JOS 



AfRPUMP POR A PERFECT VACUUW, 



Airpamp for 

 » perfect Ta- 



p. '2t5.) It may not be amiss to obserre here, that it seero* 

 necessary, that a correction for this should be made, in 

 taking observations with the barometer ; as the mercurial 

 atmoephere will react upon the surface of the top of the 

 column, and prevent it from rising to the full height, ta 

 which it would otherwise attain. 



By the following means, a perfect vacuum, I believe, 

 may be obtained. Let A B (fig. 2) oe a tube of metal, 

 ground so as to be perfectly cylindrical in the inside: let 

 C C be the piston-rod ; and D the piston, which is solid ; and 

 Jet abeasmall metallic valve, opening outwards: also let the 

 concave and convex surfaces of the barrel, and of the piston, 

 be ground accurately to each other. Let us imagine the 

 piston to be at the top of the barrel, and that all the air is 

 expelled by means of the valve a: if the piston be now 

 forced downwards, the space above it will be a perfect va^ 

 cuum; at least with respect to air, and all evaporable 

 fluids. The valve might be placed in the piston, as re* 

 presented by the dotted lines at b ; and in this case the 

 one at a would be unnecessary. The action of this pump 

 might be rendered more secure, by closing the bottom of 

 the barrel, and inserting in it a metallic valve, opening out-^ 

 wards; and by making the piston rod to move in a collar 

 of leathers: the piston also and the bottom of the barrel 

 might be ground to each other. By these means, there 

 V would be but little danger of any air forcing itself into the 

 vacuum, which would be perfect above the piston, and 

 invhich some Tiearly, if not quite, so below it. This addition is shown by 

 pcriments the dotted lines. If the barrel were made of glass, we 

 should then have it in our power, to observe the appearance 

 of the electric fluid, in a perfect vacuum ; which, I believe, 

 has never yet been the case. The bottom of the piston also 

 roi.^ht be formed of different metals, and exposed to the 

 action of a burning glass, or of a galvanic battery. If 

 any elastic fluid were generated by the process", it woul4 

 be easy to collect and to ascertain its nature. Many other- 

 solid substances might be acted upon, by fixing them ac«s 

 turately into the bottom of the piston, so as to form a part 

 l^f it ; and by using the same agents, as in the former case. 

 The principle upon whi9h this pump is constructed I 



cosjfSid?? 



•light be 



