Theory of the Pneumatic Paradox. 309 



tube. This proposition is subject to one or two limitations. 

 When, in Experiments III and lY, the water in the reservoir was 

 allowed to discharge itself to the level of the tube, it ceased to 

 escape in the form of a jet, though not to drop from the lateral 

 holes, after its head was reduced to a few inches, and in the case 

 of tubes not having an ajutage, after it was reduced to a little 

 more than a foot. Also, if descending tubes of considerable 

 length be employed, there will be no lateral jet beyond a certain 

 depth, since in obedience to the laws of falUng bodies, the de- 

 scent of the water will be constantly accelerated by gravity ; and 

 this effect will finally become such as to overcome the mutual 

 adhesion of the particles, and the lower portions becoming detach- 

 ed from those above, will leave void spaces, into which, if lateral 

 holes be made, the surrounding air will rush, and be carried down 

 with the stream. 



I have described the foregoing experiments with considerable 

 minuteness, on account of the importance they possess, inde- 

 pendently of their connexion with the main object of the pre- 

 sent article, as proofs of a controverted if not new principle of 

 hydraulics. They do not prove that the lateral pressure of water 

 flowing through cylindrical tubes is equal to that of water in a 

 state of rest under the same head. It is undoubtedly less, but to 

 determine in what precise degree it is less, would be an exceed- 

 ingly difficult problem, the solution of which is not necessary to 

 my present purpose. 



It remains to be shown that Mr. Spencer's proposition is false, 

 as it respects aeriform fluids as well as liquids. The experiments 

 with the tubes of tissue paper, described in the first part of this 

 article, prove not only that air in a state of motion is not, as a 

 necessary consequence, in a rarer state than the surrounding at- 

 mosphere, but that, when it is blown through tubes of a uniform 

 bore, and having orifices equal to that bore, it exerts a lateral pres- 

 sure. The following additional experiments corroborate the same 

 conclusion. 



Exp. I. Make with care several holes of various sizes, from a 

 twentieth to an eighth of an inch in diameter, and at intervals of 

 an inch or two, in a brass tube one eighth of an inch in diameter ; 

 on blowing strongly through it, the air will issue in an oblique di- 

 rection from either hole, when the others are stopped, with such 

 force as to extinguish a common lamp. Likewise, if a narrow strip 



Vol. XXXIX, No. 2.— July-September, 1840. 40 



