MUTUAL CHANGES COMMON TO ALL MATTER 



65 



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are limited in this and many other similar investigations by the 

 enormous stress which exists between the earth and all bodies 

 near it. We cannot, therefore, 

 easily prove the equivalence of 

 mutual changes. In all the pre- 

 vious experiments the displacement 

 which has been measured has been 

 a displacement in the direction in 

 which this stress acts, i.e. in a ver- 

 tical direction. In the case of 

 bodies moving without friction over 

 a smooth horizontal surface, there 

 is no displacement possible from the 

 stress between them and the earth, 

 and the displacements due to mutual 

 changes will not be interfered with. 

 Glass balls of different sizes rolling 

 over a horizontal glass plate would 

 approximate to these conditions, 

 and with them we can prove roughly 

 that a small fast-moving body can 

 produce the same effect as a large 

 slow-moving body. By causing 

 balls of various sizes to strike others 

 at different rates this may be shown 

 in a rough manner. 



We may obtain similar results 

 by fastening two unequal masses 

 to a long cord passing over a pulley. 

 If the heavier mass is supported, 

 and the smaller mass allowed to fall 

 for some time, and so acquire a 

 speed before it begins to tighten the cord, it will be found 

 that a rapidly-moving small mass is able to raise a much 

 larger mass. The arrangement shown in fig. 20 will suffice for 

 a number of observations. Similar results are observable in 

 the use of a hammer or a pile-driver, 



Fig. 20. 



