MR. WATT'S STEAM ENGINE, 
188 
no ſteam. The vacuum also, made by the condensation, may be as 
perfect as we please, because there is no occasion to be sparing of cold 
water for the condensation. This circumstance gives a great mecha 
nical superiority to this construction. For it has been found by long 
experience, that in the old engine, it is adviseable to work against no 
greater load than six or seven pounds on every square inch of the 
piston, because the quantity of cold water that must be employed, 
for producing a greater unbalanced pressure on the piston, cools the 
cylinder so much, that more loss is sustained by the expence of fuel to 
replace the wasted steam, than is gained by the increase of power. In 
Mr. Watt’s construction, the engine will work very tolerably against a 
load of 134 pounds, and is never loaded with less than 111; but its 
most conspicuous advantage is, that an engine of this construction 
consumes not more, and generally a good deal less, than one third of 
the fuel used in the very best common engine of the fame size. 
Mr. Watt still farther improved this engine, by making it exert an 
active stroke in both directions. This was very obvious and easy. 
He had only to make both the upper and under parts of the cylinder 
communicate with the boiler and condenser. When the upper part 
communicates with the boiler, and the lower with the condenser, the 
piston is strongly pressed down; and when these communications are 
reversed, it is as strongly pressed up. Moreover, by an artful cutting 
off, or greatly diminishing the supply of steam, before the piston has 
completed its movement, he has enabled a given quantity of steam to 
produce a much greater effect, and at the fame time has brought the 
engine so much, and so instantaneously, under command, that it is 
really more manageable than the exertion of any animal. But the ex 
planation of these particulars is not in our department, but that of the 
mechanician. 
I muſt not, however, omit an important chemical observation 
which occurred to Mr. Watt in the course of those improvements, 
and particularly in the prosecution of a project which he had formed