312 



Theory of the Pneumatic Paradox. 



sage and barrel, and then in a full gush into the receiver through 

 the tube, when a disk of card three inches in diameter was imme- 

 diately blown oif. I repeated the experiment v/ith a very excel- 

 lent air-pump, made by Mr. Joseph Wightman of this city, capa- 

 ble of producing the degree of exhaustion just described, and hav- 

 ing a passage for admitting air into the receiver one fifth of an 

 inch in diameter. The tube to which the perforated disk was 

 adapted being of the same diameter with the air passage, and the 

 latter being closed with the thumb, and the air exhausted to the 

 one two hundred and fortieth part, on suddenly removing the 

 thumb a circular card three inches in diameter, and on another 

 trial, a disk of tinned sheet iron two inches in diameter and 

 weighing forty nine and a quarter grains, were blown nearly to 

 the top of the receiver. By reducing the size of the disk, a less 

 degree of exhaustion becomes necessary in order to blow it off. 

 If it does not exceed one inch in diameter, it is not necessary to 

 exhaust the air to more than a sixtieth part, though I have found 

 it impossible, when the apparatus is adapted to the stop-cock of a 

 condensing chamber, the tube being one eighth of an inch in di- 

 ameter, to blow off a movable disk one inch in diameter by 

 means of a very powerful current of highly condensed air. 



Having completed the ungrateful labor of pointing out the 

 errors of the various theories that have been proposed to explain 

 the pneumatic paradox — which seem- y\«. 6. 



ed however a prerequisite to the undis- 

 puted admission of the correct one — 

 I now proceed to adduce the proofs of 

 the theory contained in the first part 

 of this article. 



Experiment I. In order to show that 

 rarefaction may be produced in a space 

 having a free communication with the 

 external air, I cite the following expe- 

 riment of Hauksbee. 



In two opposite sides of a small box 

 holes were made, one somewhat larger 

 than the other, and to these holes were 

 adapted the short tubes A and B, seen 

 in the accompanying figure. Through the top passed in an air- 

 tight manner a barometer tube, the lower end being immersed 



