engines. 
manner as in Savary’s engine. This valve 
is generally loaded with one or two pounds 
in the square inch, so that it allows the steam 
to escape when its elasticity is one-tenth 
greater than that of the atmosphere, dims 
all risk of bursting the boiler is avoided, the 
pressure outwards being very moderate. 
When the cistern tor the injection-water 1 , 
cannot be supplied by pipes from some more 
elevated source, water is raised by the ma- 
chine itself. A small lifting-pump, s, is 
worked by a rod v, suspended from a small 
arch upon the great beam ; this forces water 
through the pipe t into the injection-cistern. 
The parts of the engine being now de- 
scribed, the operation is as follows : 
Suppose the piston and lever in the po- 
sition represented in the plate, and the water 
in the boiler in a state of ebullition, the steam 
and injection-cocks being shut. Having 
opened the steam-cock, n, tiie steam from the 
boiler will immediately rush in, and dying 
all over the cylinder, will mix with the air. 
Much of it will be condensed by the cold 
surface of the cylinder and piston, and the 
water produced from it will trickle down the 
sidles, and run olf by the eduction-pipe. 
Tips condensation and waste of steam will 
go on until the whole cylinder and piston be 
be made as hot as boiling-water. 
When this happens, the steam will begin to 
issue through the snifting-valve, slowly at 
first, and cloudy, being mixed with much 
air ; but, by degrees, it will become more 
transparent* having carried off the greatest 
part of the air which tilled the cylinder. 
When the attendant perceives that the 
blast at the snifting-valve is strong and 
steady, and the boiler is supplied with a steam 
of proper strength, appearing by the renewal 
of its discharge at tire safety-valve, which 
had stopped while the cylinder was filling, 
he shuts the steam-cock, n, and opens the 
injection-cock, 1- The pressure of water in 
the injection-pipe forces some out into the 
cylinder, which condenses the steam and 
forms a partial vacuum, as explained above.. 
The upper side of the piston is now ex- 
posed to the whole pressure of the atmo- 
sphere, which, not being counterbalanced on 
the under side, will act with its whole force 
on the piston, and, provided there ire not too 
much weight on the other end, will raise it, 
the piston going to the bottom of the cylin- 
der. 
When the piston has gone down as low as 
necessary, the injection-cock is shut, and the 
steam-cock opened. The steam, which has 
been accumulating above the water in the 
boiler, during the time of the descent of the 
piston, and is now issuing through the pup- 
pet-clack, as soon as tiie steam-cock is 
opened, rushes violently into the cylinder, 
having a greater elasticity than that of the 
air. It therefore immediately blows open 
the snifting-valve, through which it drives 
out the air that had been disengaged from 
the injection- water. 
At the same time, the water which had 
been injected before, and the condensed 
steam, run out through the eduction-pipe, g, 
and lifting up tiie valve, r, flow into the hot- 
well. 
Hv the admission of the steam under tiie 
piston, the pressure of the atmosphere on 
tiie top is counterbalanced, and the piston is 
free to move upwards or downwards. 
VOL. I. 
I But the other end of the beam being 
I broader, ho as to be heavier than the piston, 
i now raises it to the top of the cylinder, 
i whence it is again forced downwards by the 
| pressure of the atmosphere, as soon as a \a- 
j cuum is formed under it by the admission of 
| the injection-water. In this manner the -ope- 
; ration is repeated ; the piston being forced 
! down bv the weight of the atmosphere, 
| raises tiie other end of the beam, with what- 
1 ever is attached to it ; and, on the other 
hand, when the pressure ot the atmosphere 
is counterbalanced by tiie steam under the 
piston, the superior weight ot the pump-end 
of tiie beam brings the piston up again. 
We now see tiie difference between Sa- 
vary’s and Newcomen’s engine, in respect 
to principle. Savary’s was an engine, that 
raised water by tiie force of steam ; but New- 
comen’s raises water entirely by the pres- 
sure of the atmosphere ; and the strain is em- 
ployed merely as the most expeditious mode 
of producing a vacuum, into which tiie atmo- 
spherical pressure may impel the first mover 
of his machine. 
We see also tiie great superiority of this 
new machine. We have no need of steam 
of great and dangerous elasticity ; and we 
operate by means of very moderate heats, 
and consequently with a much smaller quan- 
tity of fuel ; and* there are no bounds to the 
power of this machine. How deep soever a 
mine may be, a cylinder may be employed 
of such dimensions, that tiie pressure of tiie 
air may exceed, in any degree, the weight 
of tiie .column of water to be raised. And 
lastly, this form of the machine renders it 
! applicable to almost every mechanical pur- 
i pose; because a skilful mechanic can readily 
find a method of converting the reciprocat- 
ing motion of tiie working-beam into a mo- 
tion of any kind which may suit his purpose. 
Savary’s engine could hardly admit ot such a 
general application, and seems almost re- 
stricted to raising water. 
Newcomen’s engine was first offered to 
the public in 1705. But many difficulties 
occurred in the execution of it, which were 
removed one by one; and it was not till 
1712, that the engine seemed to give confi- 
dence in its efficacy. 
I'lie most exact and unremitting attention 
was required, to open and shut tire cocks 
precisely at the proper time; for neglect 
might be ruinous to the machine, by the con- 
fined steam beating out the bottom ot tiie 
cylinder, or allowing the piston to be wholly 
drawn out of it. Slops were contrived to 
prevent these .accidents ; then strings were 
used to connect the handles of the cocks with 
the beam, so that they should be turned 
whenever it was in certain positions, these 
were gradually changed, and improved into 
detents and catches ot different shapes ; at 
last, Mr. Brighton, a very ingenious and well- 
informed artist, simplified the whole of these 
subordinate movements, and otherwise very 
much improved the machine. 
The greatest improvement that has since 
been made on N ewcomen’s engine, has been 
in the manner of placing the boiler. Instead 
of placing it underneath the cylinder, it is 
built at some distance from it, and sometimes 
in a separate building. 
About 1762, Mr. Watt began to turn his 
attention to this machine, which he has since 
brought to so great a degree ot perfection. 
4 K 
625 
But before we explain Mr. Watt’s engines, 
it is necessary to premise a short account of 
tiie imperfections of the old steam-engines, 
and their causes. 
The steam or vapour which arises from 
water confined in a close vessel, and heated 
a few degrees above the point at which it 
boils in the open air, becomes an elastic fluid, 
unborn), and transparent, about lvalt the 
gravity of atmospheric air, very much greater 
in bulk than the water of which it is com- 
posed, and capable of being again reduced 
to water when brought into contact with 
matter ot a less degree of heat than itself. 
The pressure ot the atmosphere, or any 
equivalent resistance, prevents the produc- 
tion of steam, until the water is heated to 
212 degrees of Fahrenheit’s thermometer; 
but when that pressure is removed, or the 
water is placed in a vessel exhausted of air, 
steam is produced from it when it is colder 
than the human blood. On the contrary, if 
water is pressed upon by air or steam, which 
are more compressed than the atmosphere, 
a degree of heat above 212 degrees is neces- 
sary for the production of steam ; and the 
difference of heats at which water boiis under 
di lie rent pressures, increases in a less pro- 
portion than tiie pressures themselves; so 
that a double pressure requires less than a 
double increase of sensible heat. 
The experiments which have been pub- 
lished concerning the bulk of wafer when, 
converted into steam, are erroneous ; and the 
conclusions drawn from them make that bulk 
greater than it really is. It has been known 
tor some time, that water would boil in au 
exhausted receiver at a low degree of heat. 
If we consider the common steam-engine, 
we shall find it defective; first, because the 
vacuum is produced by throwing cold water 
into the cylinder to condense the steam: 
that water becomes hot, and, being in a ves- 
sel partially exhausted, produces a steam, 
which in part resists the pressure of the atmo- 
sphere upon the piston, and lessens the power 
of the engine. r l he second defect is the de- 
struction of steam, which unavoidably hap- 
pens upon attempting to fill a cold cylinder 
with that fluid ; for the injection-water, at the 
same time that it condenses tiie steam, not 
•only cools the cylinder, but remains there 
until it is extruded at the eduction-pipe by 
the steam which is let in to fill tiie cylinder 
for the next stroke ; and that steam will be 
condensed into water as fast as it enters, 
until all the matter it comes in contact with 
is nearly as hot as itself. 
Every attempt to make the vacuum more 
perfect by tiie addition of injection -water, 
will cool the cylinder more effectually, and 
cause a greater destruction ot steam m tiie 
next filling; and if the engine has already a 
proper load, tiie destruction of steam will 
proceed in a greater ratio than tiie increase 
of power bv the amendment of the vacuum. 
Though "it appears that tiie constructors of 
steam-engines have never investigated these 
causes, yet they havebeen so sensible of the 
effects, that a judicious engineer docs not at- 
tempt to load his engine with a column of 
water heavier than seven pounds for each 
square inch of the area of tiie piston. 
Mr. Watt’s improvements are founded 
upon these, and some other collateral obser- 
vations. He preserves an uniform heat in 
the cylinder of his engines, by suffering no 
