t 54* ] 
"Explanatory Example. 
Let us fuppofe, for example, that the heat of the 
fteam in the boiler of a fire-engine is now 300 de- 
grees of Fahrenheit’s thermometer ; it is evident, 
that if the fame fleam could, by any art, be heated to 
600 degrees, its expanfive force would be greatly 
increafed ; fo that a much fmaller quantity of fleam 
thus heated would overcome the preffure of the air, 
and elevate the piflon of the fire-engine, than is now 
applied for that purpole. And this fmaller quantity 
of fleam might be raifed in fmaller veffels, and with 
lefs fuel than is now ufed in the working that engine. 
Praflical Obfervrtions. 
The heat of the fleam now applied mufl be nearly 
the fame with the heat of the water, from which it 
is raifed. r i he heat of boiling water, in open veffels, 
is found, at a medium, about 21 2 degrees in Fahren- 
heit’s thermometer; in clofe veffels, it is often con- 
iiderably greater; but, in the boiler of the fire engine, 
can fcarce exceed 200 degrees; it is certain, that it 
never approaches near to the heat of melted lead, 
fince the tides of the boilers are often made of that 
metal. And it is obferved, that the fire, when it 
touches the tides of the leaden boiler, where it is 
only tilled with fleam, does not melt the lead ; the 
fleam having the fame effedl with water in keeping 
the lead cool, to which the fire is applied. 
From the following inflance it neverthelefs feems 
probable, that the fleam of water may be brought 
to fo great a heat, as to melt lead, to which it is ap- 
plied. The pipe, which fupplied the bailer of a fire- 
Vol. 49. Z z z engine 
