46 



MECHANICS. 



ble of moving directly upwards or 

 downwards, but not of revolving. The 

 nut F is worked by the lever D E. 

 Every complete revolution of the nut 

 urges the screw upward, through a 

 space equal to the distance between 

 two contiguous threads. 



The proportion of the weight to the 

 power, or the mechanical efficacy of the 

 screw, depends on the proportion of 

 the circumference described by the 

 power, to the distance between two con- 

 tiguous threads. Hence, it is evident 

 that the efficacy of the screw may be 

 increased, either by increasing the 

 length of the lever by which the power 

 acts, or by diminishing the distance be- 

 tween the threads. To both of these 

 there are, however, practical limits, si- 

 milar to those mentioned in the case of 

 the wheel and axle (58.) 



If the leverage of the power be very 

 much increased, the power will work 

 through an inconveniently great space, 

 and the machine will become unwieldy. 

 If, on the other hand, the thread of the 

 screw be made very small and fine, it 

 will be torn off by a great resistance in 

 passing through the nut. 



These inconveniences have been ob- 

 viated by a contrivance of Mr. Hunter, 

 the Surgeon, which, while it preserves 

 all the requisite strength and compact- 

 ness in the machine, gives it an almost 

 unlimited degree of mechanical efficacy. 



A perspective drawing of this con- 

 trivance is given in /#/85 : E E is a 

 Fig. 85. 



strong frame in which a board D moves, 

 so that when it is forced towards the 

 bottom, it will exert a pressure on any 

 substance placed between it and the 



bottom. To this moveable board D is 

 attached a cylinder B, on which the 

 thread of a screw is raised. C is a fixed 

 nut through which a screw A plays. 

 The screw A is a hollow cylinder, the 

 interior of which is a nut adapted to re- 

 ceive the screw B. "When the screw 

 A is turned once round, it advances 

 through the nut C, through a space 

 equal to the distance between two con- 

 tiguous threads ; so that, if the screw B 

 were not supposed to act, the board D 

 would advance towards the bottom, 

 through a distance equal to the distance 

 between the threads of the screwA. But 

 while the screw A advances through the 

 nut C by its revolution, the very same 

 cause makes the screw B move towards 

 C through a space equal to the distance 

 between two contiguous threads of B ; 

 or, by turning A, the nut contained in 

 the inner concave surface of A is turned 

 upon the screw B. Now, if the threads 

 of the two screws A and B were per- 

 fectly equal, the effect of these two 

 motions would be, that the board D 

 would retain its position, inasmuch as 

 the effect of one screw, in moving it 

 downwards, would be exactly equal to 

 the effect of the other screw in moving 

 it upwards, 



But if we suppose the interval be- 

 tween the threads of the screw B to be 

 somewhat less than the interval between 

 the threads of the screw A, the effect 

 will be different. In this case, one re- 

 volution will move the screw A down" 

 wards, through a space equal to the 

 interval between its threads, while the 

 screw B will be moved within the screw 

 A and upwards, through a space equal 

 to the interval between its threads. The 

 combined effect will be, that the screw 

 B, and the board C to which it is 

 attached, will be moved downwards 

 through a space equal to the difference 

 of the distances between the threads of 

 the two screws. 



Thus, if the screw A have twenty 

 threads in an inch, and the screw B 

 have twenty-one ; in one revolution, A 

 is moved downwards through the twen- 

 tieth of an inch. Suppose that the nut 

 A did not, in this motion, turn on B, 

 it is plain, then, that B and the board C 

 would be moved down through the 

 o'oth of an inch. Suppose, then, that 

 the screw B was turned round once 

 within the nut A, the screw B and the 

 board C would be raised through the 

 ^tth of an inch. Its position would 

 then be below its original position by the 



