298 Theory of the Ptieumatic Paradox. 



and in the preceding experiment, the tube should be at least a 

 quarter of an inch in diameter. 



The most recent explanation of the pneumatic paradox that has 

 come to my knowledge, is extracted in the number of the An- 

 nals of Electricity, &c., for August, 1838, from a work on '' The 

 Causes of Planetary Motions," by Jabez Allies, Esq., who infers 

 from the adhesion of the disks, the general principle, that "air in a 

 state of agitation, or currents, whether it be cold, as from bellows, 

 or warm, as from the mouth, is in a more rarefied state than the 

 surrounding atmosphere," This is certainly a very summary 

 method of solving the difficulty. So far is this principle from 

 being true, that it may, I think, be satisfactorily shown, that when 

 air is expelled from the lungs or from a pair of bellows, it is in a 

 denser state than the surrounding atmosphere. The reason is 

 obvious ; the air, both in the bellows and in the mouth, is con- 

 densed at the moment of its expulsion by mechanical pressure, 

 and, as it issues into the open air, it meets with resistance to its 

 expansion; so that not only the current itself must be in a denser 

 state than the surrounding atmosphere, but it must condense in a 

 greater or less degree the air against which it is impelled. These 

 reasonings arc verified by the following experiment. Make upon 

 a small, square rod, a tube of tissue paper, which, when the rod 

 is withdrawn, will retain its form. Fasten this to a tube of me- 

 tal, or other hard substance, of at least equal bore, and blow 

 strongly through it, either with the bellows or the lungs. The 

 sides of the paper tube will become convex, and, of course, the 

 tube itself nearly cylindrical. This proves conclusively, that the 

 current of air in the tube is more dense than the exterior air, since 

 the tube, instead of being compressed, as would be the case, if 

 the contained air were in a rarefied state, gains, according to the 

 doctrines of isoperimetrical geometry, an actnal increase of capa- 

 city by its change of form. Instead of a square tube, a cylin- 

 drical one may be used. In this case, any rarefaction of the 

 contained air, would be indicated by a greater or less degree of 

 compression of the tube, which does not take place. The idea 

 most naturally suggested on observing the adhesion of the disks, 

 and the correct one, is, that it must be caused by the rarefaction 

 of the interposed air ; but the difficulty of assigning any satisfac- 

 tory reason for the rarefaction, seems to have driven all who have 



