M E C H 
It is a matter of indifference-whether the piltons S and Z 
rife with the outer or inner end of the beam ; but it is ra¬ 
ther better that they rile with the inner end. They are 
otherwife drawn here, in order to detach them from the 
reft, and (how them more diltinftly. 
Such is Mr. Watt’s fecond engine. Let us examine its 
principles, that we may fee the caufps of its avowed and 
great fuperiority over the common engines. We have al¬ 
ready fee n one ground of fuperiority, the full operation of 
the counter-weight. We are authorifed by careful exa¬ 
mination to fay, that in the condition engines at leaft one- 
half of the counter-weight is expended in counteracting 
an unbalanced preflure of the air on the pifton during its 
afcent. In many engines, which are not the worlt, this 
extends to^tlvof the whole preflure. This is evident 
from the examination of the engine at Montrehix by 
Boffut. This makes a very great counter-weight necef- 
fary, which exhaufts.a proportional part of the moving 
force. But the great advantage of Mr. Watt’s form is 
the almoft total annihilation of the watte of fteam by con- 
denfation in the cylinder. The cylinder is always boil¬ 
ing hot, and therefore perfeffly dry. This nuift be evi¬ 
dent to any perfon who underltands the fubjefh By the 
time that Mr. Watt had completed his improvements, his 
experiments on the production of fteam had given him a 
pretty accurate knowledge of its denlity ; and he found 
himfelf authorifed to fay, that the quantity of fteam em¬ 
ployed did not exceed twice as much as would fill the cy¬ 
linder, fo that not above one-half was unavoidably wafted. 
But, before he could bring the engine to this degree of per¬ 
fection, he had many difficulties to overcome. He in- 
clofed the cylinder in an outer wooden cafe at a fmall dif- 
tance from it: this diminiihed the expen fe of heat by com¬ 
munication to furrounding bodies. Sometimes fie allowed 
the fteam from the boiler to occupy this interval : this 
undoubtedly prevented all diflipation from the inner cy¬ 
linder; but in its turn it difiipated much heat by the 
outer cafe, and a very fenfible condenfation was obferved 
between them. This has occafioned him to omit it in 
fome of his beft engines. We believe it was omitted in 
the Albion-mills. The greateft difficulty was to make the 
great pifton tight. The old and effectual method, by wa¬ 
ter lying on it, was inadmillible. He was therefore obliged 
to have his cylinders molt nicely bored, perfectly cylin¬ 
drical, and finely poiifhed ; and he made numberlefs trials 
of different foft fubftances for packing his pilton, which 
fhould be tight without enormous trillion, and which 
fhould long remain f'o, in a fituation perfeCfly dry, and hot 
almoft to burning. After all that Mr. Watt has done in 
this refpeCf, he thinks that the greateft part of the watte 
qf fteam which he (fill perceives in his engines arifes from 
the unavoidable efc.ape by the fides of the pifton during 
its defeent. 
But the faff is, that an engine of this conftruffion, of 
the fame dimenffons with a common engine, making the 
fame number of ftrokes of the fame extent, does not con- 
furne above one-fourth part of the fuel that is confumed 
by the beft engines of the common form. It is alfo a very 
fortunate circumftance, that the performance of the en¬ 
gine is not immediately deltroyed, nor indeed fenfibly di- 
rninifhed, by a fmail want of tightnefs in the pifton. In 
the common engine, if air get in, in this way, it imme¬ 
diately'puts a flop to the work; but, although even a 
confrderable quantity of Iteam get paft the pifton during 
its defeent, the rapidity of condenfation is fuch, that 
hardly any diminution of preflure can be obferved. 
Mr. Watt’s penetration foon difeovered another mott 
valuable property of this engine. When an engine of the 
common form is erefted, the engineer muft make an accu¬ 
rate eftimate of the work to be performed, and muft pro¬ 
portion his engine accordingly. He muft be careful that 
it be fully able to execute its talk 5 but its power muft 
not exceed its load in any extravagant degree. This 
would produce a motion which is too rapid, and which, 
being alternately in oppofite directions, would occafion 
A N I C S. 779 
jolts which no building or machinery could withftand. 
Many engines have been fluttered by the pumps drawing 
air, or a pump rod breaking; by which accidents the 
fteam-pifton delcends with fuch rapidity, that every thing 
gives way. But in moft operations of mining, the t.ifk of 
the engine increafes ; and it muft be fo conliruCted at firft 
as to be abie to bear this addition. It is very difficult to 
manage*an engine that is much fuperior to its talk; and 
the eafieft way is, to have it almoft full loaded, and to 
work it only during a few hours each day, and allow the 
pit-water to accumulate during its repofe. This increafes 
the firft coif, and waftes fuel during the inaction of the 
engine. But this new engine can at all times be exaitly 
fitted (at lealt during the working-ftroke) to the load of 
work that then happens to be on it. We have only to ad- 
minifter (team of a proper elafticity. At the firft erection 
the engine may be equal to twice its talk, if the fteam ad¬ 
mitted above the cylinder be equal to that ot common 
boiling water; but, when once the ebullition is fairly 
commenced, and the whole air expelled from all parts of 
the apparatus, it is evident, that, by damping the fire, 
Iteam of half this elafticity may be continually fupplied, 
and the water will continue boiling although its tempe¬ 
rature does not exceed 185° of Fahrenheit’s thermometer. 
But the method now propofea has agreat inconvenience. 
While the fteam is weaker than the atmofphere, there is 
an external force tending to fqueeze in the fides and bot¬ 
tom of the boiler. This could not be refifted when the 
difference is confiderable; and common air would rufh in 
through every crevice of the boiler, and foon choke the 
engine ; it muft therefore be given up. But the fame 
effeift will be produced by diminifhing the paffage for the 
fteam into the cylinder. For this purpofe, the puppet- 
valve by which the fteam enters the cylinder was made in 
the form of a long taper fpigot, and it was lodged in a 
cone of the fame fhape; conlequently the paffage could 
be enlarged or contracted at pleafure by the diftance to 
which the inner cone was drawn up. In this way feveral 
engines were conftruCfed, and the general purpofe of fuit- 
ing the power of the engine to its tafk was completely an- 
fu'ered ; but (as the mathematical reader will readily per¬ 
ceive) it was extremely difficult to make this adjuitment 
preciie and conftant. In agreat machine, like this, going 
. by jerks, it was hardly pofiible that every fuccellive mo¬ 
tion of the valve fhould be precifely the fame. This oc¬ 
cafioned very fenfible irregularities in the motion of the 
engine, which increafed and became hazardous when the 
joints worked loofe by long ufe. Mr. Watt’s genius, al¬ 
ways fertile in refources, found out a complete remedy for 
all thefe inconveniences. Making the valve of the ordi¬ 
nary form of a puppet-clack, he adjulted the button of 
its Italk or tail fo that it fhould always open full to the 
fame height. lie then regulated the pins of the plusr- 
frame in fuch a manner that the valve fhould fliut the 
moment that the pifton had defeended a certain propor¬ 
tion (fuppofe one-fourth, one-third, one-half, &c.) of 
the cylinder. So far the cylinder was occupied by fteam 
as elaftic as common air. In prefling the pifton farther 
down, it behoved the fteam to expand, and its elafticity 
to diminifh. It is plain that this could be done in any 
degree we pleafe, and that the adjuitment can be varied 
in a minute, according to the exigency of the cafe, by 
moving the plug-pins. In the mean time, it muft be ob¬ 
ferved, that the .preflure on the pifton is continually 
changing, and conlequently the accelerating force. The 
motion therefore will no longer be uniformly accelerated : 
it will approach much falter to uniformity ; nay, it may 
be retarded, becaufe, although the preflure on the pilton 
at the beginning of the ftroke may exceed the refiltance 
of the load, yet, when the pifton is near the bottom, the 
refiltance may exceed the preflure. Whatever may be the 
law by which the preflure on the pilton varies, an inge¬ 
nious mechanic may contrive the connecting machinery 
in fuch a way, that the chains or rods at the outer end-of 
the beam fhall continually exert the fame preflure, or fliall 
vary 
