74 



MECHANICS. 



[Lesson III. 



one of the same length. Let B E repre- 

 sent a curved lever, which is .supported at 

 F, having the weight (W) attached at B, 

 and the power (P) applied at E. If we 

 wish to find the momentum of the weight, 

 we have only to multiply its weight by 

 the ideal lines A F, or B C ; and the 

 momentum of power will be found by 

 multiplying its weight by the ideal lines 

 D E, or F G. 



[See Q. 137 and 138, p. 25.] 



31. T. What you have stated leads me 

 to suppose that levers do not always act in 

 the same direction. Is this the case? 



P. Yes. All straight levers are sup- 

 posed to have their powers or forces acting 

 at right angles with them ; but bent levers 

 act obliquely, and not at right angles, which 

 is the reason bent levers are not so advan- 

 tageous as straight ones, because the ob- 

 liquity in their action diminishes the me- 

 chanical gain. 



32. T. How is the mechanical ad- 

 vantage of the power and weight of a lever 

 described ! 



P- By a line drawn from the fulcrum, 

 at right angles to the direction in the 

 which the forces are respectively acting. 



GENERAL QUESTIONS ON LESSON II. 



1. What is the difference between theory 

 and practice ? 



2. What is a lever? 



3. What is essentially necessary in a 

 lever ? 



4. Describe what is meant by a fulcrum ? 



5. What is the use of levers ; and how 

 many kinds of levers are there ? 



6. Describe and give examples of levers 

 of the first kind. 



7. How do levers of the first, second, 

 and third class differ ? 



8. How is the advantage of the power 

 and weight of a lever represented ? 



LESSON III. 



FERGUSON tried many experiments with levers, to prove that the theories he had con- 

 structed were correct It would be well for us, if we always followed his example in 

 this respect. He applied weights to the levers in various ways, and was thus enabled 

 to determine the advantages gained or lost. "" We must examine the principles of the 

 lever experimentally, and endeavour to establish facts. One thing must always be borne 

 in mind, that what if gained in power is lost in velocity ; we cannot create either power or 

 velocity ; but we may substitute the one for the other. 



QUESTIONS. 



33. T. Why is the power applied at a 

 greater distance from the fulcrum of a lever, 

 than that at which the weight bears ? 



P. In order to overcome the resistance 

 more effectually. 



34. T. Suppose that a pole, twelve feet 

 long, resting in the centre upon a block of 

 wood, is used as a lever, and that a weight 

 of ten pounds is placed at each end, what 

 will be the effect ? 



P. Provided the pole is of uniform 

 gravity, it will remain in a state of equili- 

 brium, or in other words, it will lie balanced ; 

 but if one end of the pole is heavier than 

 the other, it will descend, and the lighter 

 end be driven upwards. 



35. T. If one weight is removed, and a 



four pound weight placed there instead, the 

 fulcrum being situated nearer to the ten 

 pound weight than to the lesser one, what 

 will be the effect ? 



P. Possibly the same ; because it will 

 depend upon how near the fulcrum is 

 placed. For example ; if a weight of 

 twenty pounds is placed on the short arm 

 of a lever, at the distance of six inches from 

 the fulcrum, it will require a weight of six 

 pounds to be placed twenty inches from the 

 fulcrum, to balance it. 



30. T. How do you know that this is 

 correct ? 



P. By experiment and calculation ; be- 

 sides, it is a general rule, that the force of 

 the lever increaset in proportion an the dis- 



