MECHANICS* 
3G-motion; the fly*wheel will he whirling with an uniform 
celerity, and with a. force capable of Continuing that cele- 
rfty when there is any relaxation in the impelling power. 
After a fhort reft the animal renews his efforts; but the 
machine is now moving with its former velocity, and thefe 
frefh efforts will have a tendency to increafe that velocity. 
The fly, however, now acts as a refilling power, receives 
the greateft part of the fuperfluous motion, andcaufestlie 
machinery to preferve its original celerity. In this way 
the fly feeures to the engine an uniform motion, whether 
the animal takes occafional relaxations or exerts his force 
with redoubled ardour. 
We have already obferved that a defultory or variable 
motion frequently arifes from the inequality of the refin¬ 
ance, or work to be performed. This is particularly ma- 
liifeft in thrafhing-mills, on a fmall fcale, which are driven 
by water. When the corn is laid unequally on the feed¬ 
ing-board, fo that too much is taken in by the fluted rol¬ 
lers, this increafe of refillance inftanlly affefils the machi¬ 
nery, and communicates a defultory or irregular motion 
even to the water-wheel, or firft mover. This variation in 
the velocity of the impelling power may bediftinftiy per¬ 
ceived by the ear in a calm evening when the machine is 
at work. The belt method of correflir.g thefe irregularities 
is to employ a fly-wheel, which will regulate the motion 
of the machine when the refillance is either augmented or 
diminifhed. In machines built upon a large fcale there is 
no iiecefiffy for the interpofition of a fly, as the inertia of 
the machinery fupplies its place, and refifts every change 
of motion that may be generated by an unequal admiffion 
of the corn. 
A variation in the velocity of engines arifes alfo from 
the nature of the machinery. Let us fuppofe that a weight 
of iooo pounds is to be railed from the bottom of a well 
50 feet, by means of a bucket attached to an iron chain 
which winds round a barrel or cylinder, and that every 
foot length of this chain weighs two pounds. It is evi¬ 
dent that the refinance to be overcome in the firft moment 
is 1000 pounds added to 100 pounds, the weight of this 
chain, and that this refillance diminifhes gradually as the 
chain coils round the cylinder, till it is only 1000 pounds 
when the chain is completely wound lip. The refinance 
therefore decreafes from 1100 to 1000 pounds ; and, if the 
impelling power is inanimate, the velocity of the bucket 
will gradually increafe; but, if an animal is employed, it 
will generally proportion its affion to the refilling load, 
and miift therefore pull with a greater or lefs force accord¬ 
ing as the bucket is near the bottom or top of the well. 
In this cafe, however, the aflillance of a fly may be dif- 
penfed with, becaufe the refillance diminiflies uniformly, 
and may be rendered conflant by making the barrel co¬ 
nical, fo that the chain may wind upon the part neareft 
the vertex at the commencement of the motion, the dia¬ 
meter of the barrel gradually increafing as the weight di- 
roinifhes. In this way the variable refifiance will he 
equalized much better than by the application of a fiy- 
w heel; for the fly, having no motion of its own, mull ne- 
ceffarily watte the impelling power. 
Vauloue’s pile-engine, of which we fliall give a more 
particular defeription farther on, is a ftriking inltance of the 
utility of fly-wheels. The ram Q is raffed between the 
uprights RR by means ot horfes acting againfl the levers SS; 
but, as foon as tlie ram is elevated to the top of the frame, 
and difeharged from the follower G, the refillance againft 
which the liorfes have been exerting their force is fud- 
denly removed, and they would inftantaneoufly tumble 
down, were it not for tire fly O. This fly is connected 
with the drum B by means of the trundle X ; and, as it is 
moving with a very great force, it oppofes a fufficient re¬ 
fillance to the action of the horfes, till the ram is again 
taken up by the follower. See Plate XVI. fig. 65. 
When the moving-power is a man adding with a handle 
cr winch, it is fubjedl to great inequalities. The greateft 
force is exeited when the man pulls the handle upwards 
6G7 
from the height of his knee, and he adds w ith the lead force 
when the handle, being in a vertical pofition, is thru It from 
him in a horizontal direction. The force is again increafed 
when the handle is pufhed downwards by the man’s weight; 
and it is diminifhed when the handle, being at its lowed 
point is pulled towards him horizontally. But, when a fly 
is properly connected with the machinery, thefe irregular 
exertions are equalized, the velocity becomes uniform, and 
the load is railed with an equable and Ready motion. 
This regulating power of the fly is without bounds, and 
may be ufed to render uniform a motion produced by the 
molt defultory and irregular power. It is thus that the 
molt regular motion is given to mills that are driven by a 
fingle-flroke fleam-engine, where for two or even three 
feconds there is no force p re fling round the mill. The 
communication is made through a maflive fly of very great 
diameter, whirling with very great rapidity. As foon as 
the impulfe ceafes, the fly, continuing its motion, urges 
round the whole machinery with almoft unabated fpeecl. 
At this inftant all the teeth, and all the joints, between 
the fly and the firft mover, are heard to catcli in the op- 
pofite diredtion. If any permanent change fhould happen 
in the impelling power, or in the refillance, the fly makes 
no obftade to its producing its full effect on the machine ; 
and it will be obferved to accelerate or retard uniformly, 
till a new general (peed is acquired exadily correfponding 
with this new power and refinance. 
Many machines include in their conftruclion move¬ 
ments which are equivalent with this intentional regu¬ 
lator. A flour-mill, tor example, cannot be better regu¬ 
lated than by its mili-ftone; but in the Albion-mills a 
heavy fly was added with great propriety; for, if the mills 
had been regulated by their mill-ftones only, then, at every 
change of ltroke in the fleam-engine, the whole train of 
communicatious between the beam, which is the firft mover, 
and the regulating millftone, which is the very laft mover, 
would take in the oppofite diredtion. Although each 
drop in tlve teeth and joints be but a trifle, the whole, 
added together, would make a conliderable jolt. This is 
avoided by a regulator immediately adjoining to the beam. 
This continually preffts the working machinery in one 
diredtion. So judicioufly were the movements of that no¬ 
ble machine contrived, and fo nicely were they executed, 
that not the leaft noife was heard, nor the flighted: tremor 
felt in the building. 
The fpindles, cards, and bobbins, of a cotton-mill, are 
alfo a fort of flies. Indeed all bulky machines of the ro¬ 
tative kind tend to preferve their motion with fome degree 
of fteadinefs, and their great momentum of inertia is as 
ufeful in this refpedt as it is prejudicial to the acceleration 
or any reciprocation when w'anted. But there is another 
kind of regulating fly, confiding of wings whirled brifkly 
round till the refinance of the air prevents any great acce¬ 
leration. This is a very bad one fora working-machine, 
for it produces its effedl by really waiting a part of the 
moving power. Frequently it employs a very great and 
unknown part of it, and robs the proprietor of much 
work. It fhould never be introduced into any machine 
employed in manufactures. 
Souft rare cafes occur where a very different regulator 
is required : where a certain determined velocity is found 
neceffary. In this cafe the-machine is furnifhed, at its' 
extreme mover, with a conical pendulum, confuting of 
two heavy bails hanging by rods, which move in very nice 
and fleady joints at the top of a vertical axis. This new 
kind of fly h is been ingenioufly employed by Mr. Watt for 
procuring a determinate velocity at the working point cf 
liis fleam-engine. It is reprefented in fig. 10S. where AB 
is a vertical axis moving upon pivots, and driven by means 
of a rope paffing from the axis of the large fly over the 
iheave E F. The large balls M, N, are fixed to the rods 
N G, M II, which have an angular motion round P, and 
are connected, by joints at G and II, with the rods GK, 
KK, attached to the extremity of the lever KL, whole 
3 centre 
