M E C H A N I C S, 
778 
snents, which, when underftood, will at the fame time 
leave no part of this more fimple form unexplained. There 
is, however, a beautif ul engraving of it in Dr. Rees’s Cy¬ 
clopaedia. 
During the progrefs of thefe improvements Mr. Watt 
made many experiments on the quantity and denfity of 
the Iteam of boiling water. Thefe fully convinced him, 
that, although he had greatly diminifhed the wafte of 
Iteam, a great deal yet remained, and that the fleam ex¬ 
pended during the rife of the pi it on was at leaft three times 
more than what would fill the cylinder. The caufe of this 
was very apparent. In the fubfequent defcent of the 
•pifton, covered with water much below the boiling tem¬ 
perature, the whole cylinder was neceffarily cooled and 
expofed to the air. Mr. Watt’s fertile genius immedi¬ 
ately fuggefted to him the expedient or employing the 
elafficity of the fleam from the boiler to impel the piflon 
down the cylinder, in place of the prefTure of the atmo¬ 
fphere ; and thus he reftored the engine to its fir ft prin¬ 
ciples, making it an engine really moved by 'f.cam. As 
this is a new epoch in its hiftory, we fhall be more parti¬ 
cular in tire description ; at the fame time Hill reflrifling 
ourlelves to the effential circumftances, and avoiding 
every peculiarity which is to be found in the prodigious 
varieties which Mr. Watt has .introduced into the ma¬ 
chines which he has ereffed, every individual of which 
has been adapted to local circumftances, or diveriined by 
the progrefs of Mr. Watt’s improvements. 
Let A (fig. 116.) reprefent the boiler. This has re¬ 
ceived great improvement from his complete acquaintance 
with the procedure of nature in the production of fleam. 
In fome of his engines the fuel has been placed in the 
tnidfl of the water, lurrounded by an iron or copper veliel, 
while the exterior boiler was made of wood, which tranf- 
anits, and therefore waftes, the heat very flowly. In others, 
the flame not only plays round the whole outfide, as in 
common boilers, but alfo runs along feveral flues which 
are conducted through the inidft of the water. By fuch 
contrivances the fire is applied to the water in a mo ft ex- 
tenfive furface, and for a long time, fo as to impart to it 
the greateft part of its heat. So lkilfully was it applied in 
the Albion-mills, that, although it was perhaps the largeft 
engine in the kingdom, its unconfumed fmoke was infe¬ 
rior to that of a very frnall brevv-houfe. In this fecond 
engine of Mr. Watt, the top of the cylinder is fhut up by 
a ltrong metal plate g h, in the middle of which is a collar 
or box of leathers k l, formed in the ufual manner of a 
jack-head pump, through which the pifton-rod PD, nicely 
turned and polifhed, can move up and down, without al¬ 
lowing any air to pafs by its Tides. From the dome of 
the boiler proceeds a large pipe B C I O Q, which, after 
reaching the cylinder with its horizontal part B C, de¬ 
scends parallel to its fide, fending off two branches, viz. 
I M to the top of the cylinder, and O N to its bottom. 
At I is a puppet-valve opening from below upwards. 
At L, immediately below this branch, there is a Similar 
valve, alfo opening from below upwards. The pipe de¬ 
scends to Q, near the bottom of a large ciftern c d ef, filled 
with cold water conftantly renewed. The pipe is then 
continued horizontally along the bottom of this ciftern 
(but not in contafl), and terminates at R in a large 
pump ST. The pifton S has clack-valves opening pip- 
wards 3 and, its rod Ss, pa fling through a collar of lea¬ 
thers at T, is fufpended try a chain to a Small arched head 
on the outer arm ot the beam. There is a valve R in the 
bottom of this pump, as ufual, which opens w'hen prefied 
in the direction QR, and fhuts againlt a contrary prefTure. 
This pump delivers its contents into another pump X Y, 
by means of the Small pipe (X, which proceeds from its 
top. This Second pump has a valve at X, and a clack in 
iis pifton Z as ufual; arid the pifton-rod Zz is fufpended 
from another arched head on the outer arm of the beam. 
The two valves I and L are opened and flnit by means of 
fpanners and handles, which are put in motion by a plug- 
frame, in the fame manner as in Newcomen’s engine, 
4 
Laftly, there may be obferved a crooked pipe a b 0, which 
enters the upright pipe laterally a little above Q. This 
has a Small jet-hole at 0; and the other end a, which is 
coniiderably under the furface of the water of the con- 
denflng-ciftern, is covered with a puppet-valve v s whole 
long ftalk vu rifes above the ftater, and may be raifed or 
lowered by hand or by the plug-beam. The valves R 
and X, and the clacks in the piltons S and Z, are opened 
or Dim by the preflures to which they are immediately 
expofed, 
This figure is not an exaft copy of any of Mr. Watt’s 
engines, but has its parts fo difpofed that all may come 
ditrinfDy into view, and exactly perform their various 
funftions. It is drawn in its quiefeent pofition, the outer 
end of the beam preponderating by the counter-weight, 
and the pifton P at the lop of the cylinder, and tl.e piltons 
S and Z in their loweft fituations. In this fituation let 
us fujppofe that a vacuum is (by any means) produced in 
all tiie fpace below the pifton, the valve I being fhut. It 
is evident that the valve R will alfo be fhut, as alfo the 
valve v. Now let the valve I be opened. The fleam 
from the boiler, as elaftic as common air, will rufh into 
the fpace above the pifton, and will exert on it a prefTure 
as great as that of the atmofphere. It will therefore prefs 
it down, raife the outer end of the beam, and caufe it to 
perform the fame work as an ordinary engine. When the 
pifton P has reached the bottom of the cylinder, the plug- 
frame fhuts the valve I, and opens L. By fo doing, the 
communication is open betw een the top and bottom of the 
cylinder, and nothing hinders the fleam which is above 
the pifton from going along the pafl'age ML ON. The 
pifton is now equally affected on both tides by the fteam, 
even though a part of it is continually condenfed by the 
cylinder, and in the pipe I O Q. Nothing therefore hin¬ 
ders the pifton from being dragged up by the counter¬ 
weight, which acts with its whole force, undiminifhed 
by any remaining unbalanced elafticity of fteam. Here 
therefore this form of the engine has an advantage (and by 
no means a frnall one) over the common engines, in which 
a great part of the counter-weight is expended in over¬ 
coming unbalanced atmofpheric prefTure. Whenever the 
pifton P arrives at the top of the cylinder, the valve L is 
fhut by the plug-frame, and the valves I and v are opened. 
All the fpace below the pifton is at this time occupied by 
the fteam which came from the-upper part of the cylinder. 
This, being a little wafted by condenfation, is not quite a 
balance for the prefTure of the atmofphere. Therefore, 
during theafcent of the pifton, the valve R was (hut, and 
it remains fo. When, therefore, the valve v is opened, 
the cold water of the ciftern mult fpout up through the 
hole 0, and condenfe the fteam. To this mult be added 
the coldnefs of the whole pipe O Q R. As faft as it is con¬ 
denfed, its place is fupplied by Iteam from the lower parr 
of the cylinder. We have already remarked, that this 
fuccellive condenfation is accompliihed vvith aftonifhing 
rapidity. In the mean time, fteam from the boiler preffes 
on the upper furface of the pifton. It muft therefore de- 
feend as before, and the engine muft perform a fecond 
working-ftroke. But in the mean time the injection-wa¬ 
ter lies in the bottom of the pipe O Q R, heated to a con- 
fiderable degree by the condenfation of the fteam ; alfo a 
quantity of air has been dif'engaged from it and from the 
water in the boiler. Flow is this to be dilcharged ? This 
is the office of the pumps ST and X Y. The capacity of 
ST is very great in proportion to the fpace in which the 
air and water are lodged. When, therefore, the pifton S 
has got to the top of its courfe, there muft be a vacuum in 
the barrel of this pump j and the water and air mult open 
the valve R, and come into it. When the pifton S comes 
down again in the next returning-ftroke, this water and 
air gets through the valve of the pifton ; and in the next 
working-ftroke they are dilcharged by the pifton into the. 
pump XY, and raifed by its pifton. The air efcapes 
at Y 3 and as much of the water as is necefl'ary is delivered 
into the boiler by a frnall pipe Y m to fuppiy its wafte. 
