M E C H A N I C S. 
782 
Shaded than the furrounding parts. This is the feat of 
the valve, and is 3 brafs or bell-metal ring turned conical 
on the outfide, fo as to fit exa£Hy into a conical part of 
the pipe GG. Thefe two pieces are fitted by grinding; 
and, the cone being of a long taper, the ring flicks firmly 
in it, efpecially after having been there for fome time, and 
united by ruff. The clack itfelf is a ftrong brafs plate D, 
turned conical on the edge, fo as to fit the conical or 
floping inner edge of the feat. Thefe are very nicely 
ground on each other with emery. This conical joining 
is much more obtufe than the outer fide of the ring ; fo 
that, although the joint is air-tight, the two pieces do not 
flick ltrongly together. The clack has a round tail DG, 
which is freely moveable up and down in the hole of a 
crofs-piece F F. On the upper fide of the valve is a ftrong 
piece of metal DC firmly joined to it, one fide of which 
is formed into a toothed rack. A is the feftion of an iron 
axle which turns in holes in the oppofite fides of the valve- 
box, where it is nicely fitted by grinding, fo as to be air¬ 
tight. Collets of thick leather, well loaked in melted 
tallow and rofin, are fcrewed on the outfide of thefe holes 
to prevent all ingrefs of air. One end of this axis pro¬ 
jects a good way without the box, and carries a fpanner 
or handle, which is moved by the plug-frame. To this 
axis is fixed a ftrong piece of metal B, the edge of which 
is formed into an arc of a circle having the axis A in 
its centre; and is cut into teeth, which work in the teeth 
of the rack DC. K is a cover which is fixed by fcrews 
to the top of the box HJJH, and may be taken off in 
order to get at the valve when it needs repairs. From 
this defcription it is eafy to fee that, by turning the 
handle which is on the axis A, the fefior B mud lift up 
the valve by means of its toothed rack DC, till the upper 
end of the rack touch the knob or button K. Turning 
the handle in the oppofite direction brings the valve down 
again to its feat. 
This valve is extremely tight. But, in order to open it 
for the paflage of the (team, we muft exert a force equal 
to the preflure of the atmofphere. This in a large engine 
is very great. A valve of fix inches diameter fuftains a 
preflure not lefs than 400 pounds. But this force is quite 
momentary, and hardly impedes the motion of the en¬ 
gine; for, the inftant the valve is detached from its feat, 
although it has not moved the 100th part of an inch, the 
preflure is over. Even this little inconvenience has been 
removed by a delicate thought of Mr. Watt. He has put 
ihe fpanner in fuch a pofition when it begins to raife the 
valve, that its mechanical energy is almoft infinitely great. 
Let QR (fig. i22.) be part of the plug-frame defcending, 
and P one of its pins juft going to lay hold of the fpan¬ 
ner NO, moveable round the axis N. On the fame axis 
is another arm NM connected by a joint with the leader 
ML, which is connected alfo by a joint with the fpanner 
LA that is on the axis A of the fe£tor within the valve- 
box. Therefore, when the pin P pufhes down the fpan¬ 
ner N O, the arm N M moves fidewife, and pulls down the 
fpanner A L by means of the connefling-rod. Things 
are fo difpofed, that, when the cock is (hut, LM and 
MN are in one ftraight line. The intelligent mechanic 
will perceive, that in this pofition the force of the lever 
ONM is infuperable. It has this further advantage, that, 
if any thing fliould tend to force open the valve, it would 
be ineffectual; for no force exerted at A, and tranfmitted 
by the rod LM, can poflibly pufh the joint M out of its 
pofition. 
We poftponed any account of .the office of the fly XX, 
(fig. 119.) as it is not of ufe in an engine regulated by the 
fly VV. The fly XX is only for regulating the recipro¬ 
cating motion of the beam when the lteam is not admitted 
during the whole defcent of the pifton. This it evidently 
muft render more uniform, accumulating a momentum 
equal to the whole preflure of the full fupply of the 
fleam, and then (haring it with the beam during the reft 
©f the defcent ot the pilton. 
When a perlon properly lkilled in mechanics and che- 
miftry reviews thefe different forms of Mr. Watt’s fteaiti- 
engine, he will eafily perceive fhem fufceptible of many 
intermediate forms* in which any one or more of the dif- 
tinguifh'ing improvements may be employed. The firft 
great improvement was the condenfalion in a feparate veJjeL 
This increafed the original powers of the engine, giving 
to the atmofpheric preflure and to the counterweight 
their full energy, at the fame time the walte of fleam is 
greatly diminifhed. The next improvement, by employ¬ 
ing the prejfure of the Jltam injlead of that of the atmofphere , 
aimed only at a (till farther diminution ot the watte; but 
was fertile in advantages, rendering the machine more 
manageable, and particularly enabling us at all times, and 
without trouble, to fuit the power of the engine to its 
load of work, however variable or increafing ; and brought 
into view a very interefling propofltion in the mechanical 
theory of the engine, viz. that the whole performance of 
a given quantity of fleam may be augmented by admit¬ 
ting it into the cylinder only during a part of the pifton’s 
motion. Mr. Watt has varied the application of this pro¬ 
pofltion in a thoufand ways; and there is nothing about 
the machine which gives more employment to the fagacity 
and judgment of the engineer. The third improvement 
of the double impulfe may be confidered as the finifhing 
touch given to the engine, and renders it as uniform in 
its action as any water-wheel. In the engine’s molt perfeft 
form, there does not feem to be above one-fourth of the 
fleam wafted by warming the apparatus; fo that it is not 
poflible to make it one-fourth part more powerful than it 
is at prefent. The only thing that feems fufceptible of 
confiderable improvement is the great beam. The enor¬ 
mous ftrains exerted on its arms require a proportional 
ftrength. This requires a vafl mafs of matter, not lefs 
indeed in an engine with a cylinder of fifty-four incites 
than three tons and a half, moving with the velocity of 
three feet in a fecond, which muft be communicated in 
about half a fecond. This mafs muft he brought into 
motion from a ftate of reft, muft again be brought to reft, 
again into motion, and again to reft, to complete the 
period of a llroke. This confumes much power; and 
Mr. Watt has not been able to load an engine with more 
than ten or eleven pounds on the inch, and preferve a fuf- 
ficient quantity of motion, fo as to make twelve or fifteen 
fix-feet ftrokes in a fecond. Many attempts have been 
made to ltffen this mafs by ufing a light framed wheel, ora 
light frame of carpentry, in place ot a folk! beam. Thefe 
have generally been conltrufted by perfons ignorant of 
the true fcientific principles of carpentry, and have fared 
accordingly. Mr. Watt has made fimilar attempts; but 
found, that, although at firft they were abundantly ftrong, 
yet, after a fhort time, the ltraps and bolts with which the 
wooden parts were connected cut their way into the wood, 
and the framing grew loofe in the joints, and, without 
giving any warning, went to pieces in an inftant. A folid 
maffy fimple beam, of fufficient ftrength, bends,and fenfibly 
complains (as the carpenters exprefs it) before it breaks. 
In all great engines, therefore, fuch only are employed; 
and in fmaller engines he fometimes ufes call-iron wheels 
or pulleys ; nay, he frequently ufes no beam or equivalent 
whatever, but employs the fleam pillon-rod to drive the 
machinery to which the engine is applied. 
It may not be here improper to ftate the atflual perfor¬ 
mance of fome of thefe engines, as they have been afeer- 
tained by experiment. An engine having a cylinder of 
31 inches in diameter, and making 17 double ftrokes per 
minute, performs the work of forty horfes working night 
and day (for which three relays or 120 horfes muft be 
kept), and burns 11,000 pounds of Stafford (hire coal per 
day. A cylinder of 19 inches, making 25 ftrokes of 4 feet 
each per minute, performs the work of 12 horfes working 
conftantly, and burns 3700 pounds of coals per day. A 
cylinder of 24 inches, making 22 ftrokes of 5 feet, burns 
5500 pounds of coals, and is> equivalent to the conftant 
work of 20 horfes. And the patentees think themfelves 
authorifed by experience to fay in general, that thefe en¬ 
gines 
