OF CORNWALL. 173 
which I cannot but mention one in particular which is, that as this 
engine flood formerly, if the fire-men chanced to nod, the violence 
of the motion increafing with the fire, the weighty bob, O I, beat, 
fhocked, and endangered the whole machine, and the fabrick it is 
inclofed in ; but now when the fire is at the extreme height, and 
the bob begins to beat and flrike the fprings, it lets fall a trigger 
into a notch and flops the injeCtion-cock, and the whole movement 
is flopped, till the injection of the cold water into the cylinder is 
reflored ; fo that this engine is now brought to fuch perfection, that 
in a great meafure it tends, regulates, frees, and checks itfelf; 
feveral fubordinate members, wires, clacks, and valves are all 
moved, opened, and fhut by the force of the fleam, and the 
motion of the piflon ; inafmuch as that by enlarging the cy- 
linder, and other parts in proportion, few Cornifh mines are fubject 
to more water than this engine will mailer : its power is in propor- 
tion to the diameter of the cylinder principally, the flrength of the 
fleam, and the depth it draws. This, here exhibited Plate xix. Fig. 
1. and 11. is the fire-engine which, in the year 1746, belonged to 
the pool-mine, Plate xvm. and the cylinder’s diameter from the outer 
edge, was but three feet; but they make them much larger now; 
and it is imagined y , that if they were flill to increafe the diameter 
of the cylinder, and make it alfo fhorter than they do now, the 
force would be augmented, and though the column of water ex- 
haufted would be fhorter, yet might this be well remedied by 
increafing the number of tubes, which the greater preflure on the 
piflon would eafily manage. A cylinder of forty-feven inches bore 
at Ludgvan-lez work, in the parifli of Ludgvan, making about 
fifteen flrokes in a minute, ufually drew through pit-barrels of fif- 
teen inches diameter, from a pump thirty fathoms deep, about an 
hogfhead at each flroke, that is, fifteen hogfheads of water in each 
minute ; fo that the quantity of water raifed in a given time, is as 
the fquare of the diameter of the pit-barrels, O O X, Plate xvm. 
Fig. 1. and the height and number of the flrokes in that time. But 
the cylinders may be made much larger; that at Herland (or Dre- 
nack) mine, in the parifh of Gwinear, is feventy inches in diame- 
ter, and will draw a greater flream of water at any equal depth, 
in proportion to the fquare of its diameter. The only objections 
to this engine are the great expences in ereCting, and vafl confump- 
tion of coals in working it. To obviate thefe expences feveral 
methods have been fuggefled of increafing the elaflicity of the fleam 
and reducing the fize of the boiler z , which can be decided only by 
experience, and to that we mufl refer them. 
y See Philofophical Tranfaaions, vol. XLVII. page 197, for tjie years 1751 and 1752. 1 Ibid, 
vol. XLIX. part ii. page 539. . 
Y y Explanation 
