300° Theory of the Pneumatic Parador. 
continually expands, so that with the same velocity, it would the 
next moment filla 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 form.t 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 they 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. J 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 explavations, 
one by the distinguished philosopher Prof. Robert Hare, of Phil- 
adelphia, one by Prof. James P. Espy, since extensively know? 
_* For an account of a secondary rarefaction additional to that described in the 
the latter part of the article. ; 
analogous fact respecting jets of fluids striking against an obstacle of equal 
y s stated by Dr. Young in his Lectures on Natural Philosophy, Vol. I, 
Ei 
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