78 



MECHANICS. 



[Lesson IV. 



P. In this diagram we shall see that it [ 

 has been applied to the boiler of a steam- ' 

 engine. The valve p, which covers and 

 closes the opening of the boiler, is forced 

 up by the pressure of the steam, but this 



Fig. 13. 



pressure is balanced by a smaller force 

 (the weight u>, which acts downwards), be- 

 cause it acts at a longer arm than the 

 pressure on the under surface of the valve. 

 In this case the fixed point of the lever a 

 is the fulcrum. 



51. T. What is meant by a lever of the 

 third kind ? 



P. It is a lever with the power placed 

 between the fulcrum and the weight or 

 resistance (Tig. 13). The fulcrum is 

 placed at the extremity of the short arm, at 



AvJ Fig. IS. 



F ; the weight w is suspended from the 

 end of the long arm ; and the power, p, is 

 placed between them. 



52. T. Is this form of lever advanta- 

 geous or not ? 



P. What is lost in power is gained in 

 velocity ; therefore it is advantageous for 

 some purposes, because a small power will 

 cause the long arm of the lever to move 

 over a great space. 



53. T. How is it that you tell me that 

 power is lost in this kind of lever ; and 

 then say, that a small power will cause the 

 long arm to move with greater velocity ? 



P. I spoke comparatively, because we 

 know that a grr-ater power must be actually 

 required than would be the case if the 

 power was applied directly to the weight ; 

 and therefore this kind of lever is not used 



to overcome resistance, hut to move a 

 weight with great speed, or else for :-ome 

 sanicular purpose. 



54. T. Give me some familiar examples 

 of levers of this kind. 



P. The treddle of a turning-lathe is one 

 of the most common, the end which rests 

 on the floor is the fulcrum ; the foot of the 

 man, which presses on the board near to 

 he fulcrum, is the power; and .the crank 

 upon the axis of the fly-wheel, which is at- 

 tached to the other end, is the weight. A 

 man using a flail with two hands is another 

 example. But the most interesting ex- 

 amples of the application of such levels 

 are to be found in the structure of animals, 

 particularly the arm and forearm of man ; 



Fig. 14.* 



and, although there is a mechanical dis- 

 advantage in the action of the biceps, yet 

 there is a corresponding increase in 

 velocity. The lower jaw also furnishes us 



1? 



Fiy. 15. 



with an excellent example of the force 

 exerted by muscular action, which, at times, 

 cannot be less than three hundred pounds 

 in man, and even more in large animals. 

 riif. 15 shows the arrangement of this kind 

 of lever ; A represents the masseter muscle, 



* Fig. 14. a, the humerux, or arm-hone; ft, <?, 

 bones of the forearm ; d, the muscle which bends 

 the forearm, called biceps flexor cubiti, and is in- 

 scried at e into the posterior part of the tubercle 

 of the rodius c ; and/, the weight. 



