296 Theory of the Pneumatic Paradox. 



Art. VI. — An Attempt to determine hy Experimental Research 

 the true Theory of the Pneumatic Paradox ; by Jos. Hale 

 Abbot, of Boston, Mass. 



/In the year 1828, the following experiment, sometimes denom- 

 inated the Pneumatic Paradox, was published in various journals 

 in this country. "Cut a couple of cards each into a circle of 

 about two inches in diameter ; perforate one of these in the cen- 

 tre, and fix it on the top of a tube, say a common quill ; make 

 the oiher ever so little concave, and place it over the first, the 

 orifice of the tube being thus under and almost in contact with 

 the upper card. Try to blow off the upper card — you will find 

 it impossible."* To it was appended a statement, presumed to 

 be apochryphal, that the Royal Society of London had offered a 

 reward of one hundred guineas for a satisfactory explanation of 

 it. Several have been published, both in this country and in 

 England, but none that I have seen seem to me admissible, either 

 because the experiment may be so modified as wholly to set them 

 aside, or because ihe principle on which they are founded may be 

 demonstrated to be false. 



One explanation attributes the adhesion of the disks to the rare- 

 faction of the air, as it issues from the mouth, on account of its 



* It is not material that the movable disk should be concave. The whole of 

 the apparatus may be made of sheet brass or tinned sheet iron, which has the ad- 

 vantage of admitting a short pin to be soldered to the centre of the movable disk, 

 to prevent it from sliding out of place. In this case, the disk may be perfectly 

 plane. The diameter of the movable disk need not exceed that of the tube more 

 than three or four times, and either disk may be placed uppermost. The phe- 

 nomenon was observed almost simultaneously in the latter part of the year 1826, 

 by several individuals in France, and by Mr. Roberts, of Manchester, in Eng- 

 land. I extract the following interesting accoiint of the circumstances which led 

 the latter gentleman to make the discovery, from a late number of the London 

 Mechanics' Magazine. " Several years ago, Mr. Roberts had constructed an ap- 

 paratus for ventilating his manufactory. A pipe conveying a blast of air termina- 

 ted close to the wall of the principal apartment ; and with a view to regulate the 

 quantity of air to be introduced, he placed a valve over the terminus of the blast 

 pipe, but found to his surprise that the most powerful blast of air would not lift 

 the valve, (which was merely a piece of flat board,) but that it even held the valve 

 firmly down over the mouth of the pipe, so that the strength of one man was 

 insufficient to withdraw it. This experiment has often been shown to visitors as 

 one of the curiosities of Manchester." 



