THE INCLINED PLANE, WEDGE, AND SCREW. 



289 



thread, but at right angles to the length of the cylinder A B, or, what is to the 

 same effect, parallel to the base H G ; therefore the proportion of the power 

 to the weight will be, according to principles already explained, the same as 

 that of C H to the space through which the power moves parallel to H G in 

 one revolution of the screw. H C is evidently the distance between the suc- 

 cessive positions of the thread as it winds round the cylinder ; and it appears, 

 from what has been just stated, that the less this distance is, or, in other words, 

 the finer the thread is, the more powerful the machine will be. 



In the application of the screw, the weight or resistance is not, as in the 

 inclined plane and wedge, placed upon the surface of the plane or thread. The 

 power is usually transmitted by causing the screw to move in a concave cyl- 

 inder, on the interior surface of which a spiral cavity is cut, corresponding 

 exactly to the thread of the screw, and in which the thread will move by turn- 

 ing round the screw continually in the same direction. This hollow cylinder 

 is usually called the nut or concave screw. The screw surrounded by its spiral 

 thread is represented in fig. 8 ; and a section of the same playing in the nut 

 is represented in fig. 9. 



Fig. 8. 



Fig. 9 



There are several ways in which the effect of the power may be conveyed 

 to the resistance by this apparatus. 



First, let us suppose that the nut A B is fixed. If the screw be continually 

 turned on its axis, by a lever E F inserted in one end of it, it will be moved 

 in the direction C D, advancing every revolution through a space equal to the 

 distance between two contiguous threads. By turning the lever in an oppo- 

 site direction, the screw will be moved in the direction D C. 



If the screw be fixed, so as to be incapable either of moving longitudinally 

 or revolving on its axis, the nut A B may be turned upon thp screw by a lever, 

 and will move on the screw toward C or toward D, according to the direction 

 in which the lever is turned. 



In the former case, we have supposed the nut to be absolutely immoveable, 

 and in the latter case, the screw to be absolutely immoveable. It may happen, 

 however, that the nut, though capable of revolving, is incapable of moving 



VOL,. II.— 19 r 



