238 Royal Society : — 



its top and bottom are secured to it air-tight by screws ; in it works 

 a plunger of boxwood well varnished 10*4 high, and moving so loosely 

 that mercury may pass it easily. The plunger is wrought by a rod 

 passing through the collar of leather II. In the top of the cylinder 

 is a stopcock E, to which is fixed a tube of vulcanized caoutchouc 

 (varnished with a solution of caoutchouc in benzidine), which is 

 shown hanging down ; it has a coupling to connect it with an 

 ordinary air-pump. There is also in the top a screw S for admitting 

 air. One end of the bell's cock communicates with the atmosphere, 

 the other with the receiver-plate II. This is of glass 2 inches in 

 diameter, 0*75 thick, and is cemented on the top of the iron pillar P. 

 Through it are drilled the passages shown in fig. 3 ; in t is ground 

 the glass tube, shown in fig. 1 by T, the end of which is in contact 

 with the cock, and their junction made air-tight by a tube of Para 

 caoutchouc ; in g and h are similarly ground the siphon-gauge G 

 and the glass cock K. These all communicate with the receiver by 

 the passage v, and by removing the tubes can be easily dried or 

 cleaned. The cock K is connected by elastic tube with the catch-jar 

 N, which is supported in a small mercurial trough M. 



The operation of this machine as an air-pump is as follows : — The 

 receiver being placed on R, open the screw S, press down the plunger 

 nearly to the bottom of the cylinder, remove the key of the bell- cock, 

 and pour through the opening which it leaves as much mercury as 

 will fill the bell to the bore of the cock. In this one 10 lbs. are 

 required. Raise the plunger to the top, and the metal will subside 

 from the bell till only 0*3 of an inch remains on the top of B, filling 

 the space left vacant in D by the rising of the plunger. The length 

 of the plunger and the height of B must be adjusted to this condi- 

 tion. Replace the key ; turn it to communicate with the atmosphere 

 (which position I call («)), and depress the plunger. The mercury 

 will rise again in the bell, filling it, and expelling the air from it, till 

 at last a little mercury will appear in the bore of the cock. To 

 prevent this from being splashed about, a bit of bent tube v is ground 

 on the end of the cock, which receives it, and when it has too 

 much is removed and emptied into D through S. Secondly, turn 

 the key to shut off the bell (position (o)) ; draw up the plunger, 

 close S, open E, and couple it to an air-pump, with which exhaust D. 

 This pump may be of the commonest description, for an exhaustion 

 of one or two inches is quite sufficient. The mercury will sink in 

 the bell, leaving above it a Torricellian vacuum. Close E, and 

 turn the key to communicate with the receiver (position (r)) ; its air 

 or gas will expand into the bell. 



These three operations form the cycle of operation, and must be 

 repeated till the required exhaustion be obtained, with one modifi- 

 cation of the first one. In it, at the second and all subsequent strokes, 

 the key is to be at (o) and S opened ; thus the atmospheric pressure 

 will raise the. mercury and do much of the plunger's work ; that must 

 then be depressed and the key set at (a) ; the other two steps are as 

 at first. 



When the instrument is to be used as a gas-holder, either the 



