[ r 5* ] 
velocity, when the wind increafed, the fhaft mio-ht 
then be let to turn in the ufual manner. But there 
would be no danger of the fhaft taking fire by any 
degree of velocity, whilft it turned on thefe wheels, 
as it would not then rub at all. 
There have been many ingenious attempts, and 
fome confiderable improvements made, with refpedt 
to the having of fuel necefiary to work a fire engine, 
which is an article of great expence : but I do not 
find the diminution of fridlion has been confidered 
as any ways material in this point, although it muff 
necellarily 1 educe the quantity of fuel in proportion. 
The power of a fire engine is eftimated by the di- 
ameter of the cylinder and pifton ; on which the at- 
mofphere prefies, when there is a vacuum made by 
the condenfation of the fleam with which the cylin- 
der has been filled. This power, or preffure, is 
deemed equal to 15 pound per inch fquare on ame- 
oium : but I fhould imagine, that the fleam, with 
which the cylinder is filled, being water expanded 
into 4000 times its bulk by the adlion of fire, when 
reduced to its original ftate by a flrong injedlion of 
cold water dafhing againft the bottom of the pifton, 
and mixing with it, muft occupy fuch a fpace in the 
cylinder, as to hinder a perfedt vacuum, which ap- 
pears, in fome meafure, from the effedts ; for the 
power of the atmofphere on a fire-engine is feldom 
found to raife 7 pound per inch, and it can hardly 
require 8 pound per inch to overcome the fridlion of 
the feveral parts of the engine, and alfo to give a 
proper degree of velocity to the leaver. 
The fridlion of the piflon moving up and down in 
the cylinder, and of the forcers or working rods, is 
in 
