Theory of tlie Pneumatic Paradox, 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 diff"usion are quickly retarded, and hence pro- 

 duce a rarefaction to only 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. 41 



