144 Account of a New Air Pump. 
tions. I have never yet been able to get an iron barrel, as 
it cannot be procured here, and numerous avocations have 
prevented its being obtained elsewhere. . 
Figs. Ist and 2d correspond in their lettering. 
In fig. Ist. (see plate Il.) AB, CD, EF, represent a ver- 
tical section of the instrument. G is a barrel of cast iron or 
glass. screwed firmly to the table EF, in it is the solid pis- 
ton H moved by the rack work I. K isa glass globe rest- 
ing upon the table CD, of a little less capacity than the 
barrel G with which it communicates the glass tubes 
L and M firmly cemented into the peice N and into the 
bottom of the barrel G. To the top of the globe K is ce- 
mented the thick cap O, through which are made two ap- 
ertures, into one of which is screwed the stop-cock P com- 
municating with the plate of the pump R; over the other 
aperture rests the valve S opening nto the atmosphere, 
eect is seen in fig. 2d.) Jo the globe K isa stiff 
> ascending into the cock P a short distance, and on it is 
screwed the valve T; the other end descends into the tube 
L, and to it is attached the wooden or cork ball U. e 
will now suppose the piston H withdrawn, and the barrel G 
filled with quicksilver; the tubes L and M being open will 
be filled to the height of the dotted line. Put the piston 
carefully in so that no air shall be between it and the mer- 
cury As the piston descends, the mercury rises, and when 
it reaches the ball U it floats it, and by means of the wire 
forces the valve T against the aperture that communicates 
with the receiver R, and as the mercury continues to rise, 
the air driven before it has no way of escaping but through 
the valve S. The piston is now at the bottom of the barrel, 
and the globe is full of mereury,—if the piston be now drawn 
up, a vacuum would be formed in the barred, but the mercury 
in the globe must descend as it is above the level of the 
piston the whole height T, and the vacuum in the globe K 
would be Torricellian were there not a communication be- 
tween it and the receiver R. When the mercury again as- 
cends into the globe, it expels every particle of air provided 
the mercury rises into the aperture at S; and to ensure tbis 
the cap O is formed into a rim so as always to supply the 
contraction or waste, and it is admitted towards the end of 
the exhaustion by raising the valve S with the finger. The 
air is admitted through a hole a in the cock P, a section 1S” 
