74 



MECHANICAL WORK. 



reference to mechanical work, this fact may be thus 

 stated: The quantity of icork which a falling body con- 

 tains is exactly the same as that which would be sufficient 

 to raise it to its original height. 



It is somewhat difficult to prove this fact experi- 

 mentally in the case of bodies falling freely ; but it may 

 be more easily demonstrated by means of bodies which, 

 are allowed to descend by the force of gravity along a 

 path which is not vertical, provided that friction and 

 other resistances to the motion are so small that the 

 work expended in overcoming them may be left out of 

 consideration. A body suspended by a fine thread may 

 be constrained to move along the arc of a circle, and this 

 circular motion is very convenient for illustrating the 

 above statement. 



Fio. 53 (^ rtl sine). 



Remove the clamp from a retort-stand and attach to it, with a 

 fow tacks or drawing-pins, a rectangular piece of cardboard, FT, 

 fig. 53, about 55 cm long, and ll cm high, having previously made four 

 horizontal holes, a, &, c, d, in the vertical rod of the stand, at dis- 

 tances of 41, 31, 21, ll cm respectively from the foot. Pass a long 

 pin of iron wire, sufficiently thick to fit tightly, through the upper- 

 most hole, and let the head project about 1 or l cm> .5 from the rod ; 



