302 Theory of the Pneumatic Paradoz. 
tected from the impulse of the blast, will be sustained against its 
gravity by the afflux of air against its under side. 
Mr. Spencer’s very ingenious explanation is founded on the 
principle, to use his own words, “that currents of fluid, whether 
elastic or non-elastic, exert no force but in the direction in which 
they move; the latter is fully proved by forcing air or water 
through a cylindrical tube ; if holes be made through the sides of 
the tube, none of the fluid will escape.” A late writer, in a Lon- 
don scientific journal, expresses the same principle thus: ‘It isa 
well known property of fluids that they transmit their pressure 
equally in all directions; but this law applies only to fluids in a 
state of rest. When they are in motion, they are subject to the 
laws which regulate the motions of solids, and do not transmit 
any lateral pressure, except where they meet obstacles to their 
onward motion.” Hence it is inferred, that the currents which 
radiate from the centre of the disks, exert no pressure against their 
internal surfaces, and that, as the impact of the blast is not sufli- 
cient to overcome the exterior atmospheric pressure, the movable 
disk consequently adheres. 
- ‘This inference seems necessarily to follow, if we admit the cor- 
rectness of the alledged principle of hydraulics from which it is 
deduced. The only proofs of the principle,.as applied to liquids, 
I have seen cited—not by Mr. Spencer, who cites none—but by 
~ others, are the experiments of Bossut and Venturi, both philoso- 
phers preeminently distinguished for their experimental researches 
and discoveries in hydrodynamics. It may however be asserted 
without fear of contradiction, that whoever carefully reads Ven- 
turi’s original work “on the Lateral Communication of Motion 
in Fluids,” cannot fail to perceive not only that he nowhere ad- 
‘vances such a general proposition, but that it is irreconcilable 
‘with his reasonings in several parts of the work. . He describes 
but two or three experiments with cylindrical tubes that lend 
any semblance of support to it, and it is hardly necessary to Say; 
that the very novel and interesting results he obtained with tubes 
of a different form, or with long, descénding, cylindrical tubes, 
are so inseparably connected with the peculiar form, or the post 
‘tion of the tubes employed, as to furnish no evidence of the gene 
Principle alledged by Mr. Spencer. 
