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. 
rom 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 theori- 
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 whole 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 contiguous half of the cylindrical part, as 
was shown by flame being drawn into lateral holes made in it. 
A similar effect 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 seco rarefaction seems not to have been suspected, either 
by Mr. Abbot or Professor Espy, which accounts for the latter 
# 
8 supposing the distance from the centre at which 
