PULLEYS, INCLINED PLANES, AND WEDGES 41 



multiply the length of rope unwound from the axle by the diameter of 

 the wheel and divide by the diameter of the axle. 

 Or in the above problem, 



24 : 12 

 24 X 5 



12 



10ft. 



In the derrick (Fig. 25), the hoisting mechanism is a form 

 of double wheel and axle in which the axle of the first works 

 upon the wheel of the second by 

 means of gears. It is used for 

 raising heavy weights. 



41. Inclined Planes. Another 

 simple machine, called an inclined 

 plane, is a slope used to enable 

 a small force, such as the strength 

 of a man, to overcome the weight 

 of a large body. When, for ex- 

 ample, it is necessary to move 

 heavy boxes, barrels, etc., from a 

 sidewalk to a wagon or from a 

 wagon to the sidewalk the team- 

 ster usually places a plank between the two distances, thus 

 making an inclined plane and pushes the barrel or box onto 

 the wagon. If a wagon bed is 4 ft. above the ground and 

 a board 8 ft. long is placed against it, a man can then roll 

 the barrel up the inclined plane with one-half the force he 

 would have to exert when lifting, but in twice the time, as 

 the distance covered is twice that of the vertical or upright 

 height. 



The mechanical power gained on an inclined plane is 



FIG. 25. Derrick. 



