THE. ELEMENTS Or MACHINERY. 



107 



to overcome the friction in drawing a heavy load up the steep incline, we 

 must add additional force to overcome the gravity, which acts parallel with 

 the inchned plane of the road, and tends constantly to make the load roll 

 back to the bottom of the slope. This force increases most rapidly with the 

 steepness, and consequently requires an immense expenditure of power. 

 An equal power expended on a road gently winding round the hill, with an 

 increase of speed, would gain the same elevation in much less time. 



An inteUigent driver, in ascending a steep hill on which there is a broad 

 road, winds from side to side, since by so doing he diminishes the abruptness 

 of the ascent (the plane being made longer in proportion to its height), and 

 thus favors the horses. 



Our common stairs are inclined planes, the steps being merely for the pur- 

 pose of obtaining a good foot-hold. 



234. In the inclined plane, as in all other simple machines, 

 a gain in power is attended \Yith a corresponding loss of time. 

 A body, in ascending an inclined plane, has a greater space 

 to pass over than if it should rise perpendicularly. The time, 

 therefore, of its ascent will be greater, and it will thus oppose 

 less resistance, and consequently require less power. 



What Is a 235. The Wedge is a movable 



Wedge? inclined plane. It is also defined 

 to be two inclined planes united at their bases, 

 as A B, Fig. 89. 



In the inchned plane, the weight moves upon the plane, 



which remains stationary ; but in the wedge, the plane itself 



is moved under the weight. 



236. The cases in which wedges are most 

 In what cases „ , . , , . , . , 



are Wedges generally used m the arts, are those m which 



ns(!d ia the j^^ intense force is required to be exerted through a very small 



space. It is, therefore, used for splitting masses of wood, or 



stone, for blocking up buildings, raising vessels in docks, and pressing out the 



oil from seeds. In this last instance, the seeds are placed in bags, between 



two surfaces of hard wood, which are pressed together by wedges. 



How is power 

 gained at the 

 expense oftime 

 in the inclined 

 plane ? 



237. The usefnlness of the wedge depends 

 on friction ; for if there were no friction, the 

 wedge would fly back after each stroke of the 

 driving force. 



238. The power of the wedge increases as 

 the length of its back, compared with that of 

 its sides, is diminished. Hence, it follows that 



the power of the wedge is in proportion to its sharpness. 



The power commonly used in the case of the wedge, is not pressure, but 

 percussion. Its edge being inserted mto a fissure, the wedge is driven in by 



Upon what 

 does the influ- 

 ence of the 

 Wedge de- 

 pend? 



How does the 

 power of the 

 Wedge ia. 

 crease? 



