as 
Mr. Patten’s Air Pump, Gazometer, &c. 95 
drilled as at h, fig. 2nd, and which when it is in the posi- 
tion it now is, opens a communication between the receiver 
and the barrel ;—we ‘will now now pid the piston close 
to Me bottom as in the figure, if the co eC 
as the aperture at h passes the hole leading from the 
recuver K, all communication between the barrel, and i 
is cut off, the air therefore in the barrel is driven out 
through the valve at I. There is upon the piston about the 
eighth of an inch of mercury represented by the dots, but 
before this can come in contact with the top of the barrel 
it must have expelled every particle of air, and if the 
ton ascends any higher this mercury is itself expelle led 
through the same aperture and is retained by the rim as 
represented by the dots, and the valve I, floats upon it. If 
the piston be now made to descend, ‘the mercury runs 
back until the weight of the valve brings it in 
with the aperture ; it then is effectually closed, swith still a 
quantity of mercury round it: the piston continues to de- 
scend until it gets nearly to the bottom, when the tube A 
coinciding with the hole in the tube aaa lets in the air 
from the receiver K. Fig. 2nd shows the manner of con- 
necting the tube aaa wiih the collar F, and also the diago- 
nal hole L through the piston rod used instead of a valves 
The lower part of the barrel with its valves H and G, acts as 
a common pump, and the air : admitted though the cock M. 
_ Another aor a I enclose for your inspection is 
used iastead of Pepys’ ingenious gas holder ABC 
the common gazometer, E is the outer cylinder with it 
frame AB. is the inner cylinder, which instead of bei 
soldered tight to the bottom as usual is screwed a 
and at H the wire goes through a collar, and bas a thumb 
crew upon it. ‘To use it, press down the cylinder E as 
sa: turn the thumb screw H, which opens the valve G, 
Pe in water and the air all escapes through the tube K, 
Jet the gas enter at D and the cylinder E rises, when it is 
fall close the aperture at D and press down: the cylin- 
