Lesaon IX.] 



MECHANICS. 



89 



riONs. 



110. T.~- What is the wedge? 



P It is a simple machine of a triangular 

 (brin, consisting of a solid mass of iron, 

 v _ wood, or some other hard 



/material, and is generally 

 described as being com- 

 posed of two inclined 

 planes, united at their 

 bases, or placed back to 

 \ back, as represented in 



n t w. Fig. 32, B A being the 



line of union of their bases. 



111. /'. Is the wedge always sloped on 

 both sides ? 



P. No ; it is sometimes flat on one 

 side, and sloped or inclined on the other, 

 as B, A 31. 



112. /'. WUi is the use of the wedge ? 

 P. It is generally employed to divide 



solid bodies, the edge A (fig. 32) being 

 driven against 



1^"*) - - -- t!iem b y lorce 



m tffr^K' :ii>plied to the 



5 T\ / 



^J L^ V.I I 33 represents a 



B^LPl \ \ 1 ,1 wedge enterin.r 



! a piece of wood, 



L 





blows, you ob- 

 serve 1 1: 



en into 



the substance of 

 the wood, so as 

 H0.M- to cause it to 

 p. 34. 



r How does the wedge act, so as 

 to divide the wood thus ? 



/*. When the wedge is urged by re- 

 peated blow* to enter the substance of tin- 

 wood, the fibres of the wood ar.- com; 



the two sides of t illy, as 



a ted, the p< 

 *till furthrr space that at firtt 



Mi- point A, is ol . 



eparatr, no a* to allow D c to enter it. 

 11 \. I Isthewedjj. 



solid bodies ? 

 /'.- 



the docks, wedges being 

 keels. It has been used to restore an 



inclined building to a perpendicular posi- 

 tion. It is the principal agent in the oil- 

 mill ; the seeds from which the oil is to be 

 extracted are placed between pieces of 

 hard wood, and wedges are inserted between 

 the bags, by allowing heavy beams to fall 

 on them. The key-stone of an arch is a 

 wedge ; planes, knives, razors, nails, pins, 

 needles, awls, and most cutting and piercing 

 instruments are wedges. A most useful 

 application of the wedge, is that of fastening 

 large timbers together by means of a wedge- 

 shaped mortice. 



115. T. How can we calculate the 

 of the wedge ? 



P. We cannot do so satisfactorily, be- 

 cause the exact force applied will depend 

 upon the numbet of blows, and the resist- 

 ance opposed by the sides ; but we may 

 lay it down as a rule, that in proportion as 

 the angle is greater, so must be the power to 

 overcome the resistance, which increase* 

 with the inclination, the same as in the 

 inclined plane. Therefore, a long thin 

 wedge has a greater power than a short 

 thick one, because it does not require so 

 great a force to be applied. 



llii. 7\ How is it when a wedge is 

 driven into a block of wood, that K 

 generally forced out again by the wood ? 



P. Because the friction, which is very 

 great, acts upon the wedge in th< 



r that a ratchet wheel does upon the 

 wheel and axle. It is to this principle that 

 nails owe their efficacy ; for, if it were not 

 for the friction which arises between their 



and the wood into which ti. 

 driven, they would recoil from their places. 



: IONS ON LESSON IX. 



1. What is the difference between A fixed 



and a movable inclined plane f 



ft some illustrations of both, 

 the wedge and its vat 

 MOW are wedges used ? 

 i > what may their action beasn 



ve some familiar examples of the 

 useful applicntinn of the wedge. 



7. Is it possible to estimate the power 



hy arc not wedges forced out of 

 the substances into which they arc dl 



