172 N A T j ü R: A L <;H I S T O R Y
s e c t . xiv» - . .T ^ f i j p o f t ^ R P ^ e r E J l ^ i y q l l ,a ^ ö G q ^ ^ i } | | e n g i n e i h i t h e r t b dm ïeB tfe d
O f thefire». "jS' fhe fire-enginel (TCliis engine^hQvy.yfeJJ^ggn. tq.tlj0 :learned,
power and ‘ hut as. their books do not reach svery^frere, and tfrisi machine is
pSüctothe elpecially. feryiceable for the worjuug-of degp • mines % and.of great
' advajit^^.iöj-the publiek revenue, a general explication of it’sprincipal
parts^ iï’s. pp^rsj and profit, to therg§veïpment,uma.y net
be improper. The principal mepibers of this engine arc? .exhibited
in the plate annexed, N°. XIX. v iz, the. ciftern. or oofter X»
a ;^Fig. ii.) j&e cylindèr1 P, and the/bObO, I, iuinih^ "öfr .'an axis
which refis" on the1-\middle of frlié wall* Y. 1 x pe ,fotlötving is
the proèefs- o f it’s feveral operations : The ciftern x j full of
boiling water, fiipplies fteam (by ïrieans óf'an upright, tube, and
valve which fhuts and opens) to fill the 'hollqiv; cylindér P,‘ "aiid expel
the air threrugh a horizontal tiibë’S, placed' at it’§ bottom':
the cylinder,’ as the fteam rifts, and the weight 'of the mine-water
depending from I, K,- Ii,; preponderates, ‘ begirri t o fill with Vapour,
and the pifton which plays up and' down in the ’Cylinder and
when -it is gotnear th e to p opéüè a clack by ,' wmch, 8ola paper is
injeéted and cöndenfes the Vapour into, nearly' thb ri^^m ,moufanotn
Ipace which it before occupied, and the cylinder being,,th;eH heady
empty, the pifton of iron edgedVtith1 tow* and covered 'VVith ^p.eEi
(to prevent - any air front! abdvé\getiihg intó tthe pylihdétj rfs driven
down by the preffiire of the atmolphefe (with the fófce óf about17
pounds -f- c«i every lquare ftipefficM inch); nearly to tnè^bqtrapi o f
the cylinder^ at this. inftant ft bbéhs' the~vMw%hipï Jejl'In me
fteam firom the boiler T , and then thepiftött'hfüëhds till ïrfèpëös
the condenfing.clackraboyu, which _ brings^ it dövVn again to. Open
the under.dack and admit the fteam, and thus cOnrinu.e| amending
and defcending as long as ^the man^erafriinkprOpgr; thisj ptocefs is-
quick, or otherwife,as the fteam is' by increaifrpr- fubftradtidn of fire
made more or lets violent, to drive the engine fafter pr.flower. TO ibis'
pifton the end of the 'bob O, is fattened by ran frapt chain, jsthd as>
the pifton 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, afeends, and by ,a chain, ! K , draw® 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. i.) a column o f
water out of the mine, equal in diameter to thé. bore, of' that- tube,
and in height' to each .ftfpker:,. or motion of the pifton in'the . cylinder
P, and the frveep;of the bob,' !K . Many improvements have
lately been added to . this excellent piece of méchanifin, among l
x By aft of parliament the duty of cdals ex- f Seventeen pounds eight ouneps and 347 grains
pended in the working fire-engines in Cornwall is —according to Heliham.
remitted. ’ ... ri f. , }P tetter X , Plate xvlit.' Fig. '1. -
which
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