94 



MECHANICS. 



[Lesson XII. 



feet, the lath being of the same thickness 

 as in the present case, what would be the 

 effect of placing it on the stones without 

 the weight ? 



P. It would bend with its own weight. 

 As we shorten its length, we increase its 

 strength, because the thicker and shorter a 

 beam is, the stronger it will be ; and if a 

 beam is twice as broad as another of the 

 same length, it will be twice as strong. 



140. T. Why does a beam increase in 

 strength, when it increases in size ? 



P. Because it contains a greater number 

 of resisting particles. If we double the 

 depth of a beam we render it four times as 

 strong, because the number of fibres are 

 doubled, and the lever is increased. 



141. T. You said that the form of 

 materials employed required consideration. 

 Explain this. 



P. We know that the arch is the 

 strongest form we can construct, and this 

 we learn from Nature, when we observe 

 the roof of the skull, eggs, or cylindrical 

 bodies. 



142. T. How is it that the arch is the 

 strongest form ? 



P. Because the particles of which it is 

 composed, bear upon each other like a 

 great many wedges with their narrow ends 

 pointed towards the hollow part of the 



arch, therefore the particles, or the concave 

 or hollow part, are more compressed than 

 on the convex or rounded part ; con- 

 sequently the more pressure we apply to 

 the upper or convex part in a perpendicular 

 direction, the greater will be the compres- 

 sion and resistance to the weight. 



143. T. How does bulk influence the 

 employment of materials ? 



P. Because, after a body has attained a 

 certain size, the additional increase of 

 bulk only adds to the weight without in- 

 creasing its power of endurance, for the 

 weight increases more rapidly than the 

 strength ; and on this account we are 

 obliged to limit the size of our machines, 

 which would not be able to support their 

 own weight if made too large. 



GENERAL QUESTIONS ON LESSON XI. 



1. "What are the desirable objects in the 

 construction of a machine ? 



2. What materials are machines generally 

 constructed of? 



3. Are all materials of uniform strength ? 



4. What is meant by the strength of a 

 body, and how is it classified ? 



5. How does position affect the materials 

 used in machines ? 



6. What influence has the form and bulk 

 of materials on machines ? 



LESSON XII. 



A MACHINE is made up of many parts, which are so connected, as to be rendered 

 available for the particular purpose the machine was designed for ; and the manner in 

 which one mechanic power acts upon another, may be changed 

 to suit certain particular purposes. Power may be transmitted 

 (liuuDr? ky belts, as in Fig. 43, where it is obvious that the wheel c is 

 turned by the belt B u, which receives its motion from A ; but, 

 as it is smaller, the velocity is increased. The motion of 

 machinery may be regulated so as to produce a uniform velocity ; or a quick motion 

 may be transformed into a slow motion, or vice versa ; motion may also be made to 

 alternate by means of eccentric wheels, or wheels that have the axis nearer to the 

 circumference than the centre. 



QUESTIONS. 



144. T. What are the chief agents in 

 the transmission of power ? 



P. Tooihed wheels, bevel wheels, shafts, 



and pulleys, which are arranged according 

 to the direction of the motion required to be 

 conveyed. Fig. 44 enables us to understand 



