300 Theory of the Pneumatic Paradox. 



continually expands, so that with the same velocity, it would the 

 next moment fill a much larger space, BB, CC. The same effect 

 takes place with respect to every other portion of air, as it recedes 

 from the centre. The interposed air becoming thus rarefied, does 

 not possess sufficient elasticity to counteract the exterior atmos- 

 pheric pressure, and the two disks consequently adhere. The 

 rarefaction is maintained during the continuance of the air-blast, 

 by the impulse of the outward currents against the surrounding 

 atmospheric air, which prevents it from rushing into the space 

 between the disks to restore the equilibrium.* 



It is to be further observed, that the reaction of the radiating 

 currents against the air in the circle AAN, together with the col- 

 lision of the air-blast, must cause a certain quantity of air to re- 

 main stationary at the centre of the disk, and assume a some- 

 what conical forra.f The air-blast strikes obliquely against this 

 conical mass of air, and consequently acts with only a part of its 

 force to separate the disks. 



The experiment explained above, suggests a very important 

 caution in regard to the form of safety valves to steam boilers. 

 If the}?' are so constructed, that the steam, when it escapes, must 

 radiate from the centre of and between two parallel surfaces, they 

 will adhere with such force, that instead of being efficient safe- 

 guards against explosions, they will serve merely to delude into a 

 false security, not to avert the danger of those dreadful catastro- 

 phes. 



Addition. — The preceding part of this article was written seve- 

 ral months since, and I was not then aware that any other expla- 

 nation of the pneumatic paradox, than those of which I have 

 endeavored to expose the fallacy, had been published. I was 

 subsequently informed by a friend, that one appeared several years 

 ago in the Journal of the Franklin Institute. On consulting that 

 work, I found, in the number for July, 1828, three explanations, 

 one by the distinguished philosopher Prof. Robert Hare, of Phil- 

 adelphia, one by Prof James P. Espy, since extensively known 



* For an aocount of a secondary rarefaction additional to that described in the 

 text, see the latter part of the article. 



t An analogous fact respecting jets of fluids striking against an obstacle cf equal 

 diameter, is stated by Dr. Young in his Lectures on Natural Philosophy, Vol. I, 

 p. 302, and represented by figure 273. 



