316 Theory of the Pneumatic Paradox. 



and care must be used not to confound the occasional adhesion of 

 the tissue paper after it becomes moist, with the exterior atmos- 

 pheric pressure. The substantial accuracy of the preceding re- 

 sults was verified by holding the plate in a nearly vertical posi- 

 tion, and applying a very small flame close to the holes. 



From the results I have described the following inferences 

 seem to me incontrovertible. 



1. Between the disk, there is a thin ring of rarefied air, the 

 inner circumference of which coincides with that of the, ori- 

 fice of the tube, and the breadth of which, in the experiments 

 last described, varied from a little more than a half to a little 

 more than three quarters of an inch. The rarefaction of this 

 ring is a compound result due to two causes. The first is the 

 expansion of the radiating currents, as explained in the first part 

 of this article, in consequence of their- filling more and more 

 space as they recede from the centre, which I shall call the pri- 

 mary rarefaction : the second is a sort of retrograde action of the 

 primary rarefaction, by which it becomes in its turn a cause of 

 additional rarefaction nearer the orifice of the tube, and which I 

 shall call the secondary rarefaction. I discovered this retrograde 

 action by means of my experiments with the compound tube, 

 in the first of which it has been seen, that on blowing, previously 

 to the juncture of the two parts, through the cylindrical tube, 

 jets of air issued from lateral holes in any part, proving the air 

 to be condensed throughout its ivhole extent ; yet, on adding the 

 conical diverging termination and blowing through the compound 

 tube thus formed, the air became rarefied, not only in the conical 

 part, but also in the co7itiguous half of the cylindrical part, as 

 was shown by flame being drawn into lateral holes made in it. 

 A similar efl"ect must be produced in the experiment of the pneu- 

 matic paradox, with this modification, that, in consequence of the 

 blast being arrested by the movable disk, the secondary rarefac- 

 tion extends only to the orifice — not into the tube itself. In what 

 proportion the secondary rarefaction contributes, in comparison 

 with the primary, to the adhesion of the disks, it is impossible to 

 decide, as they necessarily coexist, so that one cannot be produced 

 without at the same time producing the other. The existence of 

 the secondary rarefaction seems not to have been suspected, either 

 by Mr. Abbot or Professor Espy, which accounts for the latter 

 gentleman's supposing the distance from the centre at which 



