MECHANICS, 
776 
the machine are contained in one compact frame of car¬ 
pentry, and have little or no connection with the flight 
walls of the building, which is merely a cafe to hold the 
machine, and protect it from the weather. 
Such is the ftste in which Newcomen’s fteam-engine 
had continued in ufe for 60 years, negleCted by the phi- 
lofopher, although it is the molt curious object which 
human ingenuity has offered to his contemplation, and 
abandoned to the efforts of the unlettered artilt. Its ufe 
had been entirely confined to the railing of water. Mr. 
Keane Fitzgerald indeed publiflied in the Philofophical 
Tranfaftions a method of converting its reciprocating 
motion into a continued rotatory motion, by employing 
the great beam to work a crank or a train of wheel-work. 
As the real aftion of the machinery is confined to its 
working-ftroke, to accompliflt this, it became neceflary 
to conned with the crank or wheeled work a very large 
and heavy flv, which (hould accumulate in itfelf the 
yvhole preffure of the machine during its time of action, 
and therefore continue in motion, and urge forward, the 
working machinery, while the (team-engine was going 
through its inactive returning-lfroke. This will be the 
cafe, provided that the refiftance exerted by the working- 
machine during the whole period of the working and re¬ 
turning ftroke of the fteam-engine, together with the 
fridion of both, does not exceed the whole preffure ex¬ 
erted by the (team-engine during its workir.g-itroke ; and 
provided that the momentum of the fly, arifing from its 
great weight and velocity, be very great, fo that the re- 
iiltance of the work during one returning-ltroke do not 
make any very fenfible diminution of the velocity of the 
fly. This is evidently poflible and eafy. The fly may 
be made of any magnitude; and, being exadly balanced 
round its axis, it will foon acquire any velocity confif- 
tent with the motion of the engine. During the work- 
ing-ftroke, it is uniformly accelerated, and by its ac¬ 
quired momentum it produces in the beam the moyement 
of the returning-ltroke ; but, in doing this, its momen¬ 
tum is (hared with the inert matter of thg fteam-engine, 
and conlequently its velocity diminiftied, but not entirely 
taken away. The next working-ftroke, therefore, by 
prefling on it afreili, increafes its remaining velocity by 
a quantity nearly equal to the whole that it acquired 
during the firft: ftroke. We fay nearly , becaufe the time 
of the fecond working-ftroke muft be fhorter than that of 
the firft, on account of the velocity already in the ma¬ 
chine. In this manner the fly will be more and more 
accelerated every fucceeding ftroke, becaufe the prefl'ure 
of the engine during the working-ftroke does more than 
reftore to the fly the momentum w hich it loll in producing 
the returning-movement of the fteam-engine. Now fup- 
pofe the working-part of the machine to be added. The 
acceleration of the fly during each working-ftroke of the 
fleam engine will be lets than it was before, becaufe the 
impelling prefl'ure is now partly employed in driving the 
working-machine, and becaufe the fly will lofe more of 
its momentum during the rcturning-ftroke of the fteam- 
engine, part of it being expended in driving the working- 
machine. It is evident, therefore, that a time will come 
when the fucceflive augmentation of the fly’s velocity will 
ceate; for, on the one hand, the continual acceleration 
diminilhes the time of the next working-ftroke,and there¬ 
fore the time of action of the accelerating power. The ac¬ 
celeration muft diminifli in the tame proportion; and, on 
the other hand, the refiftance of the working-machine 
generalq , though not always, increafes with its velocity. 
The acceleration ceafes whenever the addition made to 
the momentum of the fly during a working-ftroke is juft 
equal to w hat it lofes by driving the machine, and by pro¬ 
ducing the returning-movement of the fteam-engine. 
This muIt be acknowledged to be a very important ad¬ 
dition to the engine; and, though fufficiently obvious, 
it is ingenious, and requires confiderable (kill and addrefs 
to ii! .ke it effective. The movement of the working- 
machine, or mill of whatever kind, muft be in foihe de¬ 
gree hobbling or unequal. But this may be made quite 
infenfible, by making the fly very large, and difpofing the 
greateft part of its weight in the rim. By thefe means its 
momentum may be made, fo great, that, the whole force 
required for driving the mill and producing the return- 
ing-movement of the engine may bear a very Tina 11 pro¬ 
portion to it. The diminution of its velocity will then 
be very trifling. No counter-weight is neceflary here, 
becaufe the returning-movement is produced by the in¬ 
ertia of the fly. A counter-weight may, however, be em¬ 
ployed, and fhould be employed, viz. as much as will 
produce the returning-movement of the fteam-engine. 
It will do this better than the fame force accumulated 
in the fly ; for this force muft be accumulated in the fly 
by the intervention of rubbing-parts, by which foine of 
it is loft; and it muft be afterwards returned to the en¬ 
gine with a fimilar lofs. But, for the fame real'on, it 
would be improper to make the counter-weight alfo able 
to drive the mill during the returning ftroke. By this 
contrivance Mr. Fitzgerald hoped to render the fteam- 
engine of molt extendve ufe; and he, or others affociated 
with him, obtained a patent excluding all others from em¬ 
ploying the fleam-engine for turning a crank. They alfo 
publilhed propofais for ereffing mills ot all kinds driven 
by Iteam-engines, and Itated very fairly their powers and 
their advantages. But their propolals do not feem to have 
acquired the confidence of the public ; for we do not 
know of any mill ever having been erected under this 
patent. 
The date of the patent is not mentioned in this ac¬ 
count, which is taken from the Encycloptedia Britannica ; 
but Dr. Olinthus Gregory fays, “ The firft notice we 
ever had ot a patent being obtained for that invention 
was about the year 1778, when a young man at Briftoj, 
Mr. Matthew Walbrongh, obtained letters patent for that 
purpole, and did apply it to his own works for turning- 
lathes, 8 cc. and alfo one at Southampton at Mr. Taylor’s 
works, uefides two or three for grinding corn. However, 
this was previoufly to Mr. Watt’s patent for that purpofe; 
and, little as it was thought of at that penod, it is now 
become of mighty confequence to the kingdom at large. 
This rotatory motion, when produced by a reciprocating 
motion, requires fome contrivance to render it uniform, 
or nearly fo. The ufual method of equalizing is (as we 
have feen) by attaching a fly-wheel to fome part of the 
machinery; but Mr. Arthur Woolf has invented an ap¬ 
paratus to be fubltituted for the fly in fleam-engines, 
which poffeffes the advantage of equalizing the motion, 
with the property of ireing flopped and let to work at any 
part of the ftroke. At fig. 114, A reprelents part of the 
engine-beam ; B, the connecting rod ; C, the crank-arm ; 
D, a cog-wheel, working into another cog-wheel E, of 
half the fize; F, a crank-arm on the fliaft of the Imall 
wheel; G, a cylinder clofed at bottom, in which a iolid 
or unperforated pifton moves, leaving a vacuum beneath. 
This a£ts (imply inftead of a weight on the crank F, by the 
conftant preffure of the atmofphere ; and the diameter of 
the pilton mult be fuch as nearly to equal one-third of the 
power of the engine. In fig. 115. the outer circle is the 
line deferibed by the crank; the circumference of the 
inner circle is equal to twice the diameter of the outer, 
and the fquare has the fame circumference : this laft ex¬ 
hibits the inequality Hill remaining, which by this method 
is reduced to about one fifth; but, by the afliftance of a 
fmall fly on the fecond motion, the effect will become 
nearly the lame as that of a rotative engine, with the ad¬ 
vantages here mentioned. 
The great obftacle to the exfenfive ufe of the fteam- 
engine is the prodigious expenfeof fuel. An engine hav. 
ing a cylinder of tour feet diameter, working night and 
day, confumes about 3400 chaldrons (London) of good 
coals in a year. This circumftance limits the ule of fleam- 
engines exceedingly. To draw water from coal-pits, 
where they can be flocked with unfaleable fmall coal, they 
are of univerfal employment; alfo for valuable mints, for 
fupplying 
