^team Engine ^ ^9 
we shall despair of having these objects regarded as they 
were formeri) ; semblance being for the greatest part the 
order of the day* 
Mr. Watt has adopted a gage (very improperly^ called 
a barometer,) to indicate the degree of vacuum in his en- 
gines ; and we deem it of important consequence to the 
well- going of the engine, the profit of the proprietor, and 
the credit of the engineer ; yet in many engines in Lon- 
don we see this important instrument either out of repair^ 
or wholly laid aside. The form is given at fig. 3. They 
have been made of glass; but if the quicksilver is not 
very pure, the alloy with which the venders of this article 
adulterate it is by constant action brought to the surface, 
and that and the vapour together make the tube so foul 
that no precision can be obtained. Iron therefore is the 
best material : both parts of the tube should be correct- 
ly of one diameter, or else the result will be erroneous. 
This tube must communicate with the condenser by a 
small copper pipe, and a stop-cock be placed between 
the gage and condenser. The index in this instriimeiit 
is a light deal rod, which is put into the shorter tube; 
and quicksilver being poured into it within three inches 
of the end, the rod is put into the tube, and it floats on 
the quicksilver. It is almost needless to remark, that the 
graduations on this instrument most be inverted with re- 
gard to those of a single tube. 
Perhaps it should have been noticed before, that the 
rod of the discharging pump is comiected with the lever 
at some point determinable by the length of the stroke ; 
and in this figure it rises w ith the piston, and brings up the 
air and water with it, both of which arc discharged at the 
branch /?, in which is a hanging valve opening outward. 
The valve /, the office of which is to open or shut the 
commiinicatlon between the cylinder and the boiler, is sO' 
* I not see why. T. C/ 
