self-adjustment, in the method by which the strength of the fire is
regulated. The governor regulates the supply of steam to the engine,
and proportions it to the work to be done. With this work, therefore,
the demands upon the boiler increase or diminish, and with these
demands the production of steam in the boiler ought to vary. In fact,
the rate at which steam is generated in the boiler, ought to be equal
to that at which it is consumed in the engine, otherwise one of two
effects must ensue: either the boiler will fail to supply the engine
with steam, or steam will accumulate in the boiler, being produced in
undue quantity, and, escaping at the safety valve, will thus be wasted.
It is, therefore, necessary to control the agent which generates the
steam, namely, the fire, and to vary its intensity from time to time,
proportioning it to the demands of the engine. To accomplish this,
the following contrivance has been adopted:--Let T, _fig. 146._,
be a tube inserted in the top of the boiler, and descending nearly
to the bottom. The pressure of the steam confined in the boiler,
acting upon the surface of the water, forces it to a certain height
in the tube T. A weight F, half immersed in the water in the tube, is
suspended by a chain, which passes over the wheels P P′, and is
balanced by a metal plate D, in the same manner as the stone float,
_fig. 145._, is balanced by the weight A. The plate D passes
through the mouth of the flue E as it issues finally from the boiler;
so that when the plate D falls it stops the flue, suspending thereby
the draught of air through the furnace, mitigating the intensity of
the fire, and checking the production of steam. If, on the contrary,
the plate D be drawn up, the draught is increased, the fire is rendered
more active, and the production of steam in the boiler is stimulated.
Now, suppose that the boiler produces steam faster than the engine
consumes it, either because the load on the engine has been diminished,
and, therefore, its consumption of steam reduced, or because the
fire has become too intense; the consequence is, that the steam,
beginning to accumulate in the boiler, will press upon the surface of
the water with increased force, and the water will be raised in the
tube T. The weight F will, therefore, be lifted, and the plate D will
descend, diminish, or stop the draught, mitigate the fire, and retard
the production of steam, and will continue to do so until the rate
at which steam is produced shall be commensurate to the wants of the
engine. If, on the other hand, the production of steam be inadequate
to the exigency of the machine, either because of an increased load,
or of the insufficient force of the fire, the steam in the boiler will
lose its elasticity, and the surface of the water not sustaining its
wonted pressure, the water in the tube T will fall; consequently the
weight F will descend, and the plate D will be raised. The flue being
thus opened, the draught will be increased, and the fire rendered
more intense. Thus the production of steam becomes more rapid, and is
rendered sufficiently abundant for the purposes of the engine. This
apparatus is called the _self-acting damper_.
(307.) When a perfectly uniform rate of motion has not been attained,
it is often necessary to indicate small variations of velocity. The
following contrivance, called a _tachometer_[4], has been invented
to accomplish this. A cup, _fig. 147._, is filled to the level
C D with quicksilver, and is attached to a spindle, which is
whirled by the machine in the same manner as the governor already
described. It is well known that the centrifugal force produced by this
whirling motion will cause the mercury to recede from the centre and
rise upon the sides of the cup, so that its surface will assume the
concave appearance represented in _fig. 148._ In this case the
centre of the surface will obviously have fallen below its original
level, _fig. 147._, and the edges will have risen above that
level. As this effect is produced by the velocity of the machine, so
it is proportionate to that velocity, and subject to corresponding
variations. Any method of rendering visible small changes in the
central level of the surface of the quicksilver will indicate minute
variations in the velocity of the machine.
[4] From the Greek words _tachos_ speed, and _metron_ measure.
A glass tube A, open at both ends, and expanding at one extremity into
a bell B, is immersed with its wider end in the mercury, the surface
of which will stand at the same level in the bell B, and in the cup
C D. The tube is so suspended as to be unconnected with the cup.
This tube is then filled to a certain height A, with spirits tinged
with some colouring matter, to render it easily observable. When the
cup is whirled by the machine to which it is attached, the level of
the quicksilver in the bell falls, leaving more space for the spirits,
which, therefore, descends in the tube. As the motion is continued,
every change of velocity causes a corresponding change in the level
of the mercury, and, therefore, also in the level A of the spirits.
It will be observed, that, in consequence of the capacity of the bell
B being much greater than that of the tube A, a very small change in
the level of the quicksilver in the bell will produce a considerable
change in the height of the spirits in the tube. Thus this ingenious
instrument becomes a very delicate indicator of variations in the
motion of machinery.
(308.) The governor, and other methods of regulating the motion of
machinery which have been just described, are adapted principally to
cases in which the proportion of the resistance to the load is subject
to certain fluctuations or gradual changes, or at least to cases in
which the resistance is not at any time entirely withdrawn, nor the
energy of the power actually suspended. Circumstances, however,
frequently occur in which, while the power remains in full activity,
the resistance is at intervals suddenly removed and as suddenly again
returns. On the other hand, cases also present themselves, in which,
while the resistance is continued, the impelling power is subject to
intermission at regular periods. In the former case, the machine would
be driven with a ruinous rapidity during those periods at which it is
relieved from its load, and on the return of the load every part would
suffer a violent strain, from its endeavour to retain the velocity
which it had acquired, and the speedy destruction of the engine could
not fail to ensue. In the latter case, the motion would be greatly
retarded or entirely suspended during those periods at which the moving
power is deprived of its activity, and, consequently, the motion which
it would communicate would be so irregular as to be useless for the
purposes of manufactures.
It is also frequently desirable, by means of a weak but continued
power, to produce a severe but instantaneous effect. Thus a blow may
be required to be given by the muscular action of a man’s arm with a
force to which, unaided by mechanical contrivance, its strength would
be entirely inadequate.
In all these cases, it is evident that the object to be attained is,
an effectual method of accumulating the energy of the power so as
to make it available after the action by which it has been produced
has ceased. Thus, in the case in which the load is at periodical
intervals withdrawn from the machine, if the force of the power could
be imparted to something by which it would be preserved, so as to be
brought against the load when it again returned, the inconvenience
would be removed. In like manner, in the case where the power itself
is subject to intermission, if a part of the force which it exerts in
its intervals of action could be accumulated and preserved, it might
be brought to bear upon the machine during its periods of suspension.
By the same means of accumulating force, the strength of an infant,
by repeated efforts, might produce effects which would be vainly
attempted by the single and momentary action of the strongest man.
(309.) The property of inertia, explained and illustrated in the third
and fourth chapters of this volume furnishes an easy and effectual
method of accomplishing this. A mass of matter retains, by virtue of
its inertia, the whole of any force which may have been given to it,
except that part of which friction and the atmospheric resistance
deprives it. By contrivances which are well known and present no
difficulty, the part of the moving force thus lost may be rendered
comparatively small, and the moving mass may be regarded as retaining
nearly the whole of the force impressed upon it. To render this method
of accumulating force fully intelligible, let us first imagine a
polished level plane on which a heavy globe of metal, also polished, is
placed. It is evident that the globe will remain at rest on any part of
the plane without a tendency to move in any direction. As the friction
is nearly removed by the polish of the surfaces, the globe will be
easily moved by the least force applied to it. Suppose a slight impulse
given to it, which will cause it to move at the rate of one foot in
a second. Setting aside the effects of friction, it will continue to
move at this rate for any length of time. The same impulse repeated
will increase its speed to two feet per second. A third impulse to
three feet, and so on. Thus 10,000 repetitions of the impulse will
cause it to move at the rate of 10,000 feet per second. If the body to
which these impulses were communicated were a cannon ball, it might,
by a constant repetition of the impelling force, be at length made to
move with as much force as if it were projected from the most powerful
piece of ordnance. The force with which the ball in such a case would
strike a building might be sufficient to reduce it to ruins, and yet
such force would be nothing more than the accumulation of a number
of weak efforts not beyond the power of a child to exert, which are
stored up, and preserved, as it were, by the moving mass, and thereby
brought to bear, at the same moment, upon the point to which the force
is directed. It is the sum of a number of actions exerted successively,
and, during a long interval, brought into operation at one and the same
moment.
But the case which is here supposed cannot actually occur; because
we have not usually any practical means of moving a body for any
considerable time in the same direction without much friction, and
without encountering numerous obstacles which would impede its
progress. It is not, however, essential to the effect which is to be
produced, that the motion should be in a straight line. If a leaden
weight be attached to the end of a light rod or cord, and be whirled by
the force of the arm in a circle, it will gradually acquire increased
speed and force, and at length may receive an impetus which would
cause it to penetrate a piece of board as effectually as if it were
discharged from a musket.
The force of a hammer or sledge depends partly on its weight, but much
more on the principle just explained. Were it allowed merely to fall
by the force of its weight upon the head of a nail, or upon a bar of
heated iron which is to be flattened, an inconsiderable effect would be
produced. But when it is wielded by the arm of a man, it receives at
every moment of its motion increased force, which is finally expended
in a single instant on the head of the nail, or on the bar of iron.
The effects of flails in threshing, of clubs, whips, canes, and
instruments for striking, axes, hatchets, cleavers, and all instruments
which cut by a blow, depend on the same principle, and are similarly
explained.
The bow-string which impels the arrow does not produce its effect at
once. It continues to act upon the shaft until it resumes its straight
position, and then the arrow takes flight with the force accumulated
during the continuance of the action of the string, from the moment it
was disengaged from the finger of the bow-man.
Fire-arms themselves act upon a similar principle, as also the air-gun
and steam-gun. In these instruments the ball is placed in a tube, and
suddenly exposed to the pressure of a highly elastic fluid, either
produced by explosion as in fire-arms, by previous condensation as in
the air-gun, or by the evaporation of highly heated liquids as in the
steam-gun. But in every case this pressure continues to act upon it
until it leaves the mouth of the tube, and then it departs with the
whole force communicated to it during its passage along the tube.
(310.) From all these considerations it will easily be perceived that
a mass of inert matter may be regarded as a magazine in which force
may be deposited and accumulated, to be used in any way which may be
necessary. For many reasons, which will be sufficiently obvious, the
form commonly given to the mass of matter used for this purpose in
machinery is that of a wheel, in the rim of which it is principally
collected. Conceive a massive ring of metal, _fig. 149._,
connected with a central box or nave by light spokes, and turning on
an axis with little friction. Such an apparatus is called a fly-wheel.
If any force be applied to it, with that force (making some slight
deduction for friction) it will move, and will continue to move until
some obstacle be opposed to its motion, which will receive from it a
part of the force it has acquired. The uses of this apparatus will be
easily understood by examples of its application.
Suppose that a heavy stamper or hammer is to be raised to a certain
height, and thence to be allowed to fall, and that the power used
for this purpose is a water-wheel. While the stamper ascends, the
power of the wheel is nearly balanced by its weight, and the motion
of the machine is slow. But the moment the stamper is disengaged and
allowed to fall, the power of the wheel, having no resistance, nor any
object on which to expend itself, suddenly accelerates the machine,
which moves with a speed proportioned to the amount of the power,
until it again engages the stamper, when its velocity is as suddenly
checked. Every part suffers a strain, and the machine moves again
slowly until it discharges its load, when it is again accelerated,
and so on. In this case, besides the certainty of injury and wear, and
the probability of fracture from the sudden and frequent changes of
velocity, nearly the whole force exerted by the power in the intervals
between the commencement of each descent of the stamper and the next
ascent is lost. These defects are removed by a fly-wheel. When the
stamper is discharged, the energy of the power is expended in moving
the wheel, which, by reason of its great mass, will not receive an
undue velocity. In the interval between the descent and ascent of the
stamper, the force of the power is lodged in the heavy rim of the
fly-wheel. When the stamper is again taken up by the machine, this
force is brought to bear upon it, combined with the immediate power
of the water-wheel, and the stamper is elevated with nearly the same
velocity as that with which the machine moved in the interval of its
descent.
(311.) In many cases, when the moving power is not subject to
variation, the efficacy of the machine to transmit it to the working
point is subject to continual change. The several parts of every
machine have certain periods of motion, in which they pass through a
variety of positions, to which they continually return after stated
intervals. In these different positions the effect of the power
transmitted to the working point is different; and cases even occur
in which this effect is altogether annihilated, and the machine is
brought into a predicament in which the power loses all influence
over the weight. In such cases the aid of a fly-wheel is effectual
and indispensable. In those phases of the machine, which are most
favourable to the transmission of force, the fly-wheel shares the
effect of the power with the load, and retaining the force thus
received directs it upon the load at the moments when the transmission
of power by the machine is either feeble or altogether suspended. These
general observations will, perhaps, be more clearly apprehended by an
example of an application of the fly-wheel, in a case such as those now
alluded to.
Let A B C D E F, _fig. 150._, be a
_crank_, which is a double winch ((252.) and _fig. 89._), by
which an axle, A B E F, is to be turned. Attached
to the middle of C D by a joint is a rod, which is connected
with a beam, worked with an alternate motion on a centre, like
the brake of a pump, and driven by any constant power, such as a
steam-engine. The bar C D is to be carried with a circular motion
round the axis A E. Let the machine, viewed in the direction
A B E F of the axis, be conceived to be represented
in _fig. 151._, where A represents the centre round which the
motion is to be produced, and G the point where the connecting rod
G H is attached to the arm of the crank. The circle through which
G is to be urged by the rod is represented by the dotted line. In
the position represented in _fig. 151._, the rod acting in the
direction H G has its full power to turn the crank G A round
the centre A. As the crank comes into the position represented in
_fig. 152._, this power is diminished, and when the point G comes
immediately below A, as in _fig. 153._, the force in the direction
H G has no effect in turning the crank round A, but, on the
contrary, is entirely expended in pulling the crank in the direction
A G, and, therefore, only acts upon the pivots or gudgeons which
support the axle. At this crisis of the motion, therefore, the whole
effective energy of the power is annihilated.
After the crank has passed to the position represented in
_fig. 154._, the direction of the force which acts upon the
connecting rod is changed, and now the crank is drawn upward in the
direction G H. In this position the moving force has some efficacy
to produce rotation round A, which efficacy continually increases
until the crank attains the position shown in _fig. 155._, when
its power is greatest. Passing from this position its efficacy is
continually diminished, until the point G comes immediately above the
axis A, _fig. 156._ Here again the power loses all its efficacy
to turn the axle. The force in the direction G H or H G can
obviously produce no other effect than a strain upon the pivots or
gudgeons.
In the critical situations represented in _fig. 153._, and
_fig. 156._, the machine would be incapable of moving, were
the immediate force of the power the only impelling principle. But
having been previously in motion by virtue of the inertia of its
various parts, it has a tendency to continue in motion; and if the
resistance of the load and the effects of friction be not too great,
this disposition to preserve its state of motion will extricate the
machine from the dilemma in which it is involved in the cases just
mentioned, by the peculiar arrangement of its parts. In many cases,
however, the force thus acquired during the phases of the machine, in
which the power is active, is insufficient to carry it through the
dead points (_fig. 153._ and _fig. 156._); and in all cases
the motion would be very unequal, being continually retarded as it
approached these points, and continually accelerated after it passed
them. A fly-wheel attached to the axis A, or to some other part of the
machinery, will effectually remove this defect. When the crank assumes
the positions in _fig. 151._ and _fig. 155._, the power is
in full play upon it, and a share of the effect is imparted to the
massive rim of the fly-wheel. When the crank gets into the predicament
exhibited in _fig. 153._ and _fig. 156._, the momentum which
the fly-wheel received when the crank acted with most advantage,
immediately extricates the machine, and, carrying the crank beyond the
dead point, brings the power again to bear upon it.
The astonishing effects of a fly-wheel, as an accumulator of force,
have led some into the error of supposing that such an apparatus
increases the actual power of a machine. It is hoped, however, that
after what has been explained respecting the inertia of matter and the
true effects of machines, the reader will not be liable to a similar
mistake. On the contrary, as a fly cannot act without friction, and as
the amount of the friction, like that of inertia, is in proportion to
the weight, a portion of the actual moving force must unavoidably be
lost by the use of a fly. In cases, however, where a fly is properly
applied this loss of power is inconsiderable, compared with the
advantageous distribution of what remains.
[Illustration: _C. Varley, del._ _H. Adlard, sc._
_London, Pubd. by Longman & Co._]
As an accumulator of force, a fly can never have more force than has
been applied to put it in motion. In this respect it is analogous to
an elastic spring, or the force of condensed air, or any other power
which derives its existence from causes purely mechanical. In bending
a spring a gradual expenditure of power is necessary. On the recoil
this power is exerted in a much shorter time than that consumed in its
production, but its total amount is not altered. Air is condensed by a
succession of manual efforts, one of which alone would be incapable of
projecting a leaden ball with any considerable force, and all of which
could not be immediately applied to the ball at the same instant. But
the reservoir of condensed air is a magazine in which a great number of
such efforts are stored up, so as to be brought at once into action.
If a ball be exposed to their effect, it may be projected with a
destructive force.
In mills for rolling metal the fly-wheel is used in this way. The
water-wheel or other moving power is allowed for some time to act upon
the fly-wheel alone, no load being placed upon the machine. A force
is thus gained which is sufficient to roll a large piece of metal, to
which without such means the mill would be quite inadequate. In the
same manner a force may be gained by the arm of a man acting on a fly
for a few seconds, sufficient to impress an image on a piece of metal
by an instantaneous stroke. The fly is, therefore, the principal agent
in coining presses.
(312.) The power of a fly is often transmitted to the working point
by means of a screw. At the extremities of the cross arm A B,
_fig. 157._, which works the screw, two heavy balls of metal are
placed. When the arm A B is whirled round, those masses of metal
acquire a momentum, by which the screw, being driven downward, urges
the die with an immense force against the substance destined to receive
the impression.
Some engines used in coining have flies with arms four feet long,
bearing one hundred weight at each of their extremities. By turning
such an arm at the rate of one entire circumference in a second, the
die will be driven against the metal with the same force as that with
which 7500 pounds weight would fall from the height of 16 feet; an
enormous power, if the simplicity and compactness of the machine be
considered.
The place to be assigned to a fly-wheel relatively to the other parts
of the machinery is determined by the purpose for which it is used. If
it be intended to equalise the action, it should be near the working
point. Thus, in a steam-engine, it is placed on the crank which turns
the axle by which the power of the engine is transmitted to the object
it is finally designed to affect. On the contrary, in handmills, such
as those commonly used for grinding coffee, &c., it is placed upon the
axis of the winch by which the machine is worked.
The open work of fenders, fire-grates, and similar ornamental articles
constructed in metal, is produced by the action of a fly, in the
manner already described. The cutting tool, shaped according to the
pattern to be executed, is attached to the end of the screw; and the
metal being held in a proper position beneath it, the fly is made to
urge the tool downwards with such force as to stamp out pieces of the
required figure. When the pattern is complicated, and it is necessary
to preserve with exactness the relative situation of its different
parts, a number of punches are impelled together, so as to strike the
entire piece of metal at the same instant, and in this manner the most
elaborate open work is executed by a single stroke.
CHAP. XVIII.
MECHANICAL CONTRIVANCES FOR MODIFYING MOTION.