MECHANIC S. 
down from the upper condenfing-valve N. Thefe two 
eduction-pipes, thus united, go downwards, and open at 
L into a rectangular box, of which the end is feen at L. 
This box goes backward from the eye, and at its farther 
extremity communicates with the air-pump K, whole 
pilton is here reprefented in feCtion with its butterfly- 
valves. The pifton delivers the water and air laterally 
into another rectangular box M, darkly lhaded, which 
box communicates with the pump Z. The pifton-rods of 
this and of the air-pump are fufpended by chains from a 
ifmall arched head on the inner arm of the great beam. 
The lower part of the edudtion-pipe, the horizontal box 
L, the air-pump K, with the communicating box M be¬ 
tween it and the pump Z, are all immerfed in the cold 
water of the condenfing ciftern. The box L is made flat, 
broad, and (hallow, in order to increafe its i'urface, and 
accelerate the condenfation. But, that this may be per¬ 
formed with the greateft expedition, a final 1 pipe H, open 
below (but occaflonally flopped by a plug-valve), is in- 
ferted laterally into the eduCtion-pipe G, and then divides 
into two branches; one of which reaches within a toot or 
two of the upper valve N, and the other approaches as 
near to the valve F. 
As it is intended by this conftruCtion to give the pifton 
a ftrong impulfe in both directions, it will not be proper 
to fufpend its rod by a chain from the great beam; for 
it mutt not only pull down that end of the beam, but 
alfo pufh it upwards. It may indeed be fufpended by 
double chains, like the piltons of the engines for extin- 
guifhing fires; and Mr. Watt has accordingly done fo in 
fome of his engines. But, in the drawing from which 
this figure is copied, he has communicated the force of 
the pilton to the beam by means of a toothed rack O O, 
which engages or works in the toothed feCtor QQ on the 
end of the beam. The reader will underftand, without 
any farther explanation, how the impulfe given to the 
pifton in either direction is thus tranfmitted to the beam 
without diminution. The fly XX, with its pinion Y, 
which alfo works in the toothed arch Q Q, may be fup- 
pofed to be removed for the prefent, and will be confi- 
dered afterwards. 
We (hall take the prefent opportunity of defcribing 
Mr. Watt’s method of communicating the force of the 
fleam-engine to any machine of the rotatory kind. VV 
reprefents the rim and arms of a very large and heavy 
metalline fly. On its axis is the concentrical toothed 
wheel U. There is attached to the end of the great beam 
a ftrong and fti'ff rod TT, to the lower end of which a 
toothed wheel W is firmly fixed by two bolts, fo that it 
cannot turn round. This wheel is of the fame fize and 
in the fame vertical plane with the wheel U ; and an iron- 
link or (trap (which cannot be feen here, becaufe it is on 
the other fide of the two wheels) connects the centres of 
the two wheels, fo that the one cannot quit the other. 
The engine being in the pofition reprefented in the figure, 
i'uppofe the fly to be turned once round by any external 
force in the direction of the darts. It is plain, that, fince 
the toothed wheels cannot quit each other, being kept 
together by the link, the inner half (that is, the half next 
the cylinder) of the wheel U will work on the inner half 
of the wheel W, fo that, at the end of the revolution of 
the fly, the wheel W malt have got to the top of the 
wheel U, and the outer end of the beam mult be railed to 
its liighelt pofition. The next revolution of the fly will 
bring the wheel W and the beam connected with it to 
their firft pofitions; and thus every two revolutions of 
the fly will make a complete period of the beam’s recipro¬ 
cating movements. Now, inftead of fuppofing the fly to 
drive the beam, let the beam drive the fly. The motions 
mult be perfectly the fame, and the afcent or defcent of 
the pifton will produce one revolution of the fly. 
A fide-view, fomewhat enlarged, of this apparatus, is 
given in fig. 120. marked by the fame letters of reference. 
This (hows the fituation of parts which were fore-fhort- 
®neu in fig. 119. particularly the defending branch C of 
' VoL.XIV. No. 1013. 
781 
the fleam-pipe, and the fituation and communications of 
the two pumps K and Z. 8, 8, is the horizontal part of 
the (team-pipe. 9 is a part of it whole box is reprefented 
by the dark circle of fig. 119. D is the box of the fteam- 
clack; and the little circle at its corner reprefents the end 
of the axis which turns it, as will be defcribed afterwards, 
N is the place of the upper eduClion-valve. A part only 
of the upper eduChon-pipe G is reprefented, the reft being 
cut off, becaufe it would have covered the aefcending 
fteam-pipe C C. When continued down, it comes be¬ 
tween the eye and the box E of the lower ite.am-valve, 
and the box F of the lower eduCtion-valve. 
Let us now trace the operation of this machine. Re¬ 
curring to fig. 119. let us fuppofe that the lower part of 
rhe cylinder B B is exhaufted of all elaftic fluids; that 
the upper (team-valve D and the lower eduCtion-valve F 
are open, and that the lower fteam-valve E and upper 
eduction-valve N are (hut. It is evident that the piftort 
muff be preffed towards the bottom of the cylinder, and 
muff pull down the end of the working-beam by means of 
the toothed rack O O and feCtor Q Q, caufing the other end 
of the beam to urge forward the machinery with which 
it is connected. When the pilton arrives at the bottom 
of the cylinder, the valves D and F are fhftt by the plug- 
frame, and E and N are opened. By this '.aft paflage the 
(team gets into the eduCtion-pipe, where it meets with 
the injettion-water, and is rapidly condenfed. The fleam 
from the boiler enters at the fame time by E, and, prefling 
on the lower fide of the pifton, forces it upwards, and by 
means of the toothed rack OO and toothed feCtor QQ 
forces up that end of the working-beam, and caufes the 
other end to urge forward the machinery with which it 
is connected: and in this manner the operation of the 
engine may be continued for ever. The injeCtion-water 
is continually running into the eduCtion-pipe, becaufe 
condenfation is continually going on, and therefore there 
is a continual atmofpheric p re flu re to produce a jet. The 
air which is difengaged from the water, or enters by leaks, 
is evacuated only during the rife of the pilton of the air- 
pump K. When this is very copious, it renders a very 
large air-pump neceflary; and in fome fituations Mr. Watt 
has been obliged to employ two air-pumps, one worked 
by each arm of the beam. This in every cafe expends a. 
very confiderable portion of the power; for the air-pump 
is always working againlt the whole preflure of the atmo- 
fphere. It is evident that this form of the engine, by- 
maintaining an almoft conftant and uninterrupted im- 
pulfion, is much fitter for driving machinery of conti¬ 
nued motion than any of the former engines, which were 
inactive during half of their motion. It does not, how¬ 
ever, feem to have this fuperiority when employed to draw 
water: but it is equally fitted for the tafk. Let the en¬ 
gine be loaded with twice as much as would be proper 
for it if a fingle-ftroke engine, and let a fly be connected 
with it. Then it is plain that the power of the engine 
during the rife of the fteatn-pifton will be accumulated in 
the fly; and this, in conjunction with the power of the 
engine during the defcent of the fteam-pilton, will be 
equal to the whole load of water. 
In fpeaking of the fteatn and eduftion valves, we faid 
that they were all puppet-valves. Mr. Watt employed 
cocks, and alfo fliding-valves, fuch as the regulator or 
(team-valves in the old engines. But he found them al¬ 
ways lofe their tightnefs after a fhort time. This is not 
furprifing, when we confider that they are always perfectly 
dry, and almoft burning hot. He was therefore obliged 
to change them all for puppet-clacks, which, when truly 
ground and nicely fitted in their motions at firft, are not 
found to go out of order by any length of time. The 
way in which Mr. Watt opens and (huts thefe valves is 
as follows. Fig. X2i. reprefents a clack with its feat and 
box. Suppofe it one of the eduCtion-valves. H H is part 
of the pipe which introduces the lteam, and GG is the 
upper part of the pipe which communicates with the con- 
tlenfer. At E E may be obferved a piece more faintly 
5 N (haded 
