Lessor 



MECHANICS. 



LESSON XL 



WE have now duly considered the leading elements of Mechanics, and even some of 

 the compound machines formed by uniting together two of the mechanic powers, as 

 the lever and screw, and the lever with the wheel and axle. We have also examined the 

 ;>roduced by combining some of the simple machines with each other, and thus 

 producing systems of their own, as with levers or pulleys. In combining bo 

 produce mechanical effects, we have certain laws to guide us, and certain objects to 

 One thing we must ever bear in mind, that the fewer parts there are in a 

 machine, and the more simple its construction, the better. 



QUKSTIONS. 



ISO. T. When we undertake to con- 

 itruct a machine, what are the chief ob- 

 jects we have to consider? 



are four. 1st The strength 

 or durability of the materials ; 2nd. The 

 arrangement of the parts of the machine 

 in as simple a m.inner as possible ; 3rd. 

 The correct adaptation or fitting of one 

 part to another ; and, 4th. Ease, regularity, 

 and unifor ion. 



131 7'. What are machines generally 

 constructed of? 



/'.Inn. steel, brass, wood, or any 

 durable materials. 



T. Do the materials differ greatly 

 in their strength ? 



P. Yes : but independent of the dura- 

 bility of the materials employed, it is neces- 

 ary to pay 

 which they are placed, their form and bulk. 



7*. What do you mean by the 

 :th of a body ? 



"ie force wit ii . hi<-h it resists 

 the s< | rs particles, and may be 



absolute o 



134. T. What is the abtolute ttmglh 

 of a body? 



It is the force by which th> 

 resist* being torn asunder when sir 

 )rngthwy. 



'< of a piece of India-rubber 

 apart ; or when a grvat weight is sm; 



: A r ..d. whirh has the other 

 end fixed perpendicularly. 



T Upon what does the absolute 

 Strength of a body dep> 



It* diagonal tection ; that is, a 

 straight line drawn th' . /n to 



t*o i>po*it angles; and, i: 



portion to the diagonal section of the body, 

 so is its absolute strength. 



136. r. What is the relative strength of 

 a body? 



P. The force which it opposes to the 

 process of breaking. In this < 

 is applied at right angles to the direction 

 long axis, while one or both < i' its 

 are supported. 



137. T. How is the connexion between 

 the particles of a solid body overcome? 



P. BY breaking, tearing, twisti: 

 crushing the hotly. 



138. T. You said that the position of 

 the materials employed in a maehine af- 

 fected their stn-ngth. Explain this. 



P. 7.J You observe that 



I have a piece of thin wood, two feet long, 



in my band it is called a lath ; and when 



each end rests on a block of wood or stone, 



snow [places the ends of the lath 



stones,] it will require twice the 



Applied to its centre, in o- 



. than it will if only the cmlro is 



supported, and a weight suspended to each 



end; and four tin :,t to break 



it, than it will if only one n.d is firmly 



fixed or supported, and th 



to the other. Observe, that when I attach 



a weight of one pound to its centre, that it 



bends, and it this is incrra-< <1 hy 



I i >uob- 



i it 1 have in rarer, 



.ind that the l.ith does not uch as 



hecause the length of th.- !. 

 ic stones is not so great as in the 

 last experiment 



stance 

 n the ends of the lath was 



