EXHAUSTING MACHINE* 63 



X. 



T)escripft07i of an exhauslhig Machine on the Principle of 

 the Torricellian Vacuum: by Dr. Thomas Stewart 

 Traill. 



S 



OME time ago I was engaged in a series of pneiiraatic Chief defect of 

 fxperiments with the air pump, which led me to consider ^JJ^^'^'f^^'^^i^g 

 of the best means of obtaining a vacuum. The chief im- imperfect. 

 perfection of an air pump consists iu its not being capable 

 of affording a perfect vacuum. Each stroke of the piston 

 removes a portion of air in the receiver; but the remaining 

 air expands, until it occupies the same volume which the 

 whole of the included air did. The next stroke abstracts 

 an equal volume of air with the former, but as it is now - 

 less dense, the i-eal quantity is smaller; and hence every 

 succeeding stroke removes a less quantity of air than the 

 preceding. The exhaustion goes on, till the elasticity of 

 what remains in the receiver is no longer able to open the 

 valves of the machine, when it has reached the utmost li- 

 mit. But even if the machine was constructed in the most 

 perfect manner possible, it would evidently be impossible 

 to obtain a complete vacuum on the principle of the air 

 pump : for its effect is expressed by a fraction, the value of 

 which, though constantly increasing, never amounts to 

 unity : i. e. though continually approaching to, it never can 

 afford a complete vacuum. 



Impressed with these objections to the air pump, it occur- Attempts to 

 red to me, that, if there was a convenient method of using: ^P^)^' '^^'^^^^' 



. . " riceliiaa va- 



t]>; Torricellian vacuum, it would be preferable to the com- cuuin. 

 mon air pump, even when best constructed. After various 

 attempts, the annexed figure and description will give an 

 idea of the machine, which I conceive well adapted to answer 

 the end proposed. 



The object in this machine is to procure a vacuum in the Apparatus for 

 receiver D, by means of mercury, with which the receiver '^'s purpose 

 is previously filled. A (PI. I, fig. 1) is a circular plate of 

 thick glass, firmly imbedded in the wooden frame C, which 



is 



