 

 to the breadth of either of the threads, moves through a space equal to their 

 difference. Suppose the distance between the threads of A to be l-20th of 

 an inch, and of B 1-2 1st of an inch ; then in turning the screw A once, the 

 board D will descend a distance equal to the difference between l-20th and 

 l-2l3t, or the l-420th of an inch. Hence, if the circle described by the han- 

 dle be 26 inches while the screw advances l-420th of an inch, the power will 

 be to the weight as 1 to 8,400. 



247. All macliines, however complicated, are made up of combinations of 

 the six simple machines. If we examine the construction of any complex ma- 

 chine, as a steam-engine, a loom, a spinning machine, or a time-piece, we 

 shall find that they are composed of simple levers, wheels and axle.« 

 screws, etc., connected together in an endless variety of forms, to form a 



complete whole. 



In the practical application of machinery, it rarely or never 

 happens that the moving force is capable of producing directly, 

 the particular kind of motion required by the machine to per- 

 form the work to which it is adapted. Expedients must 



therefore be resorted to, by means of which the motions which the moving 



Is the moving 

 force in ma- 

 chinery ap- 

 plied directly ? 



