¢ 
* 
Theory of the Pnewmatic Parador. 317 
the air becomes “of the same density with the atmosphere,” and 
beyond which it expands, to “be not much more than double 
the diameter of the tube.” 
2. There is, exterior to the thin ring of rarefied air and sur- 
rounding it, a thin ring of condensed air, extending to the cir- 
cumference of the movable disk. ‘This ring of condensed air is 
owing to the resistance of the exterior air to the egress of the 
radiating currents, and proves the fallacy of the idea advanced 
by Prof. Espy, in order to account for the maintenance of rare- 
faction during the continuance of the air-blast, that “the atmos- 
phere makes no sensible resistance to the egress of a current.” 
The reason why the rarefaction extends to so small a distance 
only from the tube, is set in a clear point of view by the follow- 
ing experiment. Blow through a brass tube one eighth of an 
inch in diameter and six or eight inches long, and a common 
lamp may be extinguished at the distance of about two feet. 
Attempt to extinguish it at the same distance by blowing through 
_ a similar tube, having a conical termination, such as is described 
in Exp. TV, and the flame will not waver in the slightest degree, 
and it will be impossible to extinguish it, even when held quite 
near to the end of the tube. If the flame be applied close to 
lateral holes in the conical part, it will be drawn in at those only 
which are near the place of junction, and not be affected in any 
way at the remote ones. Hence it would appear, that the current 
expands as it enters the conical tube, and quickly loses its mo- 
mentum by having to overcome the inertia of a continually en- 
larging column of air, while, in the atmosphere, it is prevented 
from expanding in an equal degree by surrounding pressure, and 
consequently moving before it a less column of air, it does not 
so quickly lose its momentum. The above reasoning applies 
equally to currents radiating from the centre of the disks, which 
on account of their diffusion are quickly retarded, and hence pro- 
duce a rarefaction to oply a small distance from the tube, and 
hence too are capable of producing only a slight condensation 
before them. ; 
The principal results of the foregoing researches may be stated 
as follows: On blowing through the tube, there are, when the 
movable disk is placed underneath, three forces which urge it 
downwards—its own gravity; the impulse of the blast, weak- 
ened by striking obliquely against a somewhat conical mass of 
Vol. xxxix, No. 2.—July-September, 1840. 1 
