LeonIII.] 



MECHANICS. 



75 



tnnceof the power from the fulcrum increases, 

 proportion as the distance 

 from the fulcrum diminishes. 



T. How can we find out, by cil- 

 culation, the requisite power to employ I 



/'. \Ve must first of all establish 

 points, and then act by rule. The points 

 are 1. The resp.-ctive lengths of the long 

 and short arms of the lever. 2. The units 

 Jit and distance. 



38. r. What do you mean by the units 



it and uis-.i 



P. If we fix ounces or pounds, or any 

 other weight as the unit of the short arm 

 of the lever, we must fix the unit of power 

 of the long arm the same ; ami if we make 

 the unit of distance of the short arm to be 

 an inch, we mutt have inches as the units 

 of length of the long arm. 



39. T. What is the rule you mentioned? 

 /'.A very simple one. Multiply the 



weight by its distance from the fulcrum ; 

 iltiply the power by its distance from 

 the same point ; if the products are equal, 

 ^ht and power will counteract each 

 other. 

 [See Q. 35, bore.] 



40. T. You have not told me yet how 

 to calculate the power we must employ at 



: 1 ' ann of a lever, to balance a known 

 attached to the short 



in easily <i. 

 weight by the length of the short a: 



in the long result is the power 



of the short nrm,) gives us 240 as 

 f 240 by 24, 

 (which 



>unds we must employ ; 

 mechanical 

 because the weight 

 is twice that of the power. 



41. T. Ifwe* levertoover- 

 eome ;> preferable, a 

 long one or a hort 



A short on. 



he long one, 



/'. It i s.ii<l to be a rule, 



small weight, descending a long way in any 

 given length of time, is equal in effect to a 

 great weight descending a proper:, 

 shorter way in the same space of time. \V ill 

 you give me an example of this, and state 

 if it is correct ? 



P. It is quite true ; and it is on this 



principle that the see-saw acts. In this 



diagram we see that two boys are amusing 



Ives with a plank, which is balanced 



on a stone. The planlc A B, is a lever of 

 t class, of which the stone F is the 

 fulcrum. As the boy E is heavier than 

 the boy o, it is necessary to place him 

 nearer to the fulcrum. It is easy to see 

 that the lighter boy traverses a greater space 

 from H to D, than the heavy boy from c to 

 A. Let us suppose that E rep- 

 a weight of ten pounds, and that 



s a power of one pound. If the 



power is depressed too. the weight will be 



oc; the space B D bears the same 



proportion to c A, as the arm B F to A P. 



be ten times c A, which 



ihe power o, ol one pound, m 



, 

 only raised 



/ r U 11 < . : , qualto 



the resistance overcome. \\hieh is the cause 



saying that it retpiir. * ten pounds 



r to raise a weight of ten pounds ; 



machine was enabled to over- 



iiis resistance with a Mini! ]>< 



,tinct efforts instead of 

 only ( 



useful applic.r 



an example, uh'-n srn 



' 



a scale 



/., acting upon the arm ac, and 



