furnaces continually blown, so that the heat may never be abated
by day or night. In the extensive ironworks at Colebrook Dale,
several water-wheels were used in the different operations of the
manufacture of iron, especially in driving the blowers of the iron
furnaces. These wheels were usually driven by the water of a
river, but in the summer months the supply became so short that it
was insufficient to work them all. Steam engines were accordingly
erected to [Pg182] return the water for driving these wheels.
This application of the engine as an occasional power for the
supply of water-wheels having been found so effectual, returning
engines were soon adopted as the permanent and regular means of
supplying water-wheels. The first attempt of this kind is recorded
to have been made by Mr. Oxley, in 1762, who constructed a machine
to draw coals out of a pit at Hartley colliery, in Northumberland.
It was originally intended to turn the machine by a continuous
circular motion received from the beam of the engine; but that
method not being successful, the engine was applied to raise water
for a wheel by which the machine was worked. This engine was
continued in use for several years, and though it was at length
abandoned, on account of its defective construction, it
nevertheless established the practicability of using steam power
as a means of driving water wheels.[23]
(109.) In the year 1777, Mr. John Stewart read a paper before the
Royal Society, describing a method for obtaining a continued
circular motion for turning all kinds of mills from the
reciprocating motion of a steam engine. He proposed to accomplish
this by means of two endless chains passing over pulleys, which
should be moved upwards and downwards by the motion of the engine,
in the manner of a window sash. The joint pins of the links of the
two chains worked in teeth at the opposite sides of a cog wheel,
to which they imparted a circular motion, first by one chain, and
then by the other, acting alternately on opposite sides of the
wheel. One chain impelled it during the descent of the piston, and
the other during the ascent; but one of these chains always passed
over its pulleys so as to produce no effect on one side of the cog
wheel, whilst the other chain worked on the opposite side to turn
it round. For this purpose each chain was provided with a catch,
to prevent its circulating over its pulleys in one direction, but
to allow it free motion in the other. The cog wheel thus kept in
revolution might be applied to the axis of any mill which the
engine was required to work. Thus, if it were applied to a
flour-mill, the millstone itself would perform the office of a
fly-wheel to regulate the intermission of [Pg183] the power, and
in other mills a fly-wheel might be added for this purpose.
The hints obtained by Mr. Stewart from Papin's contrivance, before
mentioned, will not fail to be perceived. In Mr. Stewart's paper
he notices indirectly the method of obtaining a continued circular
motion from a reciprocating motion by means of a crank or winch,
which, he says, occurs naturally in theory, but in practice would
be impossible, from the nature of the motion of the engine, which
depends on the force of the steam, and cannot be ascertained in
its length. Therefore, on the first variation, the machine would
be either broken in pieces or turned back. Such an opinion,
pronounced by a man of considerable mechanical knowledge and
ingenuity, against a contrivance which, as will presently appear,
proved in practice, not less than in theory, to be the most
effectual means of accomplishing the end here pronounced to be
impossible, is sufficiently remarkable. It might cast some doubt
on the extent of Mr. Stewart's practical knowledge, if it did not
happen to be in accordance with a judgment so generally
unimpeachable as that of Mr. Smeaton. This paper of Mr. Stewart's
was referred by the council of the Royal Society to Mr. Smeaton,
who remarked upon the difficulty arising from the absolute
stopping of the whole mass of moving power, whenever the direction
of the motion is changed; and observed, that although a fly-wheel
might be applied to regulate the motion, it must be such a large
one as would not be readily controlled by the engine itself; and
he considered that the use of such a fly-wheel would be a greater
incumbrance to a mill than a water-wheel to be supplied by water
pumped up by the engine. This engineer, illustrious as he was, not
only fell into the error of Mr. Stewart in respect of the crank,
but committed the further blunder of condemning the very expedient
which has since rendered the crank effectual. It will presently
appear that the combination of the crank and fly-wheel have been
the chief means of establishing the dominion of the steam engine
over manufactures.
(110.) In 1779, Mr. Matthew Wasbrough, an engineer at Bristol,
took out a patent for the application of a steam engine [Pg184]
to produce a continuous circular motion by means of ratchet
wheels, similar to those previously used by Mr. Oxley, at Hartley
colliery; to which, however, Mr. Wasbrough added a fly-wheel to
maintain and regulate the motion. Several machines were
constructed under this patent; and among others, one was erected
at Mr. Taylor's saw-mills and block manufactory at Southampton. In
1780, one was erected at Birmingham, where the ratchet work was
found to be subject to such objections, that one of the persons
about the works substituted for it the simple crank, which has
since been invariably used. A patent was taken out for this
application of the crank in the same year, by Mr. James Pickard,
of Birmingham. It will presently appear, however, that the
suggestion of this application of the crank was derived from the
proceedings of Watt, who was at the same time engaged in similar
experiments.
(111.) The single-acting steam engine, as constructed by Watt, was
not adapted to produce continuous uniform motion of rotation, for
the following reasons:—
_First._ The effect required was that of an uniformly acting
force. The steam engine, on the other hand, supplied an
intermitting force. Its operation was continued during the
descending motion of the piston, but it was suspended during the
ascent of the piston. To produce the continued effect now
required, either its principle of operation should be altered, or
some expedient should be devised for maintaining the motion of the
revolving shaft during the ascent of the piston, and the
consequent suspension of the moving power.
_Secondly._ The action of the steam engine was rectilinear. It was
a power which acted in a straight line, viz., in the direction of
the cylinder. The motion, however, required to be produced, was a
circular motion—a motion of rotation around the axis or shaft of
the mill.
The steps by which Watt proceeded to accomplish these objects have
been recorded by himself as follows, in his notes upon Dr.
Robison's article on the steam engine:—
"I had very early turned my mind to the producing of continued
motion round an axis; and it will be seen, by reference to my
first specification in 1769, that I there described [Pg185] a
steam wheel, moved by the force of steam, acting in a circular
channel against a valve on one side, and against a column of
mercury, or some other fluid metal, on the other side. This was
executed upon a scale of about six feet diameter at Soho, and
worked repeatedly, but was given up, as several practical
objections were found to operate against it; similar objections
lay against other rotative engines, which had been contrived by
myself and others, as well as to the engines producing rotatory
motions by means of ratchet wheels.
"Having made my single reciprocating engines very regular in their
movements, I considered how to produce rotative motions from them
in the best manner; and amongst various schemes which were
subjected to trial, or which passed through my mind, none appeared
so likely to answer the purpose as the application of the crank,
in the manner of the common turning lathe; but as the rotative
motion is produced in that machine by impulse given to the crank
in the descent of the foot only, it requires to be continued in
its ascent by the energy of the wheel, which acts as a fly; being
unwilling to load my engine with a fly-wheel heavy enough to
continue the motion during the ascent of the piston (or with a
fly-wheel heavy enough to equalise the motion, even if a
counterweight were employed to act during that ascent), I proposed
to employ two engines, acting upon two cranks fixed on the same
axis, at an angle of 120° to one another, and a weight placed upon
the circumference of the fly-wheel at the same angle to each of
the cranks, by which means the motion might be rendered nearly
equal, and only a very light fly-wheel would be requisite.
"This had occurred to me very early; but my attention being fully
employed in making and erecting engines for raising water, it
remained _in petto_ until about the year 1778 or 1789, when Mr.
Wasbrough erected one of his ratchet-wheel engines at Birmingham,
the frequent breakages and irregularities of which recalled the
subject to my mind, and I proceeded to make a model of my method,
which answered my expectations; but having neglected to take out a
patent, the invention was communicated by a workman employed
to [Pg186] make the model, to some of the people about Mr.
Wasbrough's engine, and a patent was taken out by them for the
application of the crank to steam engines. This fact the said
workman confessed, and the engineer who directed the works
acknowledged it; but said, nevertheless, that the same idea had
occurred to him prior to his hearing of mine, and that he had even
made a model of it before that time; which might be a fact, as the
application to a single crank was sufficiently obvious.
"In these circumstances, I thought it better to endeavour to
accomplish the same end by other means, than to enter into
litigation; and if successful, by demolishing the patent, to lay
the matter open to every body. Accordingly, in 1781, I invented
and took out a patent for several methods of producing rotative
motions from reciprocating ones; amongst which was the method of
the sun-and-planet wheels. This contrivance was applied to many
engines, and possesses the great advantage of giving a double
velocity to the fly-wheel; but is perhaps more subject to wear,
and to be broken under great strains, than a simple crank, which
is now more commonly used, although it requires a fly-wheel of
four times the weight, if fixed upon the first axis; my
application of the double engine to these rotative machines
rendered the counterweight unnecessary, and produced a more
regular motion."
(112.) Watt's second patent here referred to, was dated 25th
October, 1781, and was entitled "A patent for certain new methods
of applying the vibrating or reciprocating motions of steam or
fire engines to produce a continued rotative or circular motion
round an axis or centre, and thereby to give motion to the wheels
of mills and other machines."
All the methods specified in this patent were intended to be
worked by the single-acting engine, already described, a
counterweight being applied to impel the machinery during the
returning stroke of the engine, which weight would be elevated
during the descent of the piston. There were five different
expedients proposed in the specification for producing a rotatory
motion; but, of these five, two only were ever applied in
practice. [Pg187]
(113.) Suppose a rod or bar attached by a pin or joint at the
upper extremity to the working end of the beam of the engine, and
by a similar pin or joint at the lower extremity to an iron wheel
fixed on the extremity of the axis of the fly-wheel. One half of
this wheel is formed of a solid semicircle of cast iron, while the
other half is constructed of open spokes, so as to be as light as
is consistent with strength. The position of the wheel on the axis
is such that during the returning stroke of the piston, when the
operation of the steam is suspended, the heavy semicircle of the
wheel will be descending, and by its weight will draw down the
connecting bar, and thereby draw down the working end of the beam,
and draw up the piston in the cylinder. When the piston descends
and is driven by the power of the steam, the heavy semicircle of
the above-mentioned wheel will be drawn upwards, and in the same
way the motion will be continued.
[Illustration: _Fig._ 32.]
(114.) The second method of producing a rotatory motion, which was
subsequently continued for many years in practical operation, was
that which was called the _Sun-and-planet Wheels_. A toothed wheel
A (_fig._ 32.), called the sun wheel, was fixed on the axle of the
fly-wheel, to which rotation was to be imparted. The wheel B,
called the planet wheel, having an equal diameter, was fastened on
the end I of the connecting rod H I, so as to be incapable of
revolving. During the descent of the piston, the working end of
the beam was drawn upwards, and the end I of the connecting rod
travelled from C to D, through the dotted semicircle C I D. The
wheel B not being capable of revolving on the centre I, would,
during this motion, drive the sun wheel A. During the ascent of
the steam piston, the working end of the beam would descend, and
the centre I of [Pg188] the planet wheel B would be driven
downwards from D to C, through the other dotted semicircle, and
would consequently continue to drive the sun wheel round in the
same direction.
This contrivance, although in the main inferior to the more simple
one of the crank, is not without some advantages; among others, it
gives to the sun wheel double the velocity which would be
communicated by the crank; for in the crank one revolution only on
the axle is produced by one revolution of the crank, but in the
sun-and-planet wheel, two revolutions of the sun wheel are
produced by one of the planet wheel; thus a double velocity is
obtained from the same motion of the beam. This will be evident
from considering that when the planet wheel is in its highest
position, its lowest tooth is engaged with the highest tooth of
the sun wheel; as the planet wheel passes from the highest
position, its teeth drive those of the sun wheel before them, and
when it comes into the lowest position, the highest tooth of the
planet wheel is engaged with the lowest of the sun wheel: but then
half of the sun wheel has _rolled off_ the planet wheel, and,
therefore, the tooth which was engaged with it in its highest
position, must now be distant from it by half the circumference of
the wheel, and must, therefore, be again in the highest position;
so that while the planet wheel has been carried from the top to
the bottom, the sun wheel has made a complete revolution.
This advantage of giving an increased velocity may be obtained
also by the crank, by placing toothed wheels on its axle.
Independently of the greater expense attending the construction of
the sun-and-planet wheel, its liability to go out of order, and
the rapid wear of the teeth, and other objections, rendered it
inferior to the crank, which has entirely superseded it.
(115.) Although by these contrivances Watt succeeded in obtaining
a continuous circular motion from the reciprocating motion of
the steam engine, the machine was still one of intermitting,
instead of continuous action. The expedient of a counterweight,
elevated during the descending stroke, and giving back the power
expended on it in the interval of the returning stroke, did not
satisfy the [Pg189] fastidious mechanical taste of Watt. He
soon perceived that all which he proposed to accomplish by the
application of two cylinders and pistons working alternately,
could be attained with greater simplicity and effect by a single
cylinder, if he could devise means by which the piston might be
impelled by steam upwards as well as downwards. To accomplish
this, it was only necessary to throw the lower end of the cylinder
into alternate communication with the boiler, while the upper end
would be put into communication with the condenser. If, for
example, during the descent of the piston, the upper end of the
cylinder communicated with the boiler, and the lower end with the
condenser; and, on the other hand, during the ascent of the
piston, the lower end communicated with the boiler, and the upper
end with the condenser; then the piston would be driven
continually, whether upwards or downwards, by the power of steam
acting against a vacuum. Watt obtained his third patent for this
contrivance, on the 12th of March, 1782.
This change in the principle of the machine involved several other
changes in the details of its mechanism.
[Illustration: _Fig._ 33.]
(116.) It was necessary, in the first place, to provide means for
admitting and withdrawing the steam at either end of the cylinder.
For this purpose let B and B′ (_fig._ 33.) be two steam-boxes, B
the upper, and B′ the lower, communicating respectively with the
top and bottom of the cylinder by proper passages D D′. Let two
valves be placed in B, one, S, above the passage D, and the other,
C, below it; and in like manner two other valves in the lower
valve-box, B′, one, S′, above the passage D′, and the other, C′,
below it. Above the valve S in the upper steam-box is an opening
at which the steam-pipe from the boiler enters, and below the
valve C is another opening, at which enters the exhausting-pipe
leading to the condenser. In like manner, above the valve S′ in
the lower steam-box enters a steam-pipe leading from the boiler,
and below the valve C′ enters an exhausting-pipe leading to
[Pg190] the condenser. It is evident, therefore, that steam can
always be admitted above the piston by opening the valve S, and
below it by opening the valve S′; and, in like manner, steam can
be withdrawn from the cylinder above the piston, and allowed to
pass to the condenser, by opening the valve C, and from below it
by opening the valve C′.
[Illustration: _Fig._ 34.]
Supposing the piston P to be at the top of the cylinder, and the
cylinder below the piston to be filled with pure steam, let the
valves S and C′ be opened, the valves C and S′ being closed as
represented in _fig._ 34. Steam from the boiler will, therefore,
flow in through the open valve S, and will press the piston
downwards, while the steam that has filled the cylinder below the
piston will pass through the open valve C′ into the exhausting-pipe
leading to the condenser, and being condensed will leave the
cylinder below the piston a vacuum. The piston will, therefore, be
pressed downwards by the action of the steam above it, as in the
single-acting engine. Having arrived at the bottom of the cylinder,
let the valves S and C′ be both closed, and the valves S′ and C be
opened, as represented in _fig._ 34. Steam will now be admitted
through the open valve S′ and through the passage D′ below the
piston, while the steam which has just driven the piston downwards,
filling the cylinder above the piston, will be drawn off through the
open valve C, and the exhausting-pipe, into the condenser, leaving
the cylinder above the piston a vacuum. The piston will, therefore,
be pressed upwards by the action of the steam below it, against the
vacuum above it, and will ascend with the same force as that with
which it had descended.
This alternate action of the piston upwards and downwards may
evidently be continued by opening and closing the valves alternately
in pairs. Whenever the piston is at the top of the cylinder, as
represented in _fig._ 33., the valves S and C′, that is, the upper
steam-valve and the lower exhausting-valve, are opened, and the
valves C and S′, that is, the upper exhausting-valve and the lower
steam-valve, are closed; and [Pg191] when the piston has arrived at
the bottom of the cylinder, as represented in _fig._ 34., the valves
C and S′, that is, the upper exhausting-valve and the lower
steam-valve, are opened, and the valves S and C′, that is, the upper
steam-valve and the lower exhausting-valve, are closed.
If these valves, as has been here supposed, be opened and closed
at the moments at which the piston reaches the top and bottom of
the cylinder, it is evident that they may be all worked by a
single lever connected with them by proper mechanism. When the
piston arrives at the top of the cylinder, this lever would be
made to open the valves S and C′, and at the same time to close
the valves S′ and C; and when it arrives at the bottom of the
cylinder, it would be made to close the valves S and C′, and to
open the valves S′ and C.
If, however, it be desired to cut off the steam before the arrival
of the piston at the termination of its stroke, whether upwards or
downwards, then the steam-valves must be closed before the arrival
of the piston at the end of its stroke; and as the exhausting-valve
ought to be left open until the stroke is completed, these valves
ought to be moved at different times. In that case separate levers
should be provided for the different valves. We shall, however,
return again to the subject of the valves which regulate the
admission of steam to the cylinder and its escape to the condenser.
(117.) It will be remembered that in the single-acting engine the
process of condensation was suspended while the piston ascended in
the cylinder, and therefore the play of the jet of cold water in the
condenser was stopped during this interval. In the double-acting
engine, however, the flow of steam from the cylinder to the
condenser is continued, whether the piston ascends or descends, and
therefore a constant condensation of steam must be produced. The
condensing jet, therefore, does not in this case, as in the former,
play with intervals of intermission. A constant jet of cold water
must be maintained in the condenser.
It will presently appear that in the double-acting engine applied
to manufactures, the motion of the piston was subject to more or
less variation of speed, and the quantity of steam [Pg192]
admitted to the cylinder was subject to a corresponding change.
The quantity of steam, therefore, drawn into the condenser was
subject to variation, and required a considerable change in the
quantity of cold water admitted through the jet to condense it. To
regulate this, the valve or cock by which the water was admitted
into the condenser was worked in the double-acting engine by a
lever furnished with an index, by which the quantity of condensing
water admitted into the condenser could be regulated. This index
played upon a graduated arch, by which the engine-man was enabled
to regulate the supply.
[Illustration: HEATHFIELD HOUSE, NEAR BIRMINGHAM, THE RESIDENCE OF
WATT.]
FOOTNOTES:
[20] These effects are explained in my Treatise on Heat; and
they have lately been verified by experiments made with
locomotive engines by M. de Pambour, who found that the steam
raised from the boiler of a locomotive engine, under a
pressure of above 50 lbs. per square inch, was in the state of
common steam as it issued from the chimney at a very diminished
pressure and at a lower pressure.
[21] It is strange that this absurdity has been repeatedly
given as unquestionable fact in various encyclopædias, as well
as in by far the greater number of treatises expressly on the
subject.
[22] Farey, Treatise on the Steam Engine, p. 122.
[23] Farey on the Steam Engine, p. 297.
[Pg193]
[Illustration: DOUBLE-ACTING ENGINE.—CITY SAW-MILLS.]
CHAP. VIII.
METHODS OF CONNECTING THE PISTON-ROD AND BEAM IN THE
DOUBLE-ACTING ENGINE. — RACK AND SECTOR. — PARALLEL MOTION. —
CONNECTING ROD AND CRANK. — FLY-WHEEL. — THROTTLE-VALVE. —
GOVERNOR. — CONSTRUCTION AND OPERATION OF THE DOUBLE-ACTING
ENGINE. — ECCENTRIC. — COCKS AND VALVES. — SINGLE-CLACK VALVE.
— DOUBLE-CLACK VALVE. — CONICAL VALVES. — SLIDE VALVES. —
MURRAY'S SLIDES. — THE D VALVE. — SEAWARD'S SLIDES. — SINGLE
COCK. — FOUR-WAY COCK. — PISTONS. — COMMON HEMP-PACKED PISTON.
— WOOLFE'S PISTON. — METALLIC PISTONS. — CARTWRIGHT'S ENGINE.
— CARTWRIGHT'S PISTON. — BARTON'S PISTON.
(118.) In the single-acting engine, the force of the piston acted
on the beam only during its descent; and this force was
transmitted from the piston to the beam, as we have seen, by a
flexible chain, extended from the end of the piston-rod, [Pg194]
and playing upon the arch head of the beam. In the double-acting
engine, however, the force of the steam pressing the piston
upwards must likewise be transmitted to the beam, so as to drive
the latter upwards while the piston ascends. This action could not
be accomplished by a chain connecting the piston with the arch
head of the beam.