THE INCLINED PLANE, WEDGE, AND SCREW. 



291 



Fig. 12. 



impression of a die is to be made upon a piece of metal, and in the same way 

 in producing the impression of a seal upon wax or other substance adapted to 

 receive it. When soft and light materials, such as cotton, are to be reduced 

 to a convenient bulk for transportation, the screw is used to compress them, 

 and they are thus reduced into hard, dense masses. In printing, formerly, 

 the paper was urged by a severe and sudden pressure upon the types by 

 means of a screw. 



As the mechanical power of the screw depends upon the relative magnitude 

 of the circumference through which the power revolves, and the distance be- 

 tween the threads, it is evident, that, to increase the efficacy of the machine, 

 we must either increase the length of the lever by which the power acts, or 

 diminish the magnitude of the thread. Although there is no limit in theory to 

 the increase of the mechanical efficacy by these means, yet practical incon- 

 venience arises which effectually prevents that increase being carried beyond 

 a certain extent. If the lever by which the power acts be increased, the same 

 difficulty arises as was already explained in the wheel and axle : the space 

 through which the power should act would be so unwieldy, that its applica- 

 tion would become impracticable. If, on the other hand, the power of the 

 machine he increased by diminishing the size of the thread, the strength of 

 the thread will be so diminished, that a slight resistance will tear it from the 

 cylinder. The cases in which it is necessary to increase the power of the 

 machine being those in which the greatest resistances are to be overcome, the 

 object will evidently be defeated if the means chosen to increase that power 

 deprive the machine of the strength which is necessary to sustain the force to 

 which it is to be submitted. 



These inconveniences are removed by a contrivance of Mr. Hunter, which, 

 while it gives to the machine all the requisite strength and compactness, allows 

 it to have an almost unlimited degree of mechanical efficacy. 



This contrivance consists in the use of two screws, the threads of which 

 may have any strength and magnitude, but which have a very small difference 

 of breadth. While the working point is urged forward by that which has the 

 greater thread, it is drawn back by that which has the less ; so that, during 

 each revolution of the screw, instead of being advanced through a space equal 

 to the magnitude of either of the threads, it moves through a space equal to 

 their difference. The mechanical power of such a machine will be the same 

 as that of a single screw, having a thread whose magnitude is equal to the 

 difference of the magnitudes of the two threads just mentioned. 



Thus, without inconveniently increasing the sweep of the power, on the one 

 hand, or, on the other, diminishing the thread until the necessary strength is 



