44 THE WEDGE AND SCREW. 



pounds will balance the five hundred. It is much less labori- 

 ous to ascend a hill by a winding gentle ascent than to climb 

 up a steep declivity. In addition to there being a greater 

 force required in ascending a hill, horses, that draw a load, 

 are placed in a position in which they can exert but a small 

 part of their usual strength. The principle of the inclined 

 plane is applied to the construction of carriage-ways, for the 

 conveyance of heavy loads up steep elevations. It is applied 

 also in rail-ways, the use of which has been hitherto confined, 

 almost exclusively, to coal-works, and other mines. Inven- 

 tions, whose only recommendations are simplicity and use- 

 fulness, are often suffered to lie long in a state of public neg- 

 lect, while others of more imposing asjjj^ct are readily adopt- 

 ed. It has been remarked with respect to Great Britain 



that the time has at length arrived, when carriage3|||taffin^ J 



on level surfaces, or on gently inclining planes, ^^||IBp^ttJ|^ "ij 



friction, and without obstructions, are fast spreading over j 



the face of the country. ^, t i 



Questions. — 1. How is the pulley farmed ? S*; What are the two 



kinds of pullies ? 3. What is said of the siSfj^ fixed pulley ? 4. What ' 



advantage is gained in a single moveable^ jiulley ? 5. How is the i 



power gained to be estimated in a system of pullies ? G. How is this  



explained and what maxim does it establish ? "^U^escvibe the wheel i 



and axle. 8. In what proportion is advantage ^nncd in this mecha- ' 



nical power ? J). What is the example ? 10. Why may the wheel i 



and axle ba|:onsidered as a perpetual lever ? 11. What application is | 



made of this power ? 12. What is an inclined plane ? 13. In what .^ 



proportion is the increase of power .? 14. What is the example for ; 



illustrating this ? 15. What application is made of the principle of ] 



{{lo inclined plane ? IG. What has been remarked concerning the use ; 



of rail-ways ? 17. With respect to Great Britain ? iS. Explain the g 



single moveable pulley by fig. 13. — system of pullies by fig. 15. 19. ^ 



Illustrate the power of the whe^ and axle by fig. 11. 20. Inclined ^ 

 plane by fig. 8. 



LESSON 22.. I 



The Wedge and Screw. . -l 



Percus'sion, the impression a body makes in falling or striking 



upon another, or the shock of two bodies in motion. ^ 



Sili'ceous, flinty ; see Lesson 63. ^ 



The wedge may be considered as two equally inclined i 

 planes united at their bases. The advantage gained by it i« * 



