MECHANICS. 
770 
water thrown Into it. A partial vacuum being thus made, 
the weight of the atmofphere preffes down the pifton, and 
raifes the other end of the ftraight lever, together with the 
water, from the well. Then immediately a hole is unco¬ 
vered in the bottom of the cylinder, by which a frefti 
quantity of hot (team rufhes in from a boiler of water be¬ 
low it, which proving a counterbalance for the atmofphere 
above the pifton, the weight of the pump-rods at the other 
end of the lever carries that end down, and raifes the pifton 
of the fteam-cylinder. The (team hole is then immedi¬ 
ately (hut, and acock opened for injefting the cold water 
into the .cylinder of fteam, which condenses it to water 
again; and, thus making a vacuum below the pifton, the 
atmofphere again preffes it down and raifes the pump-rods, 
as before; and fo on continually. 
3. The great features of improvement made by Mr. 
Watt upon the engine of Newcomen and Cawley are, as 
Mr. Nicholfon remarks, firft, that the elafticity of the 
(team itfelf is ufed as the aftive power in this engine ; and 
fecondly, that befides various other judicious arrange¬ 
ments for the economy of heat, he condenfes the fteam, 
not in the cylinder, but in a feparate veil'd. 
In the cylinder or fyringe, concerning which we have 
fpoken, in mentioning the engine of Newcomen, let us 
fuppofe the upper part to be clofed, ami the pifton-rod to 
hide air-tight through a collar of leathers. In this fitua- 
tion, it is evident that the pifton might be depreffed by 
throwing the fteam upon its upper furface, through an 
aperture at the fuperior end of the cylinder. But, if we 
fuppofe the external air to have accefs to the lower furface 
of the pifton, we (hall find, that fteam no ftronger in its 
elafticity than to equal the weight of the atmofphere 
would not move the pifton at all ; and confequently that 
this new engine would require much denfer fteam, and 
confume much more fuel, than the old engine. The re¬ 
medy for this evil is to maintain a conftant vacuum be¬ 
neath the pifton. If fuch a vacuum were originally pro¬ 
duced by (team, it is certain that its permanency could not 
be depended on, unlefs the engine contained a provifion 
for constantly keeping it up. Mr. Watt’s contrivance in 
his fimpleft engine is as follows : The fteam is conveyed 
from the boiler to the upper part of the cylinder through 
a pipe, which alfo communicates occafionally with the 
lower part, and beyond that fpace with a veffel immerfed 
in a trough of water; in which veil'd the condenlation is 
performed by an injeded dream of cold water. This wa¬ 
ter is drawn oft', not by an edmftion-pipe, but by a pump, 
of which the ftroke is lufficientiy capacious to leave room 
for the elaftic fluid, feparated during the injeftion, to fol¬ 
low and be carried out with the injeftion-water. Sup- 
pofe now the pifton to beat its greateft: elevation, and the 
communication from the boiler to the upper as well as to 
the lower parts of the cylinder to be opened. The ftearn 
will then pafs into the whole internal part of the engine, 
and will drive the air downwards into the condenfer, and 
thence through the valves of the air-pump. In this fitua- 
tion, if the communication from the boiler to the lower 
part of the cylinder be (topped, and an injeftion be made 
into the condenfer, a vacuum will be produced in that 
veil'd, and the fteam contained in the lower part of the cy¬ 
linder and communication-pipe will expand itfelf with 
wonderful rapidity towards the condenfer, fo that in a pe¬ 
riod of time too minute to be appreciated the whole of the 
fteam beneath the pifton will be practically condenfed. 
The (team which continues to aft above the pifton will 
immediately deprefs it into the vacuum beneath ; at the 
fame time that by connection with the external apparatus 
the pifton of the air-pump alfo defcends in its barrel. 
When the ftroke is nearly completed downwards, the re- 
quifite part of the apparatus (huts the communication 
with the boiler, opens that between the upper and lower 
parts of the cylinder and condenfing-Veflel, arid turns the 
injeftion-cock. At this very inftant the pifton lofes its 
tendency to defcend, becaufe the fteam prefl'es equally on 
both furfaces, and continues its equality of pretl'ure while 
the condenfation js performed. It therefore rifes; the m.- 
jeftion is flopped ; and the air-pump, making its ftroke, 
fuffers the injeftion-water and a conliderable part of the 
elaftic fluid to pafs through its lower valve. The vacuum 
is thus kept up through the whole internal capacity of the 
engine. As foon as the pifton has reached the upper part 
of the cylinder, the communication to the under part of 
the cylinder is (topped, and that with the boiler opened, 
as before ; the confequence of which is, that the pifton 
again defcends; and in this manner the alternations re¬ 
peatedly take place. 
The principal augmentation of power in this engine, 
compared with that of Newcomen, arifes from the cylin¬ 
der not being cooled by the injeftion-water, from its being 
practicable to ufe fteam which is more powerful than the 
prefi'ure of the atmofphere, and from the employing of 
this fteam both to elevate and to deprefs the pifton. In 
general, thefe engines are worked by (ream which would 
l’upport a column of four or five inches of mercury befides 
the prefi'ure of the atmofphere, and fometimes more; for 
Mr. Nicholfon fays he has fometimes feen the gauge as 
high as eight inches. Mr. Watt has made leveral fuecef- 
five modifications and additions to the engine juft defcribed. 
Come of which will be further fpoken ot in the couil'e of 
this feftion. 
It has been cuftomary, when treating of fteam-engines,. 
to prefent feveral theorems for the computation of their 
power and eft'efls. But, as all which has hitherto been 
advanced on thefe points feems to us very vague and un- 
fatisfaftory, we (hall not delude the ftudent with an ap¬ 
pearance of mathematical accuracy, when it is fo far from 
being attained. It is obvious enough, that the abfolute 
power of a fleam-engine is in the compound ratio of the 
area of the pifton, the prefi'ure upon each inch of it, the 
length of the ftroke, and the number of (trokes in any af- 
figned time; but the prefi'ure upon any portion of the 
pifton can only be afcertained by experiment and obfer- 
vation, and that with difficulty, becaufe of our uncertain 
methods of eftimating f riff ion and other fpecies of red (fi¬ 
ance ; while judicious obfervations would with much lefs 
labour determine the work affually performed, either 
when the fteam-engine works pumps, or gives motion to- 
any kind of mills. The quantity of water raifed by pumps 
in a given time may foon be eftimated ; and, when the al¬ 
ternating motion of the fttam-pifton is converted into a 
rotatory one, the real effeff the engine is capable of pro¬ 
ducing may be afcertained by obferving the velocity with 
which a given weight is raifed when lufpended from the 
axle to which the rotation is firft given. 
The ufual method of eftimating the effefts of engines 
by what are called “ horfe-powers” mult inevitably be 
very fallacious, unlefs all engineers could agree as to the 
quantity o( w’ork which they would arbitrarily affign to 
one horfe. It ftiould alfo be obferved, that, in determin¬ 
ing the comparative value of different (team-engines, it is 
not fufficient to compare the quantities of work each will 
perforin in equal times; for the expenfe of ereftion, the 
probability of repairs being more or lefs frequent on ac¬ 
count of the complexnefs or (implicity of conftiuftion, 
and the quantities of fuel confumed by each, mult like- 
wife be taken into the account. 
We' now proceed to a more particular defcription of 
thefe three kinds of (team-engines. 
1. Captain Savery’s engine, as improved and Amplified 
by himfelf, is reprefented on Plate XXiX. A (fig. in.) 
is a ltrong copper boiler properly built up in a (urnace. 
There proceeds from its top a large (team-pipe B, which 
enters into the top of another ftrong veffel R, called the 
receiver. This pipe has a cock at C, called the Jleant-coch . 
In the bottom of the receiver is a pipe F, which commu¬ 
nicates fidewife with the rifing-pipe HGK, The lower 
end H ctf this pipe is immerfed in the water of the pit or 
well, and its upper part K opens into the ciftern into 
which the water is to be delivered. Immediately below 
the pipe of communication F there is a valve G, opening 
when 
