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over to the sucking-side but against this the necessary precautions 
have been taken. 
B. 1. If the packing of the plunger is to produce no difficulties 
it is necessary that it should give a good closure with little frict- 
ion, for which reason I have replaced CaILLETet’s packing by a 
collar packing. 
B. 2. It is also necessary that the plunger should remain per- 
fectly smooth (comp. 2.2), which can only be obtained in the long 
run when a perfectly rectilinear motion independent of the packing 
is secured. This is facilitated by the use of guides and rods and 
by modifying the beam accordingly. 
%. 3. The moving of the mercury should be slow only, (from 20 to 60 
up and down movements per minute); to obtain this and also to allow 
the pump to be regularly worked by hand, the crank is connected to the 
shaft which makes 60 revolutions per minutes (with an electromotor 
up to 90) by means of chain gearing. 
€. During compression, the pump-cylinder must always be enti- 
rely filled with mercury, so that the high pressure gas remaining 
behind in the clearance space, shall not make exhaustion impossible 
when the r:ercury recedes. 
Each time that the mercury presses the gas through the forcing 
valve into the reservoir of compressed gas, the gas takes some 
mercury with it. In CAILLETET’s pump to make up for this loss, 
some mercury is admitted into the sucking chamber from the reser- 
voir above v (PL. I fig. 2). But after some time there is an uncer- 
tainty about the quantity of mercury in the high-pressure reservoir 
of the pump, more especially in consequence of the leakage of 
mercury from the apparatus, which is unavoidable with the con- 
struction of this pump. 
If too much mercury is admitted into this reservoir, it would 
overflow into the apparatus in which the compressed gas is forced. 
In the newly constructed pump the quantity of mercury to be 
used is measured precisely once for all and is not liable to diminish. 
Moreover a capillary connection has been contrived between the 
reservoir of compressed gas and the pump-cylinder, through which a 
quantity of mercury which can be exactly regulated, can flow back 
from the former into the pump body, so that during every com- 
pression there is a small excess of mercury in the latter. 
D. 1. A perfectly rectilinear up and down motion is desirable in 
order to ensure a satisfactory fit of the forcing valve, which separates 
the pump reservoir from the pump cylinder. 
D. 2. In finishing the upper end of the pump chamber we must 
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