! y 2 NATURAL HISTORY 
sect. xiv. The moft powerful as well as conflant engine hitherto invented 
Of the fire- J s the fire-engine. This engine is now well known to the learned, 
power and but as their books do not reach every where, and this machine is 
P u°bHc t0the efpecially ferviceable for the working of deep mines 11 , and of great 
advantage to the publick revenue, a general explication of it s prin- 
cipal parts, it’s powers, and profit to the government, may not 
be improper. The principal members of this engine are exhibited 
in the plate annexed, N XIX. viz. the ciftern or boiler T, 
(Fig. ii.) the cylinder P, and the bob O, I, turning on an axis 
which refts on the middle of the wall Y. The following is 
the procefs of it’s feveral operations : The ciftern T, full of 
boiling water, fupplies fteam (by means of an upright tube and 
valve which fhuts and opens) to fill the hollow cylinder P, and ex- 
pel the air through a horizontal tube S, placed at it s bottom . 
the cylinder, as the fleam rifes, and the weight of the mine-water 
depending from I, K, L, preponderates, begins to fill with vapour 
and the pifton which plays up and down in the cylinder rifes, and 
when it is got near the top opens a clack by which cold water is 
injedted and condenfes the vapour into nearly the twelve thoufandth 
fpace which it before occupied, and the cylinder being then nearly 
empty, the piflon of iron edged with tow and covered with water, 
(to prevent any air from above getting into the cylinder) is driven 
down by the preflure of the atmolphere (with the force of about 1 7 
pounds F on every fquare fuperficial inch) nearly to the bottom of 
the cylinder; at this inflan t it opens the valve which lets in the 
fleam from the boiler T, and then the piflon afcends till it opens 
the condenfing clack above, which brings it down again to open 
the under clack and admit the fteam, and thus continues afcending 
and defcending as long as the managers think proper; this procefs is 
quick, or otherwife, as the fleam is by increafe or fubftra&ion of fire 
made more or lefs violent, to drive the engine fafter or flower. To this 
piflon the end of the bob O, is faftened by an iron chain, and as 
the piflon defcends in the cylinder P, this end of the bob is drawn 
downwards, and vice versa ; as the end O is drawn down, the 
other end of the bob I, afcends, and by a chain, I K, draws up with 
it, from an iron or brafs cylindrical tube, called a pit-barrel ", through 
a tyre of wooden pumps, (O O, Plate XVIII. Fig. 1.) a column of 
water out of the mine equal in diameter to the bore of that tube, 
and in height to each flroke or motion of the piflon in the cy- 
linder P, and the fweep of the bob, I K. Many improvements nave 
■ lately been added to this excellent piece of mechanifin, among 
x By aft of parliament the duty of coals ex- t Seventeen pounds eight ounces and 347 grains 
pended in the working fire-engines in Cornwall is according to Helfnam. 
remitted * Letter X ’ P!ate XVIII> Pi S' Xl 
which 
