310 Theory of the Pneumatic Paradox. 



of tissue paper two inches long be held in the hand by one end, 

 so that the other may rest against the hole, it will be blown away 

 from it. To obtain more exact results, I used a brass triblet tube 

 similar to those already described, twelve and a half inches long, 

 and three eighths of an inch in diameter, and having a lateral 

 hole in the middle one line in diameter. The sectional area of 

 the tube, therefore, was twenty and a quarter times greater than 

 the area of the hole ; consequently, the latter would not, even 

 on the supposition that the lateral and outward forces are equal, 

 permit the issue of so much as one twentieth of the air expelled 

 from the mouth ; notwithstanding which, the lamp was readily 

 extinguished, and the strips of tissue paper repelled by the lateral 

 jet of air. 



These results prove beyond the possibility of doubt, the fact of 

 the lateral pressure of air forced through cylindrical tubes. It is 

 not important to determine the comparative intensity of the on- 

 ward and the lateral force ; the latter must be greater than the 

 exterior atmospheric pressure, since the lateral jet could not other- 

 wise take place. This fact being incompatible with the funda- 

 mental principle by which Mr. Spencer explains the adhesion of 

 the disks, his hypothesis necessarily falls to the ground. 



A writer in a late number of the London Mechanics' Magazine, 

 maintains, on the authority of an experiment of Mr. Tomlinson, 

 that the movable disk is not retained in its place during the con- 

 tinuance of the air-blast by exterior atmospheric pressure. As- 

 suming also the principle which has just been disproved, that 

 currents of air flowing through tubes exert no lateral pressure, 

 and applying it to the currents that radiate from the common 

 centre of the tube and disks, he asserts that " the disk is not re- 

 moved because there is in fact no force to effect its removal." 

 He overlooks the tendency of the impulse of the blast to separate 

 the disks, and leaves the adhesion of the movable disk against 

 its own gravity, when it is placed underneath, unaccounted for, 

 unless he means to be understood to refer it to what he calls "the 

 attractive force of the air-blast," when it is ''spread out in a thin 

 film" between the disks. The necessity of having recourse to 

 so novel a supposition, will be obviated by the results I obtained 

 in repeating the following experiment of Mr. Tomlinson. 



" Let a brass tube, open at both ends, and terminated at the 

 end by a perforated screw, be fixed to the table of an air-pump. 



