8t> OUTLINES OV NATURAL PHILOSOPHY. 



The combination of pulleys that has the most simpli- 

 city, is that in which a number of pulleys hang above 

 one another in an oblique line, each doubling the 

 power of that which is under it, so that as the num- 

 ber of pulleys increases in arithmetical progression, 

 the advantage increases in geometrical progression. 

 From being so extremely portable, and so applicable 

 to cordage, the pulley is of all the mechanical powers 

 the most useful at sea, 



Of the Wedge. 



144. The wedge is a triangular prism, made of 

 wood or of metal. It is usual to make the prism 

 isosceles, and the angle contained between the 

 equal sides more acute than the others. When 

 instead of being isosceles, the prism is rectangular, 

 and has one of the planes containing the right 

 angle, placed horizontally, the wedge coincides 

 with the inclined plane, which is sometimes con- 

 sidered as a mechanical power distinct from the 

 rest. 



a, There has been a considerable difference in the 

 rules given by mechanical writers for determining 

 the power of the wedge. This has arisen, from not 

 attending sufficiently to the direction of the resis- 

 tance, and to another circumstance, whether both 

 the resisting bodies are moveable, or one of them 

 only. The best way of considering the subject, is 

 to resolve all the forces that act upon the wedge 

 into parts parallel to two axes, at right angles to one 



another. 



