MECHANICAL PARADOXES. 



jet strikes an obstacle, it tends to drive it for- 

 cibly away. If a light disc, for instance, be 

 suspended above the jet in the position shown 

 at C (Fig. 24), this disc will be driven away 

 and prevented from falling down on the jet 

 at B. 



But if, instead of being presented to the 

 jet at some distance, as in that instance, the 

 disc be placed quite closely over another disc, 

 which is pierced with a small hole for the 

 steam-jet, we get from these changed condi- 

 tions a very different result. The upper disc 

 now, instead of being repelled by the jet of 

 steam, appears to be attracted by it. It is, at 

 any rate, pulled down by it into close contact 

 with the lower disc, as at E D F, so that to 

 separate the two discs requires considerable 

 force exerted through the strings by which 

 the upper disc is suspended. 



Now, how is it that exactly the same set 

 of circumstances can produce two such oppo- 

 site results, the only change in the conditions 

 being the provision of a second disc for the jet 

 to shoot through and the lessening of the 

 distance from which it shoots against the first 

 disc ? 



In the first case, we may consider that the 

 pipe A forms part of the boiler, and that boiler- 

 pressure in the steam extends to the inner end 

 of the small hole B, throughout the length of 

 which it is continually diminishing till it comes 



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