3<8 MECHANIC POWERS. 



When a screw acts in a wheel in this manner, 

 it is called an endless screw. 



When the screw is not employed in turning a 

 wheel, it consists of two parts : the first is called 

 the male, or outside screw, being cut in such a 

 manner as to have a prominent part going round 

 the cylinder in a spiral manner, which prominent 

 part is called the thread of the screw: the other 

 part, which is called the female, or inside screw, is 

 a solid body, containing a hollow cylinder, whose 

 concave surface is cut in the same manner as the 

 convex surface of the male screw, so that the pro- 

 minent parts of the one may fit the concave parts 

 of the other. 



A very considerable degree of friction always 

 acts against the power in a screw; but this is fully 

 compensated by other advantages : for on this ac- 

 count the screw continues to sustain a weight, even 

 after the power is removed, or ceases to act, and 

 presses upon the body against which it is driven. 

 Hence the screw will sustain very great weights ; 

 insomuch, that several screws, properly applied, 

 would support a large building, whilst the foun- 

 dation was mending, or renewed. 



OF COMPOUND MACHINES. 



Although it be evident from the principles de- 

 livered above, that any one of the mechanic 

 powers is capable of overcoming the greatest 

 possible resistance, in theory: yet, in practice, if 

 used singly, for producing very great effects, they 

 would be frequently so unwieldy and unmanageable, 

 as to render it impossible to apply them. For this 

 reason, it is generally found more advantageous to 

 combine them together; by which means the power 



