MOTION, MECHANICS, ETC. 



149 



hammer, mallet, &c. A smart blow of a hammer of half a 

 pound weight, overcoming more resistance than the weight 

 of many pounds laid on the top of the wedge. 



The Screw. (Fig. 156, as below) This piece of appa- 

 ratus consists of a screw, about 8 inches long, entering a 

 board supported on a base by two pillars ; in the top of the 

 screw there is a hole for the introduction of a lever. Fixed 

 in the same frame there is usually a brass spiral in the form 

 of the thread of a screw, having a lever, or handle, project- 

 ing through the frame, by which it is turned, and is used in 

 explaining the screw as an inclined plane, used in connexion 

 with the lever. 



Fig. 157. 



Fig. 156. 



The Wheel and Axis (Fig. 157, as above,) This ap- 

 paratus consists of a mahogany wheel, the diameter in the 

 groove being 5 inches, on the side of which is a smaller 

 wheel of 2i inches diameter; they are mounted on an axis, 

 the diameter of which near the wheel is one inch, and the 

 other end half an inch. Both of these have silk cords fixed 

 to them, for suspending the weights ; the axis is supported 

 at the ends by small pivots, moving in a brass frame, Avhich 

 is usually attached to the frame supporting the system of 

 pulleys. # 



