possessed considerable advantages over that of Savery; and even at the
present day this machine is not unfrequently used in districts where
fuel is very abundant and cheap, the first cost being considerably
less than that of a modern engine. The low pressure of the steam
necessary to work it rendered the use of the atmospheric engine
perfectly safe; there being only a bursting pressure of about 1lb. per
inch, while in Savery's there was a bursting pressure amounting to
30lbs. The temperature of the steam not exceeding 216°, did not weaken
or destroy the materials; while Savery's engines required steam raised
from water at 267°, which in a short time rendered the engine unable
to sustain the pressure.
The power of Savery's engines was also very limited, both as to the
quantity of water raised, and the height to which it was elevated
(34.). On the other hand, the atmospheric engine had no other limit
than the dimensions of the piston. In estimating the power of these
engines, however, we cannot allow the full atmospheric pressure as an
effective force. The condensing water being mixed with the condensed
steam, forms a quantity of hot water in the bottom of the cylinder,
which, not being submitted to the atmospheric pressure (17.), produces
a vapour which resists the descent of the piston. In practice we find
that an allowance of at least 3lbs. per square inch should be made for
the resistance of this vapour, and 1lb. per square inch for friction,
&c.; so that the effective force will be found by subtracting these
4lbs. per square inch from the atmospheric pressure; which, if
estimated at 15lbs., leaves an effective working power of about 11lbs.
per square inch. This, however, is rather above what is commonly
obtained.
Another advantage which this engine has over those of Savery, is the
facility with which it might be applied to drive machinery by means of
the working beam.
The merit of this engine as an invention, must be ascribed principally
to its mechanism and combinations. We find in it no new principle; the
agency of atmospheric pressure acting against a vacuum, or a partial
vacuum, was long known. The formation of a vacuum by the condensation
of steam had been suggested by Papin and Savery, and carried into
practical effect by the latter. The mechanical power derivable from
the direct pressure of the elastic force of steam was distinctly
pointed out by Lord Worcester, and even prior to his time; the boiler,
gauge-pipes, and regulator of the atmospheric engine, were evidently
borrowed from Savery's engine. The idea of working a piston in a
cylinder by the atmospheric pressure against a vacuum below, was
suggested by Otto Guericke, an ingenious German philosopher, the
inventor of the air-pump, and subsequently by Papin; and the use of a
working beam could not have been unknown. Nevertheless, considerable
credit must be acknowledged to be due to Newcomen for the judicious
combination of those scattered principles. "The mechanism contrived by
him," says Tredgold, "produces all the difference between an efficient
and inefficient engine, and should be more highly valued than the
fortuitous discovery of a new principle." The rapid condensation of
steam by the injection of water, the method of clearing the cylinder
of air and water after the stroke, are two contrivances not before in
use, and which are quite essential to the effective operation of the
engine: these are wholly due to Newcomen and his associates.
(44.) The patent of Newcomen was granted in 1705; and in 1707, Papin
published a work, entitled "A New Method of raising Water by Fire," in
which a steam engine is described, which would scarcely merit notice
here but for the contests which have arisen upon the claims of
different nations for a share in the invention of the steam engine.
The publication of this work of Papin was nine years after Savery's
patent, with which he acknowledges himself acquainted, and two years
after Newcomen's. The following is a description of Papin's steam
engine:--
An oval boiler, A (fig. 11.), is filled to about two thirds of its
entire capacity with water, through a valve B in the top, which opens
upwards, and is kept down by a lever carrying a sliding weight. The
pressure on the valve is regulated by moving the weight to or from B,
like the common steelyard. This boiler communicates with a cylinder,
C, by a syphon tube furnished with a stopcock at D. The cylinder C has
a valve F in the top, closed by a lever and weight similar to B, and a
tube with a stopcock G opening into the atmosphere. In this cylinder
is placed a hollow copper piston H, which moves freely in it, and
floats upon the water. Another tube forms a communication between the
bottom of this cylinder and the bottom of a close cylindrical vessel
I, called the _air-vessel_. In this tube is a valve at K, opening
_upwards_; also a pipe terminated in a funnel, and furnished with a
valve L, which opens _downwards_. From the lower part of the
air-vessel a tube proceeds, furnished also with a stopcock M, which is
continued to whatever height the water is to be raised.
Water being poured into the funnel, passes through the valve L, which
opens downwards; and filling the tube, ascends into the cylinder C,
carrying the floating piston H on its surface, and maintains the same
level in C which it has in the funnel. In this manner the cylinder C
may be filled to the level of the top of the funnel. In this process
the cock G should be left open, to allow the air in the cylinder to
escape as the water rises.
Let us now suppose that, a fire being placed beneath the boiler, steam
is being produced. On opening the cock D, and closing G, the steam,
flowing through the syphon tube into the top of the cylinder, presses
down the floating piston, and forces the water into the lower tube.
The passage at L being stopped, since L opens _downwards_, the water
forces open the valve K, and passes into the air-vessel I. When the
piston H has been forced to the bottom of the cylinder, the cock D is
closed, and G is opened, and the steam allowed to escape into the
atmosphere. The cylinder is then replenished from the funnel as
before; and the cock G being closed, and D opened, the process is
repeated, and more water forced into the air-vessel I.
By continuing this process, water is forced into the air-vessel, and
the air which originally filled that vessel is compressed into the
space above the water; and its elastic force increases exactly in the
same proportion as its bulk is diminished. (6.) Now, suppose that half
of the vessel I has been filled by the water which is forced in, the
air above the water being reduced to half its bulk has acquired twice
the elastic force, and therefore presses on the surface of the water
with twice the pressure of the atmosphere. Again, if two thirds of the
air-vessel be filled with water, the air is compressed into one third
of its bulk, and presses on the surface of the water with three times
the pressure of the atmosphere, and so on.
[Illustration: Pl. III.]
Now if the cock M be opened, the pressure of the condensed air will
force the water up in the tube N, and it will continue to rise until
the column balances the pressure of the condensed air. If, when the
water is suspended in the tube, and the cock M open, the vessel I is
half filled, the height of the column in N will be 34 feet, because 34
feet of water has a pressure equal to the atmosphere; and this, added
to the atmospheric pressure on it, gives a total pressure equal to
twice that of the atmosphere, which balances the pressure of the air
in I reduced to half its bulk. If two thirds of I be filled with
water, a column of 68 feet will be supported in N; for such a column,
united with the atmospheric pressure on it, gives a total pressure
equal to three times that of the atmosphere, which balances the air in
I compressed into one third of its original bulk.
By omitting the principle of condensation, this machine loses 26 feet
in the perpendicular lift. But, indeed, in every point of view, it is
inferior to the engines of Savery and Newcomen.
(45.) From the construction of the atmospheric engine by Newcomen, in
1705, for about half a century, no very important step had been made
in the improvement of the steam engine. During this time the
celebrated Smeaton had given much attention to the details of the
atmospheric engine, and brought that machine to as high a state of
perfection as its principle seemed to admit, and as it has ever since
reached.
In the year 1763, JAMES WATT, a name illustrious in the history of
mechanical science, commenced his experiments on steam. He was born at
Greenock, in the year 1736; and at the age of 16 was apprenticed to a
mathematical instrument-maker, with whom he spent four years. At the
age of 20 he removed to London, where he still pursued the same trade
under a mathematical instrument-maker in that city. After a short
time, however, finding his health declining, he returned to Scotland,
and commenced business on his own account at Glasgow. In 1757 he was
appointed mathematical instrument-maker to the university of Glasgow,
where he resided and carried on business.
This circumstance produced an acquaintance between him and the
celebrated Dr. Robison, then a student in Glasgow, who directed Watt's
attention to the steam engine. In his first experiments he used steam
of a high-pressure; but found it attended with so much danger of
bursting the boiler, and difficulty of keeping the joints tight, and
other objections, that he relinquished the inquiry at that time.
(46.) In the winter of 1763, Watt was employed to repair the model of
an atmospheric engine, belonging to the natural philosophy class in
the university--a circumstance which again turned his attention to the
subject of the steam engine. He found the consumption of steam in
working this model so great, that he inferred that the quantity
wasted, must have had a very large proportion to that used in working
the piston. His first conclusion was, that the material of the
cylinder (brass) was too good a conductor of heat, and that much was
thereby lost. He made some experiments, accordingly, with wooden
cylinders, soaked in linseed oil, which, however, he soon laid aside.
Further consideration convinced him that a prodigious waste of steam
was essential to the very principle of the atmospheric engine. This
will be easily understood.
When the steam has filled the cylinder so as to balance the
atmospheric pressure on the piston, the cylinder must have the same
temperature as the steam itself. Now, on introducing the condensing
jet, the steam mixed with this water forms a mass of hot water in the
bottom of the cylinder. This water, not being under the atmospheric
pressure, boils at very low temperatures, and produces a vapour which
resists the descent of the piston.
The heat of the cylinder itself assists this process; so that in order
to produce a tolerably perfect vacuum, it was found necessary to
introduce a quantity of condensing water, sufficient to reduce the
temperature of the water in the cylinder lower than 100°, and
consequently to cool the cylinder itself to that temperature. Under
these circumstances, the descent of the piston was found to suffer
very little resistance from any vapour within the cylinder: but then
on the subsequent ascent, an immense waste of steam ensued; for the
steam, on being admitted under the piston, was immediately condensed
by the cold cylinder and water of condensation, and this continued
until the cylinder became again heated up to 212°, to which point the
whole cylinder should be heated before the ascent could be completed.
Here, then, was an obvious and an extensive cause of the waste of
heat. At every descent of the piston, the cylinder should be cooled
below 100°; and at every ascent it should be again heated to 212°. It,
therefore, became a question whether the force gained by the increased
perfection of the vacuum was adequate to the waste of fuel in
producing the vacuum; and it was found, on the whole, more profitable
not to cool the cylinder to so low a temperature, and consequently to
work with a very imperfect vacuum, and a diminished power.
Watt, therefore, found the engine involved in this dilemma: either
much or little condensation-water must be used. If much were used, the
vacuum would be perfect; but then the cylinder would be cooled, and
would entail an extensive waste of fuel in heating it. If little were
used, a vapour would remain, which would resist the descent of the
piston, and rob the atmosphere of a part of its power. The great
problem then pressed itself on his attention, _to condense the steam
without cooling the cylinder_.
From the small quantity of water in the form of steam which filled the
cylinder, and the large quantity of injected water to which this
communicated heat, Watt was led to inquire what proportion the bulk of
water in the liquid state bore to its bulk in the vaporous state; and
also what proportion subsisted between the heat which it contained in
these two states. He found by experiment that a cubic inch of water
formed about a cubic foot of steam; and that the cubic foot of steam
contained as much heat as would raise a cubic inch of water to about
1000°. (15.) This gave him some surprise, as the thermometer indicated
the same temperature, 212°, for both the steam and the water from
which it was raised. What then became of all the additional heat which
was contained in the steam, and not indicated by the thermometer? Watt
concluded that this heat must be in some way engaged in maintaining
the water in its new form.
Struck with the singularity of this circumstance, he communicated it
to Dr. Black, who then explained to Watt his doctrine of _latent
heat_, which he had been teaching for a short time before that, but of
which Watt had not previously heard; and thus, says Watt, "I stumbled
upon one of the material facts on which that theory is founded."
(47.) Watt now gave his whole mind to the discovery of a method of
"condensing the steam without cooling the cylinder." The idea occurred
to him of providing a vessel separate from the cylinder, in which a
constant vacuum might be sustained. If a communication could be opened
between the cylinder and this vessel, the steam, by its expansive
property, would rush from the cylinder to this vessel, where, being
exposed to cold, it would be immediately condensed, the cylinder
meanwhile being sustained at the temperature of 212°.
This happy conception formed the first step of that brilliant career
which has immortalized the name of Watt, and which has spread his fame
to the very skirts of civilization. He states, that the moment the
notion of "separate condensation" struck him, all the other details of
his improved engine followed in rapid and immediate succession, so
that in the course of a day his invention was so complete that he
proceeded to submit it to experiment.
His first notion was, as we have stated, to provide a separate vessel,
called a _condenser_, having a pipe or tube communicating with the
cylinder. This condenser he proposed to keep cold by being immersed in
a cistern of cold water, and by providing a jet of cold water to play
within it. When the communication with the cylinder is opened, the
steam, rushing into the condenser, is immediately condensed by the jet
and the cold surface. But here a difficulty presented itself, viz. how
to dispose of the condensing water, and condensed steam, which would
collect in the bottom of the condenser. But besides this, a quantity
of air or permanent uncondensible gas would collect from various
sources. Water in its ordinary state always holds more or less air in
combination with it: the air thus combined with the water in the
boiler passes through the tubes and cylinder with the steam, and would
collect in the condenser. Air also would enter in combination with the
condensing water, which would be set free by the heat it would receive
from admixture with the steam. The air proceeding from these sources
would, as Watt foresaw, accumulate in the condenser, even though the
water might be withdrawn from it, and would at length resist the
descent of the piston. To remedy this he proposed _to form a
communication between the bottom of the condenser and a pump which he
called the_ AIR PUMP, _so that the water and air which might be
collected in the condenser would be drawn off_; and it was easy to see
how this pump could be worked by the machine itself. This constituted
the second great step in the invention.
To make it air-tight in the cylinder, it had been found necessary to
keep a quantity of water supplied above the piston. In the present
case, any of this water which might escape through the piston, or
between it and the cylinder, would boil, the cylinder being kept at
212°; and would thus, by the steam it would produce, vitiate the
vacuum. To avoid this inconvenience, Watt proposed to lubricate the
piston, and keep it air-tight, by employing melted wax and tallow.
Another inconvenience was still to be removed. On the descent of the
piston, the air which must then enter the cylinder would lower its
temperature; so that upon the next ascent, some of the steam which
would enter it would be condensed, and hence would arise a source of
waste. To remove this difficulty, Watt proposed to close the top of
the cylinder altogether by an air-tight and steam-tight cover,
allowing the piston-rod to play through a hole furnished with a
stuffing-box, and _to press down the piston by steam instead of the
atmosphere_.
This was the third step in this great invention, and one which
totally changed the character of the machine. It now became really a
_steam engine_ in every sense; for the pressure above the piston was
the elastic force of steam, and the vacuum below it was produced by
the condensation of steam; so that steam was used both directly and
indirectly as a moving power; whereas, in the atmospheric engine, the
indirect force of steam only was used, being adopted merely as an easy
method of producing a vacuum.
The last difficulty respecting the economy of heat which remained to
be removed, was the circumstance of the cylinder being liable to be
cooled on the external surface by the atmosphere. To obviate this, he
first proposed casing the cylinder in wood, that being a substance
which conducted heat slowly. He subsequently, however, adopted a
different method, and inclosed one cylinder within another, leaving a
space between them, which he kept constantly supplied with steam. Thus
the inner cylinder was kept continually at the temperature of the
steam which surrounded it. The outer cylinder was called the
_jacket_.[15]
[Footnote 15: It is a remarkable circumstance, that Watt used the same
means for keeping the cylinder hot as Newcomen used in his earlier
engines to cool it. (38.)]
(48.) Watt computed that in the atmospheric engine three times as much
heat was wasted in heating the cylinder, &c. as was spent in useful
effect. And, as by the improvements proposed by him nearly all this
waste was removed, he contemplated, and afterwards actually effected,
a saving of three fourths of the fuel.
The honour due to Watt for his discoveries is enhanced by the
difficulties under which he laboured from contracted circumstances at
the time he made them. He relates, that when he was endeavouring to
determine the heat consumed in the production of steam, his means did
not permit him to use an efficient and proper apparatus, which would
have been attended with expense; and it was by experiments made with
apothecaries' phials, that he discovered the property already
mentioned, which was one of the facts on which the doctrine of latent
heat was founded.
A large share of the merit of Watt's discoveries has, by some writers,
been attributed to Dr. Black, to whose instructions on the subject of
latent heat it is said that Watt owed the knowledge of those facts
which led to his improvements. Such, however, was not the case; and
the mistake arose chiefly from some passages respecting Watt in the
works of Dr. Robison, in one of which he states that Watt had been a
_pupil_ and intimate friend of Dr. Black; and that he attended two
courses of his lectures at college in Glasgow. Such, however, was not
the case; for "Unfortunately for me," says Watt in a letter to Dr.
Brewster, "the necessary avocations of my business prevented me from
attending his or any other lectures at college. In further noticing
Dr. Black's opinion, that his fortunate observation of what happens in
the formation and condensation of elastic vapour 'has contributed in
no inconsiderable degree to the public good, by suggesting to my
friend Mr. Watt of Birmingham, then of Glasgow, his improvements on
the steam-engine,' it is very painful for me to controvert any opinion
or assertion of my revered friend; yet, in the present case, I find it
necessary to say, that he appears to me to have fallen into an error.
These improvements proceeded upon the established fact, that steam was
condensed by the contact of cold bodies, and the later known one, that
water boiled at heats below 100°, and consequently that a vacuum could
not be obtained unless the cylinder and its contents were cooled every
stroke below the heat."
CHAPTER V.
WATT'S SINGLE-ACTING STEAM ENGINE.
Expansive Principle applied. -- Failure of Roebuck, and
Partnership with Bolton. -- Patent extended to 1800. -- Counter.
-- Difficulties in getting the Engines into use.
(49.) The first machine in which Watt realised the conceptions which
we mentioned in the last chapter, is that which was afterwards called
his _Single-acting Steam Engine_. We shall now describe the working
apparatus in this machine.
The cylinder is represented at C (fig. 12.)--in which the piston P
moves steam-tight. It is closed at the top, and the piston-rod being
very accurately turned, runs in a steam-tight collar B furnished with
a stuffing-box, and constantly supplied with melted tallow or wax.
Through a funnel in the top of the cylinder, melted grease flows upon
the piston so as to maintain it steam-tight. Two boxes A A, containing
the valves for admitting and withdrawing the steam, connected by a
tube of communication T, are attached to the cylinder; the action of
these valves will be presently described. Below the cylinder, placed
in a cistern of cold water, is a close cylindrical vessel D, called
the condenser, communicating with the cylinder by a tube T´, leading
to the lower valve-box A. In the side of this condenser is inserted a
tube, the inner end of which is pierced with holes like the rose of a
watering-pot; and a cock E in the cold cistern is placed on the
outside, through which, when open, the water passing, rises in a jet
on the inside.
The tube S, which conducts steam from the boiler, enters the top of
the upper valve-box at F. Immediately under it is placed a valve G,
which is opened and closed by a lever or rod G´. This valve, when
open, admits steam to the top of the piston, and also to the tube T,
which communicates between the two valve-boxes, and when closed
suspends the admission of steam. There are two valves in the lower
box, one H in the top worked by the lever H´, and one I in the bottom
worked by the lever I´. The valve H, when open, admits steam to pass
from the cylinder _above_ the piston, by the tube T, to the cylinder
_below_ the piston, the valve I being supposed in this case to be
closed. This valve I, when open, (the valve H being closed,) admits
steam to pass from below the cylinder through T´ to the condenser.
This steam, entering the condenser, meets the jet, admitted to play by
the valve E, and is condensed.
The valve G is called the _upper steam valve_; H, _lower steam valve_;
I, the _exhausting-valve_; and E, the _condensing valve_. Let us now
consider how these valves must be worked in order to produce the
alternate ascent and descent of the piston.
It is in the first place necessary that all the air which fills the
cylinder, tubes, and condenser should be expelled. To accomplish this
it is only necessary to open at once the valves G, H, and I. The steam
then rushing from F through the valve G will pass into the upper part
of the cylinder, and through the tube T and the valve H into the lower
part, and also through the valve I into the condenser. After the steam
ceases to be condensed by the cold of the apparatus, it will rush out
mixed with air through the valve M, which opens outward; and this will
continue until all the air has been expelled, and the apparatus filled
with pure steam. Then suppose all the valves again closed. The
cylinder both above and below the piston is filled with steam; and the
steam which filled the condenser being cooled by the cold surface, a
vacuum has been produced in that vessel.
The apparatus being in this state, let the upper steam valve G, the
exhausting-valve I, and the condensing valve E be opened. Steam will
thus be admitted through G to press on the top of the piston; and this
steam will be prevented from circulating to the lower part of the
cylinder by the lower steam-valve H being closed. Also the steam which
filled the cylinder below the piston rushes through the open
exhausting-valve I to the condenser, where it meets the jet allowed to
play by the open condensing valve E. It is thus instantly condensed,
and a vacuum is left in the cylinder below the piston. Into this
vacuum the piston is pressed without resistance by the steam which is
admitted through G. When the piston has thus been forced to the bottom
of the cylinder, let the three valves G, I, and E, which were before
opened, be closed, and let the lower steam-valve H be opened. The
effects of this change are easily perceived. By closing the upper
steam-valve G, the further admission of steam to the apparatus is
stopped. By closing the exhausting-valve I, all transmission of steam
from the cylinder to the condenser is stopped. Thus the steam which is
_in_ the cylinder, valve-boxes, and tubes is shut up in them, and no
more admitted, nor any allowed to escape. By closing the condensing
valve E, the play of the jet in the condenser is suspended.
Previously to opening the valve H, the steam contained in the
apparatus was confined to the part of the cylinder _above_ the piston
and the tube T and the valve-box A. But on opening this valve, the
steam is allowed to circulate above and below the piston; and in fact
through every part included between the upper steam valve G, and the
exhausting-valve I. The same steam circulating on both sides, the
piston is thus equally pressed upward and downward.
In this case there is no force tending to retain the piston at the
bottom of the cylinder except its own weight. Its ascent is produced
in the same manner as the ascent of the piston in the atmospheric
engine. The piston-rod is connected by chains G to the arch-head of
the beam, and the weight of the pump-rod R, or any other counterpoise
acting on the chains suspended from the other arch-head, draws the
piston to the top of the cylinder.
When the piston has arrived at the top of the cylinder, suppose the
three valves G, I, and E to be again opened, and H closed. Steam
passes from the steam-pipe F through the upper steam-valve G to the
top of the piston, and at the same time the steam which filled the
cylinder below the piston is drawn off through the open
exhausting-valve I into the condenser, where it is condensed by the
jet allowed to play by the open condensing valve E. The pressure of
the steam above the piston then forces it without resistance into the
vacuum below it, and so the process is continued.
It should be remembered, that of the four valves necessary to work the
piston, three are to be opened the moment the piston reaches the top
of the cylinder, and the fourth is to be closed; and on the piston
arriving at the bottom of the cylinder, these three are to be closed
and the fourth opened. The three valves which are thus opened and
closed together are the upper steam-valve, the exhausting-valve, and
the condensing valve. The lower steam-valve is to be opened at the
same instant that these are closed, and _vice versâ_. The manner of
working these valves we shall describe hereafter.
The process which has just been described, if continued for any
considerable number of reciprocations of the piston, would be attended
with two very obvious effects which would obstruct and finally destroy
the action of the machine. First, the condensing water and condensed
steam would collect in the condenser D, and fill it; and secondly, the
water in the cistern in which the condenser is placed would gradually
become heated, until at last it would not be cold enough to condense
the steam when introduced in the jet. Besides this, it will be
recollected that water boils in a vacuum at a very low temperature
(17); and, therefore, the hot water collected in the bottom of the
condenser would produce steam which, rising into the cylinder through
the exhausting-valve, would resist the descent of the piston, and
counteract the effects of the steam above it. A further disadvantage
arises from the air or other permanently elastic fluid which enters in
combination with the water, both in the boiler and condensing jet, and
which is disengaged by its own elasticity.
To remove these difficulties, a pump is placed near the condenser
communicating with it by a valve M, which opens from the condenser
into the pump. In this pump is placed a piston which moves air-tight,
and in which there is a valve N, which opens upwards. Now suppose the
piston at the bottom of the pump. As it rises, since the valve in it
opens _upwards_, no air can pass _down_ through it, and consequently
it leaves a vacuum _below_ it. The water and any air which may be
collected in the condenser open the valve M, and pass into the lower
part of the pump from which they cannot return in consequence of the
valve M opening _outwards_. On the descent of the pump-piston, the
fluids which occupy the lower part of the pump, force open the
piston-valve N; and passing through it, get _above_ the piston, from
which their return is prevented by the valve N. In the next ascent,
the piston lifts these fluids to the top of the pump, whence they are
discharged through a conduit into a small cistern B by a valve K which
opens outwards. The water which is thus collected in B is heated by
the condensed steam, and is reserved in B, which is called the hot
well for feeding the boiler, which is effected by means which we shall
presently explain. The pump which draws off the hot water and air from
the condenser is called the _air-pump_.
(50.) We have not yet explained the manner in which the valves and the
air-pump piston are worked. The rod Q of the latter is connected with
the working beam, and the pump is therefore wrought by the engine
itself. It is not very material to which arm of the beam it is
attached. If it be on the same side of the centre of the beam with the
cylinder, it rises and falls with the steam-piston; but if it be on
the opposite side, the pump-piston rises when the steam-piston falls,
and _vice versâ_. In the single-engine there are some advantages in
the latter arrangement. As the steam-piston _descends_, the steam
rushes into the condenser, and the jet is playing; and this,
therefore, is the most favourable time for drawing out the water and
condensed steam from the condenser by the ascent of the pump-piston,
since by this means the descent of the steam-piston is assisted; an
effect which would not be produced if the steam-piston and pump-piston
descended together.
With respect to the method of opening and closing the valves, it is
evident that the three valves which are simultaneously opened and
closed may be so connected as to be worked by the same lever. This
lever may be struck by a pin fixed upon the rod Q of the air-pump, so
that when the pistons have arrived at the top of the cylinders the pin
strikes the lever and opens the three valves. A catch or detent is
provided for keeping them open during the descent of the piston, from
which they are disengaged in a similar manner on the arrival of the
piston at the bottom of the cylinder, and they close by their own
weight.
In exactly the same way the lower steam-valve is opened on the arrival
of the piston at the bottom of the cylinder, and closed on its arrival
at the top by the action of a pin placed on the piston-rod of the
air-pump.
(51.) Soon after the invention of these engines, Watt found that in
some instances inconvenience arose from the too rapid motion of the
steam-piston at the end of its stroke, owing to its being moved with
an _accelerated motion_. This was owing to the uniform action of the
steam-pressure upon it: for upon first putting it in motion at the top
of the cylinder, the motion was comparatively slow; but from the
continuance of the same pressure the velocity with which the piston
descended was continually increasing, until it reached the bottom of
the cylinder, where it acquired its greatest velocity. To prevent
this, and to render the descent as nearly as possible uniform, it was
proposed to cut off the steam before the descent was completed, so
that the remainder might be effected merely by the expansion of the
steam which was admitted to the cylinder. To accomplish this, he
contrived, by means of a pin on the rod of the air-pump, to close the
upper steam-valve when the steam-piston had completed one-third of its
entire descent, and to keep it closed during the remainder of the
descent, and until the piston again reached the top of the cylinder.
By this arrangement the steam pressed the piston with its full force
through one-third of the descent, and thus put it into motion; during
the other two thirds the steam thus admitted acted merely by its
expansive force, which became less in exactly the same proportion as
the space given to it by the descent of the piston increased. Thus,
during the last two thirds of the descent the piston is urged by a
gradually decreasing force, which in practice was found just
sufficient to sustain in the piston a uniform velocity.
(52.) We have already mentioned the difficulty arising from the water
in the cistern, in which the condenser and air-pump are placed,
becoming heated, and the condensation therefore being imperfect. To
prevent this, a waste-pipe is placed in this cistern, from which the
water is continually discharged, and a pump L (called the
_cold-water-pump_) is worked by the engine itself, which raises a
supply of cold water and sends it through a pipe in a constant stream
into the cold cistern. The waste-pipe, through which the water flows
from the cistern, is placed near the top of it, since the heated
water, being lighter than the cold, remains on the top. Thus the
heated water is continually flowing off, and a constant stream of cold
water supplied. The piston-rod of the cold-water-pump is attached to
the beam (by which it is worked), usually on the opposite side from
the cylinder.
Another pump O (called the _hot-water-pump_) enters the _hot well_ B;
and raising the water from it, forces it through a tube to the boiler
for the purpose of feeding it. The manner in which this is effected
will be more particularly described hereafter. A part of the heat
which would otherwise be lost, is thus restored to the boiler to
assist in the production of fresh steam. We may consider a portion of
the heat to be in this manner _circulating_ continually through the
machine. It proceeds from the boiler in steam, works the piston,
passes into the condenser, and is reconverted into hot water; thence
it is passed to the hot well, from whence it is pumped back into the
boiler, and is again converted into steam, and so proceeds in constant
circulation.
From what has been described, it appears that there are four pistons
attached to the great beam and worked by the piston of the
steam-cylinder. On the same side of the centre with the cylinder is
the piston-rod of the air-pump, and on the opposite side are the
piston-rods of the hot-water pump and the cold-water-pump; and lastly,
at the extremity of the beam opposite to that at which the
steam-piston works, is the piston of the pump to be wrought by the
engine.
(53.) The position of these piston-rods with respect to the centre of
the beam depends on the play necessary to be given to the piston. If
the play of the piston be short, its rod will be attached to the beam
near the centre; and if longer, more remote from the centre. The
cylinder of the air-pump is commonly half the length of the
steam-cylinder, and its piston-rod is attached to the beam at the
point exactly in the middle between the end of the beam and the
centre. The hot-water pump not being required to raise a considerable
quantity of water, its piston requires but little play, and is
therefore placed near the centre of the beam, the piston-rod of the
cold-water pump being farther from the centre.
(54.) It appears to have been about the year 1763, that Watt made
these improvements in the steam engine, and constructed a model which
fully realized his expectations. Either from want of influence or the
fear of prejudice and opposition, he did not make known his discovery
or attempt to secure it by a patent at that time. Having adopted the
profession of a land surveyor, his business brought him into
communication with Dr. Roebuck, at that time extensively engaged in
mining speculations, who possessed some command of capital, and was of
a very enterprising disposition. By Roebuck's assistance and
countenance, Watt erected an engine of the new construction at a coal
mine on the estate of the Duke of Hamilton, at Kinneil near
Burrowstoness. This engine being a kind of experimental one, was
improved from time to time as circumstances suggested, until it
reached considerable perfection. While it was being erected, Watt in
conjunction with Roebuck applied for and obtained a patent to secure
the property in the invention. This patent was enrolled in 1769, six
years after Watt invented the improved engine.
Watt was now preparing to manufacture the new engines on an extensive
scale, when his partner Roebuck suffered a considerable loss by the
failure of a mining speculation in which he had engaged, and became
involved in embarrassments, so as to be unable to make the pecuniary
advances necessary to carry Watt's designs into execution. Again
disappointed, and harassed by the difficulties which he had to
encounter, Watt was about to relinquish the further prosecution of his
plans, when Mr. Matthew Bolton, a gentleman who had established a
factory at Birmingham a short time before, made proposals to purchase
Dr. Roebuck's share in the patent, in which he succeeded; and in 1773,
Watt entered into partnership with Bolton.
His situation was now completely changed. Bolton was not only a man of
extensive capital, but also of considerable personal influence, and
had a disposition which led him, from taste, to undertakings which
were great and difficult, and which he prosecuted with the most
unremitting ardency and spirit. "Mr. Watt," says Playfair, "was
studious and reserved, keeping aloof from the world; while Mr. Bolton
was a man of address, delighting in society, active, and mixing with
people of all ranks with great freedom, and without ceremony. Had Mr.
Watt searched all Europe, he probably would not have found another
person so fitted to bring his invention before the public, in a manner
worthy of its merit and importance; and although of most opposite
habits, it fortunately so happened that no two men ever more cordially
agreed in their intercourse with each other."
The delay in the progress of the manufacture of engines occasioned by
the failure of Dr. Roebuck was such, that Watt found that the duration
of his patent would probably expire before he would even be reimbursed
the necessary expenses attending the various arrangements for the
manufacture of the engines. He therefore, with the advice and
influence of Bolton, Roebuck, and other friends, in 1775, applied to
parliament for an extension of the terms of his patent, which was
granted for 25 years from the date of his application, so that his
exclusive privilege should expire in 1800.
An engine was now erected at Soho (the name of Bolton's factory) as a
specimen for the examination of mining speculators, and the engines
were beginning to come into demand. The manner in which Watt chose to
receive remuneration from those who used his engines was as remarkable
for its ingenuity as for its fairness and liberality. He required that
one-third of the saving of coals effected by his engines, compared
with the atmospheric engines hitherto used, should be paid to him,
leaving the benefit of the other two-thirds to the public. Accurate
experiments were made to ascertain the saving of coals; and as the
amount of this saving in each engine depended on the length of time it
was worked, or rather on the number of descents of the piston, Watt
invented a very ingenious method of determining this. The vibrations
of the great working beam were made to communicate with a train of
wheelwork, in the same way as those of a pendulum communicate with the
work of a clock. Each vibration of the beam moved one tooth of a small
wheel, and the motion was communicated to a hand or index, which moved
on a kind of graduated plate like the dial plate of a clock. The
position of this hand marked the number of vibrations of the beam.
This apparatus, which was called the _counter_, was locked up and
secured by two different keys, one of which was kept by the
proprietor, and the other by Bolton and Watt, whose agents went round
periodically to examine the engines, when the counters were opened by
both parties and examined, and the number of vibrations of the beam
determined, and the value of the patent third found.[16]
[Footnote 16: The extent of the saving in fuel may be judged from
this: that for three engines erected at Chacewater mine in Cornwall,
it was agreed by the proprietors that they would compound for the
patent third at 2400_l._ per annum; so that the whole saving must have
exceeded 7200_l._ per annum.]
Notwithstanding the manifest superiority of these engines over the old
atmospheric engines; yet such were the influence of prejudice and the
dislike of what is new, that Watt found great difficulties in getting
them into general use. The comparative first cost also probably
operated against them; for it was necessary that all the parts should
be executed with great accuracy, which entailed proportionally
increased expense. In many instances they felt themselves obliged to
induce the proprietors of the old atmospheric engines to replace them
by the new ones, by allowing them in exchange an exorbitant price for
the old engines; and in some cases they were induced to erect engines
at their own expense, upon an agreement that they should only be paid
if the engines were found to fulfil the expectations, and brought the
advantages which they promised. It appeared since, that Bolton and
Watt had actually expended a sum of nearly 50,000_l._ on these engines
before they began to receive any return. When we contemplate the
immense advantages which the commercial interests of the country have
gained by the improvements in the steam engine, we cannot but look
back with disgust at the influence of that fatal prejudice which
opposes the progress of improvement under the pretence of resisting
innovation. It would be a problem of curious calculation to determine
what would have been lost to the resources of this country, if chance
had not united the genius of such a man as Watt with the spirit,
enterprise, and capital, of such a man as Bolton! The result would
reflect little credit on those who think novelty alone a sufficient
reason for opposition.
CHAPTER VI.
DOUBLE-ACTING STEAM ENGINE.
The Single-Acting Engine unfit to impel Machinery. -- Various
contrivances to adapt it to this purpose. -- Double-Cylinder. --
Double-Acting Cylinder. -- Various mode of connecting the Piston
with the Beam. -- Rack and Sector. -- Double Chain. -- Parallel
Motion. -- Crank. -- Sun and Planet Motion. -- Fly Wheel. --
Governor.
In the atmospheric engine of Newcomen, and in the improved steam
engine of Watt, described in the last chapter, the action of the
moving power is an intermitting one. While the piston descends, the
moving power is in action, but its action is suspended during the
ascent. Thus the opposite or working end of the beam can only be
applied in cases where a lifting power is required. This action is
quite suitable to the purposes of pumping, which was the chief or only
object to which the steam engine had hitherto been applied. In a more
extended application of the machine, this intermission of the moving
power and its action taking place only in one direction would be
inadmissible. To drive the machinery generally employed in
manufactures a constant and uniform force is required; and to render
the steam engine available for this purpose, it would be necessary
that the beam should be driven by the moving power as well in its
ascent as in its descent.
When Watt first conceived the notion of extending the application of
the engine to manufactures generally, he proposed to accomplish this
double action upon the beam by placing a steam cylinder under each end
of it, so that while each piston would be ascending, and not impelled
by the steam, the other would be descending, being urged downwards by
the steam above it acting against the vacuum below. Thus, the power
acting on each during the time when its action on the other would be
suspended, a constant force would be exerted upon the beam, and the
uniformity of the motion would be produced by making both cylinders
communicate with the same boiler, so that both pistons would be driven
by steam of the same pressure. One condenser might also be used for
both cylinders, so that a similar vacuum would be produced under each.
This arrangement, however, was soon laid aside for one much more
simple and obvious. This consisted in the production of exactly the
same effect by a single cylinder in which steam was introduced
alternately above and below the piston, being at the same time
withdrawn by the condenser at the opposite side. Thus the piston being
at the top of the cylinder, steam is introduced from the boiler above
it, while the steam in the cylinder below it is drawn off by the
condenser. The piston, therefore, is pressed from above into the
vacuum below, and descends to the bottom of the cylinder. Having
arrived there, the top of the cylinder is cut off from all
communication with the boiler; and, on the other hand, a communication
is opened between it and the condenser. The steam which has pressed
the piston down is therefore drawn off by the condenser, while a
communication is opened between the boiler and the bottom of the
cylinder, so that steam is admitted below the piston: the piston, thus
pressed from below into the vacuum above, ascends, and in the same way
the alternate motion is continued. Such is the principle of what is
called the _Double-acting Steam Engine_, in contradistinction to that
described in the last chapter, in which the steam acts only above the
piston while a vacuum is produced below it.
It is evident that, in the arrangement now described, the condenser
must be in constant action: while the piston is descending the
condenser must draw off the steam below it, and while it is ascending,
it must draw off the steam above it. As steam, therefore, must be
constantly drawn into the condenser, the jet of cold water which
condenses the steam must be kept constantly playing. This jet,
therefore, will not be worked by the valve alternately opening and
closing, as in the single engine, but will be worked by a cock, the
opening of which will be adjusted according to the quantity of cold
water necessary to condense the steam. When the steam is used at a low
pressure, and, therefore, in a less compressed state, less condensing
water would be necessary than when it is used at a higher pressure and
in a more compressed state. In the one case, therefore, the condensing
cock would be less open than in the other. Again, the quantity of
condensing water must vary with the speed of the engine, because the
greater the speed of the engine, the more rapidly will the steam flow
from the cylinder into the condenser; and, as the same quantity of
steam requires the same quantity of condensing water, the supply of
the condensing water must be proportional to the speed of the engine.
In the double-acting engine, then, the jet cock is regulated by a
lever or index which moves upon a graduated arch, and which is
regulated by the engineer according to the manner in which the engine
works.
This change in the action of the steam upon the piston rendered it
necessary to make a corresponding change in the mechanism by which the
piston-rod was connected with the beam. In the single acting engine,
the piston-rod pulled the end of the beam down during the descent, and
was pulled up by it in the ascent. The connection by which this action
was transmitted between the beam and piston was, as we have seen, a
flexible chain passing from the end of the piston and playing upon the
arch-head of the beam. Now, where the mechanical action to be
transmitted is a _pull_, and not a _push_, a flexible chain, or cord,
or strap, is always sufficient; but if a _push_ or _thrust_ is
required to be transmitted, then the flexibility of the medium of
mechanical communication afforded by a chain, renders it inapplicable.
In the double-acting engine, during the descent, the piston-rod still
pulls the beam down, and so far a chain connecting the piston-rod with
the beam would be sufficient to transmit the action of the one to the
other; but in the ascent the beam no longer pulls up the piston-rod,
but is pushed up by it. A chain from the piston-rod to the arch-head,
as described in the single acting engine, would fail to transmit this
force. If such a chain were used with the double engine, where there
is no counter weight on the opposite end of the beam, the consequence
would be, that in the ascent of the piston the chain would slacken,
and the beam would still remain depressed. It is therefore necessary
that some other mechanical connection be contrived between the
piston-rod and the beam, of such a nature that in the _descent_ the
piston-rod may _pull_ the beam down, and may _push_ it up in the
_ascent_.
Watt first proposed to effect this by attaching to the end of the
piston-rod a straight rack, faced with teeth, which should work in
corresponding teeth raised on the arch-head of the beam as represented
in fig. 13. If his improved steam engines required no further
precision of operation and construction than the atmospheric engines,
this might have been sufficient; but in these engines it was
indispensably necessary that the piston-rod should be guided with a
smooth and even motion through the stuffing-box in the top of the
cylinder, otherwise any shake or irregularity would cause it to work
loose in the stuffing-box, and either to admit the air, or to let the
steam escape. In fact, it was necessary to turn these piston-rods very
accurately in the lathe, so that they may work with sufficient
precision in the cylinder. Under these circumstances, the motion of
the rack and toothed arch-head were inadmissible, since it was
impossible by such means to impart to the piston-rod that smooth and
equable motion which was requisite. Another contrivance which occurred
to Watt was, to attach to the top of the piston-rod a bar which should
extend above the beam, and to use two chains or straps, one extending
from the top of the bar to the lower end of the arch-head, and the
other from the bottom of the bar to the upper end of the arch-head. By
such means the latter strap would pull the beam down when the piston
would descend, and the former would pull the beam up when the piston
would ascend. These contrivances, however, were superseded by the
celebrated mechanism, since called the _Parallel Motion_, one of the
most ingenious mechanical combinations connected with the history of
the steam engine.
It will be observed that the object was to connect by some inflexible
means the end of the piston-rod with the extremity of the beam, and so
to contrive the mechanism, that while the end of the beam would move
alternately up and down in a circle, the end of the piston-rod
connected with the beam should move exactly up and down in a straight
line. If the end of the piston-rod were fastened upon the end of the
beam by a pivot without any other connexion, it is evident that, being
moved up and down in the arch of a circle, it would be bent to the
left and the right alternately, and would consequently either be
broken, or would work loose in the stuffing box. Instead of connecting
the end of the rod immediately with the end of the beam by a pivot,
Watt proposed to connect them by certain moveable rods so arranged
that, as the end of the beam would move up and down in the circular
arch, the rods would so accommodate themselves to that motion, that
the end connected with the piston-rod should not be disturbed from its
rectilinear course.
To accomplish this, he conceived the notion of connecting three rods
in the following manner:--A B and C D (fig. 14.), are two rods or
levers turning on fixed pivots or centres at A and C. A third rod, B
D, is connected with them by pivots placed at their extremities, B and
D, and the lengths of the rods are so adjusted that when A B and C D
are horizontal, B D shall be perpendicular or vertical, and that A B
and C D shall be of equal lengths. Now, let a pencil be imagined to be
placed at P, exactly in the middle of the rod B D: if the rod A B be
caused to move up and down like the beam of the steam engine in the
arch represented in the figure, it is clear, from the mode of their
connexion, that the rod C D will be moved up and down in the other
arch. Now Watt conceived that, under such circumstances, the pencil P
would be moved up and down in a perpendicular straight line.
However difficult the first conception of this mechanism may have
been, it is easy to perceive why the desired effect will be produced
by it. When the rod A B rises to the upper extremity of the arch, the
point B departs a little to the right; at the same time, the point D
is moved a little to the left. Now the extremities of the rod B D
being thus at the same time, carried slightly in opposite directions,
the pencil in the middle of it will ascend directly upwards; the one
extremity of the rod having a tendency to draw it as much to the right
as the other has to draw it to the left. In the same manner, when the
rod A B moves to the lower extremity of the arch, the rod C D will be
likewise moved to the lower extremity of its arch. The point B is thus
transferred a little to the right, and the point D to the left; and,
for the same reason as before, the point P in the middle will move
neither to the right nor to the left, but straight downwards.[17]
[Footnote 17: In a strict mathematical sense, the path of the point P
is a curve of a high order, but in the play which is given to it in
the application used in the steam engine, it describes only a part of
its entire locus; and this part extending equally on each side of a
point of inflection, its radius of curvature is infinite, so that, in
practice, the deviation from a straight line, when proper proportions
are observed in the rods, and too great a play not given to them, is
insignificant.]
Now Watt conceived that his object would be attained if he could
contrive to make the beam perform the part of A B in fig. 14., and to
connect with it other two rods, C D and D B, attaching the end of the
piston to the middle of the rod D B. The practical application of this
principle required some modification, but is as elegant as the notion
itself is ingenious.
The apparatus adopted for carrying it into effect is represented on
the arm which works the piston in fig. 15. The beam, moving on its
axis C, every point in its arm moves in the arc of a circle of which C
is the centre. Let B be the point which divides the arm A C into equal
parts, A B and B C; and let D E be a straight rod equal in length to C
B, and playing on the fixed centre or pivot D. The end E of this rod
is connected by a straight bar, E B, with the point B, by pivots at B
and E on which the rod B E plays freely. If the beam be supposed to
move alternately on its axis C, the point B will move up and down in a
circular arc, of which C is the centre, and at the same time the point
E will move in an equal circular arc round the point D as a centre.
According to what we have just explained, the middle point F of the
rod B E will move up and down in a straight line.
Also, let a rod, A G, equal in length to B E, be attached to the end A
of the beam by a pivot on which it moves freely, and let its extremity
G be connected with E by a rod, G E, equal in length to A B, and
playing on pivots at G and E.
By this arrangement the joint A G being always parallel to B E, the
three points C, A, and G will be in circumstances precisely similar to
the points C, B, and F, except that the system C A G will be on a
scale of double the magnitude of C B F: C A being twice C B, and A G
twice B F, it is clear, then, that whatever course the point F may
follow, the point G must follow a similar line,[18] but will move
twice as fast. But, since the point F has been already shown to move
up and down in a straight line, the point G must also move up and down
in a straight line, but of double the length.[19]
[Footnote 18: It is, in fact, the principle of the pantograph. The
points C, F, and G evidently lie in the same straight line, since
C B : C A :: B F : A G, and the latter lines are parallel. Taking
C as the common _pole_ of the _loci_ of the points F G, the _radius
vector_ of the one will always be twice the corresponding _radius
vector_ of the other; and therefore these curves are similar,
similarly placed, and parallel. Hence, by the last note, the point G
must move in a line differing imperceptibly from a right line.]
[Footnote 19: It is not necessary that the rods, forming the parallel
motion, should have the proportions which we have assigned to them.
There are various proportions which answer the purpose, and which will
be seen by reference to practical works on the steam engine.]
By this arrangement the pistons of both the steam cylinder and
air-pump are worked; the rod of the latter being attached to the point
F, and that of the former to the point G.
This beautiful contrivance, which is incontestably one of the happiest
mechanical inventions of Watt, affords an example with what facility
the mind of a mere mechanician can perceive, as it were instinctively,
a result to obtain which by strict reasoning would require a very
complicated mathematical analysis. Watt, when asked, by persons whose
admiration was justly excited by this invention, to what process of
reasoning he could trace back his discovery, replied that he was aware
of none; that the conception flashed upon his mind without previous
investigation, and so as to excite in himself surprise at the
perfection of its action; and that on looking at it for the first
time, he experienced all that pleasurable sense of novelty which
arises from the first contemplation of the results of the invention
of others.
This and the other inventions of Watt seem to have been the pure
creations of his natural genius, very little assisted by the results
of practice, and not at all by the light of education. It does not
even appear that he was a dexterous mechanic; for he never assisted in
the construction of the first models of his own inventions. His
dwelling-house was two miles from the factory, to which he never went
more than once in a week, and then did not stay half an hour.
(_a_) However beautiful and ingenious in principle the parallel motion
may be, it has recently been shown in the United States that much
simpler means are sufficient to subserve the same purpose. In the
engines constructed recently, under the direction of Mr. R. L.
Stevens, a substitute for the parallel motion has been introduced that
performs the task equally well, and is much less complex. On the head
of the piston-rod a bar is fixed, at right angles to it, and to the
longitudinal section of the engine. The ends of this bar work in
guides formed of two parallel and vertical bars of iron, by which the
upper end of the piston-rod is constrained to move in a straight line.
The cross-bar that moves in the guides is connected with the end of
the working beam by an inflexible bar, having a motion on two circular
gudgeons, one of which is in the working beam, the other in the
cross-bar. This is therefore free to accommodate itself to the changes
in the respective position of the piston-rod and working beam, and yet
transmits the power exerted by the steam upon the former whether it be
ascending or descending, to the latter and through it to the other
parts of the machine.--A. E.
(_b_) The most improved form of Watt's engine was reached by
successive additions to the old atmospheric engine of Newcomen and
Cawley. Hence, the working-beam, derived from the pump brake of that
engine, always formed a part; and the parallel motion, or some
equivalent contrivance was absolutely necessary. In many American
engines, and particularly in those used in steam boats, the working
beam is no longer used for the purpose of transmitting motion to the
machinery. This is effected by applying a bar, called the cross-head,
at right angles to the upper ends of the piston-rod. The ends of the
cross-head work in iron guides, adapted to a gallows-frame of wood. On
each side of the cylinder, connecting rods are applied, which take
hold of the cranks of the shafts of the water-wheel. Two other
connecting rods give motion to a short beam, which works the air and
supply pumps.
The working beam is also suppressed in engines which work
horizontally. The connecting rod is in them merely a jointed
prolongation of the piston-rod, extending to the crank, whose axis
lies in the same horizontal plane with and at right angles to the axis
of the cylinder.--A. E.
(55.) A perfect motion being thus obtained of conveying the alternate
motion of the piston to the working beam, the use of a counterpoise to
lift the piston was discontinued, and the beam was made to balance
itself exactly on its centre. The next end to be obtained was to adapt
the reciprocating motion of the working end of the beam to machinery.
The motion most generally useful for this purpose is one of _continued
rotation_. The object, therefore, was by the _alternate_ motion of the
end of the beam to transmit to a shaft or axis a _continued circular_
motion. In the first instance, Watt proposed effecting this by a
_crank_, connected with the working end of the beam by a metal
connector or rod.
Let K be the centre or axis, or shaft by which motion is given to the
machinery, and to which rotation is to be imparted by the beam C H. On
the axle K, suppose a lever, K I, fixed, so that when K I is turned
round the centre K, the wheel must be turned with it. Let a connector
or rod, H I, be attached to the points H and I, playing freely on
pivots or joints. As the end H is moved upwards and downwards, the
lever K I is turned round the centre K, so as to give a continued
rotatory motion to the shaft which revolves on that centre. The
different positions which the connector and lever K I assume in the
different parts of a revolution are represented in fig. 16.
(56.) This was the first method which occurred to Watt for producing a
continued rotatory motion by means of the vibrating motion of the
beam, and is the method now universally used. A workman, however, from
Mr. Watt's factory, who was aware of the construction of a model of
this, communicated the method to Mr. Washborough of Bristol, who
anticipated Watt in taking out a patent; and although it was in his
power to have disputed the patent, yet rather than be involved in
litigation, he gave up the point, and contrived another way of
producing the same effect, which he called the _sun and planet_ wheel,
and which he used until the expiration of Washborough's patent, when
the crank was resumed.
The toothed wheel B (fig. 17.) is fixed on the end of the connector,
so that it does not turn on its axis. The teeth of this wheel work in
those of another wheel, A, which is the wheel to which rotation is to
be imparted, and which is turned by the wheel B revolving round it,
urged by the rod H I, which receives its motion from the working-beam.
The wheel A is called the sun-wheel, and B the planet-wheel, from the
obvious resemblance to the motion of these bodies.
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 simple crank, for in the simple 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 higher 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. A little
reflection, however, on the nature of the motion, will render this
plainer than any description can. This advantage of giving an
increased velocity, may be obtained also by the simple 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 decidedly inferior to the crank, which
has now entirely superseded it.
(58.) Whether the simple crank or the sun and planet wheel be used,
there still remains a difficulty of a peculiar nature attending the
continuance of the rotatory motion. There are two positions in which
the engine can give no motion whatever to the crank. These are when
the end of the beam, the axle of the crank, and the pivot which joins
the connector with the crank, are in the same straight line. This will
be easily understood. Suppose the beam, connector, and crank to assume
the position represented in fig. 15. If steam urge the piston
downwards, the point H and the connector H I will be drawn directly
upwards. But it must be very evident that in the present situation of
the connector H I, and the lever I K, the force which draws the point
I in the direction I K can have no effect whatever in turning I K
round the centre K, but will merely exert a pressure on the axle or
pivots of the wheel.
Again, suppose the crank and connector to be in the position H I K
(fig. 16.), the piston being consequently at the bottom of the
cylinder. If steam now press the piston _upwards_, the pivot H and the
connector H I will be pressed _downwards_, and this pressure will urge
the crank I K in the direction I K. It is evident that such a force
cannot turn the crank round the centre K, and can be attended with no
other effect than a pressure on the axle or pivots of the wheel.
Hence in these two positions, the engine can have no effect whatever
in turning the crank. What, then, it may be asked, extricates the
machine from this mechanical dilemma in which it is placed twice in
every revolution, on arriving at those positions in which the crank
escapes the influence of the power? There is a tendency in bodies,
when once put in motion, to continue that motion until stopped by some
opposing force, and this tendency carries the crank out of those two
critical situations. The velocity which is given to it, while it is
under the influence of the impelling force of the beam, is retained in
a sufficient degree to carry it through that situation in which it is
deserted by this impelling force. Although the rotatory motion
intended to be produced by the crank is, therefore, not absolutely
destroyed by this circumstance, yet it is rendered extremely
irregular, since, in passing through the two positions already
described, where the machine loses its power over the crank, the
motion will be very slow, and, in the positions of the crank most
remote from these, where the power of the beam upon it is greatest,
the motion will be very quick. As the crank revolves from each of
those positions where the power of the machine over it is greatest, to
where that power is altogether lost, it is continually diminished, so
that, in fact, the crank is driven by a varying power, and therefore
produces a varying motion. This will be easily understood by
considering the successive positions of the crank and connector
represented in fig. 16.
This variable motion becomes particularly objectionable when the
engine is employed to drive machinery. To remove this defect, we have
recourse to the property of bodies just mentioned, viz. their tendency
to retain a motion which is communicated to them. A large metal wheel
called a _fly-wheel_ is placed upon the axis of the crank (fig. 15.),
and is turned by it. The effect of this wheel is to equalize the
motion communicated by the action of the beam on the crank, that
action being just sufficient to sustain in the fly-wheel a uniform
velocity, and the tendency of this wheel to retain the velocity it
receives, renders its rotation sufficiently uniform for all practical
purposes.
This uniformity of motion, however, will only be preserved on two
conditions; _first_, that the supply of steam from the boiler shall be
uniform; and secondly, that the machine have always the same
resistance to overcome or be _loaded_ equally. If the supply of steam
from the boiler to the cylinder be increased, the motion of the piston
will be rendered more rapid, and, therefore, the revolution of the
fly-wheel will also be more rapid, and, on the other hand, a
diminished supply of steam will retard the fly-wheel. Again, if the
resistance or load upon the engine be diminished, the supply of steam
remaining the same, the velocity will be increased, since a less
resistance is opposed to the energy of the moving power; and, on the
other hand, if the resistance or load be increased, the speed will be
diminished, since a greater resistance will be opposed to the same
moving power. To insure a uniform velocity, in whatever manner the
load or resistance may be changed, it is necessary to proportion the
supply of steam to the resistance, so that, upon the least variation
in the velocity, the supply of steam will be increased or diminished,
so as to keep the engine going at the same rate.
(59.) One of the most striking and elegant appendages of the steam
engine is the apparatus contrived by Watt for effecting this purpose.
An apparatus, called a _regulator_ or _governor_, had been long known
to mill-wrights for rendering uniform the action of the stones in
corn-mills, and was used generally in machinery. Mr. Watt contrived a
beautiful application of this apparatus for the regulation of the
steam engine. In the pipe which conducts steam from the boiler to the
cylinder he placed a thin circular plate, so that when placed with its
face presented towards the length of the pipe, it nearly stopped it,
and allowed little or no steam to pass to the cylinder, but when its
edge was placed in the direction of the pipe, it offered no resistance
whatever to the passage of the steam. This circular plate, called the
throttle valve, was made to turn on a diameter as an axis, passing
consequently through the centre of the tube, and was worked by a lever
outside the tube. According to the position given to it, it would
permit more or less steam to pass. If the valve be placed with its
edge nearly in the direction of the tube, the supply of steam is
abundant; if it be placed with its face nearly in the direction of the
tube, the supply of steam is more limited, and it appears that, by the
position given to this valve, the steam may be measured in any
quantity to the cylinder.
At first it was proposed that the engine-man should adjust this valve
with his hand; when the engine was observed to increase its speed too
much, he would check the supply of steam by partially closing the
valve; but if, on the other hand, the motion was too slow, he would
open the valve and let in a more abundant supply of steam. Watt,
however, was not content with this, and desired to make the engine
itself discharge this task with more steadiness and regularity than
any attendant could, and for this purpose he applied the governor
already alluded to.
This apparatus is represented in fig. 15.; L is a perpendicular shaft
or axle to which a wheel, M, with a groove is attached. A strap or
rope, which is rolled upon the axle of the fly-wheel, is passed round
the groove in the wheel M, in the same manner as the strap acts in a
turning lathe. By means of this strap the rotation of the fly-wheel
will produce a rotation of the wheel M and the shaft L, and the speed
of the one will always increase or diminish in the same proportion as
the speed of the other. N, N are two heavy balls of metal placed at
the ends of rods, which play on an axis fixed on the revolving shaft
at O, and extend beyond the axis to Q Q. Connected with these by
joints at Q Q are two other rods, Q R, which are attached to a broad
ring of metal, moving freely up and down the revolving shaft. This
ring is attached to a lever whose centre is S, and is connected by a
series of levers with the throttle-valve T. When the speed of the
fly-wheel is much increased, the spindle L is whirled round with
considerable rapidity, and by their natural tendency[20] the balls N N
fly from the centre. The levers which play on the axis O, by this
motion, diverge from each other, and thereby depress the joints Q Q,
and draw down the joints R, and with them the ring of metal which
slides upon the spindle. By these means the end of the lever playing
on S is depressed, and the end V raised, and the motion is transmitted
to the throttle-valve, which is thereby partially closed, and the
supply of steam to the cylinder checked. If, on the contrary, the
velocity of the fly-wheel be diminished, the balls will fall towards
the axis, and the opposite effects ensuing, the supply of steam will
be increased, and the velocity restored.
[Footnote 20: The _centrifugal force_.]
[Illustration: Pl. IV. WATT'S SINGLE ACTING STEAM ENGINE.]
The peculiar beauty of this apparatus is, that in whatever position
the balls settle themselves, the velocity with which the governor
revolves must be the same,[21] and in this, in fact, consists its
whole efficacy as a regulator. Its regulating power is limited, and it
is only small changes of velocity that it will correct. It is evident
that such a velocity as, on the one hand, would cause the balls to fly
to the extremity of their play, or, on the other, would cause them to
fall down on their rests, would not be influenced by the governor.
[Footnote 21: Strictly speaking, this is only true when the divergence
of the rods from the spindle is not very great, and, in practice, this
divergence is never sufficient to render the above assertion untrue.
This property of the conical pendulum arises from the circumstance of
the centrifugal force, in this instance, varying as the radius of the
circle in which the balls are moved; and when this is the case, as is
well known, the _periodic time_ is constant. The time of one
revolution of the balls is equal to twice the time in which either
ball, as a common pendulum, would vibrate on the centre, and as all
its vibrations, though the arcs be unequal, are equal in time,
provided those arcs be small, so also is the periodic time of the
revolving ball invariable. These observations, however, only apply
when the balls settle themselves steadily into a circular motion; for
while they are ascending they describe a spiral curve with double
curvature, and the period will vary. This takes place during the
momentary changes in the velocity of the engine.]
We have thus described the principal parts of the double-acting steam
engine. The valves and the methods of working them have been reserved
for the next chapter, as they admit of considerable variety, and will
be better treated of separately. We have also reserved the
consideration of the boiler, which is far from being the least
interesting part of the modern steam engine, for a future chapter.
CHAPTER VII.
DOUBLE-ACTING STEAM ENGINE (_continued_.)
On the Valves of the Double-Acting Steam Engine. -- Original
Valves. -- Spindle Valves. -- Sliding Valve. -- D Valve. --
Four-Way Cock.
(60.) The various improvements described in the last chapter were
secured to Watt by patent in the year 1782. The engine now acquired an
enlarged sphere of action; for its dominion over manufactures was
decided by the _fly-wheel_, _crank_, and _governor_. By means of these
appendages, its motions were regulated with the most delicate
precision; so that while it retained a power whose magnitude was
almost unlimited, that power was under as exact regulation as the
motion of a time-piece. There is no species of manufacture, therefore,
to which this machine is not applicable, from the power which spins
the finest thread, or produces the most delicate web, to that which is
necessary to elevate the most enormous weights, or overcome the most
unlimited resistances. Although it be true, that in later times the
steam engine has received many improvements, some of which are very
creditable to the invention and talents of their projectors, yet it is
undeniable that all its great and leading perfections, all those
qualities by which it has produced such wonderful effects on the
resources of these countries, by the extension of manufactures and
commerce,--those qualities by which its influence is felt and
acknowledged in every part of the civilized globe, in increasing the
happiness, in multiplying the enjoyments, and cheapening the
pleasures of life,--that these qualities are due to the predominating
powers of one man, and that man one who possessed neither the
influence of wealth, rank, nor education, to give that first impetus
which is so often necessary to carry into circulation the earlier
productions of genius.
The method of working the valves of the double-acting steam engine, is
a subject which has much exercised the ingenuity of engineers, and
many elegant contrivances have been suggested, some of which we shall
now proceed to describe. But even in this the invention of Watt has
anticipated his successors; and the contrivances suggested by him are
those which are now almost universally used.
In order perfectly to comprehend the action of the several systems of
valves which we are about to describe, it will be necessary distinctly
to remember the manner in which the steam is to be communicated to the
cylinder, and withdrawn from it. When the piston is at the top of the
cylinder, the steam below it is to be drawn off to the condenser, and
the steam from the boiler is to be admitted above it. Again, when it
has arrived at the bottom of the cylinder, the steam above is to be
drawn off to the condenser, and the steam from the boiler is to be
admitted below it.
In the earlier engines constructed by Watt, this was accomplished by
four valves, which were opened and closed in pairs. Valve boxes were
placed at the top and bottom of the cylinder, each of which
communicated by tubes both with the steam-pipe from the boiler and the
condenser. Each valve-box accordingly contained two valves, one to
admit steam from the steam-pipe to the cylinder, and the other to
allow that steam to pass into the condenser. Thus each valve-box
contained a steam valve and an exhausting valve. The valves at the top
of the cylinder are called the _upper steam valve_ and the _upper
exhausting-valve_, and those at the bottom, the _lower steam valve_
and the _lower exhausting-valve_. In fig. 15. A´ is the upper steam
valve, which, when open, admits steam above the piston; B´ is the
upper exhausting-valve, which, when open, draws off the steam from the
piston to the condenser. C´ is the lower steam valve, which admits
steam below the piston; and D´, the lower exhausting-valve, which draws
off the steam from below the piston to the condenser.
Now, suppose the piston to be at the top of the cylinder, the cylinder
below it being filled with steam, which has just pressed it up. Let
the _upper steam valve_ A´, and the _lower exhausting-valve_ D´ be
opened, and the other two valves closed. The steam which fills the
cylinder below the piston will immediately pass through the valve D´
into the condenser, and a vacuum will be produced below the piston. At
the same time, steam is admitted from the steam-pipe through the valve
A´ above the piston, and its pressure will force the piston to the
bottom of the cylinder. On the arrival of the piston at the bottom of
the cylinder, the upper steam valve A´, and lower exhausting-valve D´,
are closed; and the lower steam valve C´, and upper exhausting-valve
B´ are opened. The steam which fills the cylinder above the piston now
passes off through B´ into the condenser, and leaves a vacuum above
the piston. At the same time, steam from the boiler is admitted
through the lower steam valve C´, below the piston, so that it will
press the piston to the top of the cylinder; and so the process is
continued.
It appears, therefore, that the upper steam valve, and the lower
exhausting-valve, must be opened together, on the arrival of the
piston at the top of the cylinder. To effect this, one lever, E´, is
made to communicate by jointed rods with both these valves, and this
lever is moved by a pin placed on the piston-rod of the air-pump; and
such a position may be given to this pin as to produce the desired
effect exactly at the proper moment of time. In like manner, another
lever, F´, communicates by jointed rods with the upper exhausting
valve and lower steam valve, so as to open them and close them
together; and this lever, in like manner, is worked by a pin on the
piston-rod of the air-pump.
(61.) This method of connecting the valves, and working them, has been
superseded by another, for which Mr. MURRAY of Leeds obtained a
patent, which was, however, set aside by Messrs. Bolton and Watt, who
showed that they had previously practised it. This method is
represented in figs. 18, 19. The stems of the valves are
perpendicular, and move in steam-tight sockets in the top of the
valve-boxes. The stem of the upper steam valve A is a tube through
which the stem of the upper exhausting-valve B passes, and in which it
moves steam-tight; both these stems moving steam-tight through the top
of the valve-box. The lower steam valve C, and exhausting-valve D, are
similarly circumstanced; the stem of the former being a tube through
which the stem of the latter passes. The stems of the upper steam
valve and lower exhausting-valve are then connected by a rod, E; and
those of the upper exhausting-valve and lower steam valve by another
rod, F. These rods, therefore, are capable of moving the valves in
pairs, when elevated and depressed. The motion which works the valves
is, however, not communicated by the rod of the air-pump, but is
received from the axis of the fly-wheel. This axis works an apparatus
called an _eccentric_; the principle which regulates the motion of
this may be thus explained:--
D E (figs. 20, 21.) is a circular metallic ring, the inner surface of
which is perfectly smooth. This ring is connected with a shaft, F B,
which communicates motion to the valves by levers which are attached
to it at B. A circular metallic plate is fitted in the ring so as to
be capable of turning within it, the surfaces of the ring and plate
which are in contact being smooth and lubricated with oil or grease.
This circular plate revolves, but not on its centre. It turns on an
axis C, at some distance from its centre A; the effect of which,
evidently, is that the ring within which it is turned is moved
alternately in opposite directions, and through a space equal to twice
the distance (C A) of the axis of the circular plate from the common
centre of it and the ring. The eccentric in its two extreme positions
is represented in figs. 20, 21. The plate and ring D E are placed on
the axis of the fly-wheel, or on the axis of some other wheel which is
worked by the fly-wheel. So that the motion of continued rotation in
the fly-wheel is thus made to produce an alternate motion in a
straight line in the shaft F B. This rod is made to communicate by
levers with the rods E and F (figs. 18, 19.), which work the valves in
such a manner, that, when the eccentric is in the position fig. 20.,
one pair of valves are opened, and the other pair closed; and when it
is brought to the position fig. 21., the other pair are opened and the
former closed and so on. It is by means of such an apparatus as this
that the valves are worked almost universally at present.
The piston being supposed to be at the top of the cylinder (fig. 18.),
and the rod E raised, the valves A and D are opened, and B and C
closed. The steam enters from the steam-pipe at an aperture
immediately above the valve A, and, passing through the open valve,
enters the cylinder above the piston. At the same time, the steam
which is below the piston, and which has just pressed it up, flows
through the open valve D, and through a tube immediately under it to
the condenser. A vacuum being thus produced below the piston, and
steam pressure acting above it, it descends; and when it arrives at
the bottom of the cylinder (fig. 19.) the rod F is drawn down, and the
valves A and D fall into their seats, and at the same time the rod F
is raised, and the valves B and C are opened. Steam is now admitted
through an aperture above the valve C, and passes below the piston,
while the steam above it passes through the open valve B into a tube
immediately under it, which leads to the condenser. A vacuum being
thus produced above the piston, and steam pressure acting below it,
the piston ascends, and thus the alternate ascent and descent is
continued by the motion communicated to the rods E F from the
fly-wheel.
[Illustration: Pl. V. WATT'S DOUBLE-ACTING STEAM ENGINE.]
[Illustration: Pl. VI.]
(_c_) An improvement has been made in the United States in the mode of
working the puppet valve. It consists in placing them by pairs in two
different vertical planes instead of one. The rods then work through
four separate stuffing boxes, and the necessity of making two of them
hollow cylinders is avoided.--A. E.
(62.) There are various other contrivances for regulating the
circulation of steam through the cylinder. In figs. 22, 23. is
represented a section of a slide valve suggested by Mr. Murray of
Leeds. The steam-pipe from the boiler enters the valve-box D E at S.
Curved passages, A A, B B, communicate between this valve-box and the
top and bottom of the cylinder; and a fourth passage leads to the tube
C, which passes to the condenser. A sliding piece within the valve-box
opens a communication alternately between each end of the cylinder and
the tube C, which leads to the condenser. In the position of the
apparatus in fig. 22. steam is passing from the steam-pipes, through
the curved passage A A above the piston, and at the same time the
steam below the piston is passing through the passage B B into the
tube C, and thence to the condenser. A vacuum is thus formed below the
piston, and steam is introduced above it. The piston, therefore,
descends; and when it arrives at the bottom of the cylinder, the slide
is moved into the position represented in fig. 23. Steam now passes
from S through B B below the piston, and the steam above it passes
through A A and C to the condenser. A vacuum is thus produced above
the piston, and steam pressure is introduced below it, and the piston
ascends; and in this way the motion is continued.
The slide is moved by a lever, which is worked by the eccentric from
the fly-wheel.
(63.) Watt suggested a method of regulating the circulation of steam,
which is called the D valve, from the resemblance which the horizontal
section of the valve has to the letter D. This method, which is very
generally used, is represented in section in figs. 24, 25. Steam from
the boiler enters through S. A rod of metal connects two solid plugs,
A B, which move steam-tight in the passage D. In the position of the
apparatus represented in fig. 24. the steam passes from S through the
passage D, and enters the cylinder above the piston; while the steam
below the piston passes through the open passage by the tube C to the
condenser. A vacuum is thus formed below the piston, while the
pressure of steam is introduced above it, and it accordingly descends.
When it has arrived at the bottom of the cylinder, the plugs A B are
moved into the position in fig. 25. Steam now passing from S through
D, enters the cylinder below the piston; while the steam which is
above the piston, and has just pressed it down, passes through the
open passage into the condenser. A vacuum is thus produced above the
piston, and the steam pressure below forces it up. When it has arrived
at the top of the cylinder, the position of the plugs A B is again
changed to that represented in fig. 24., and a similar effect to that
already described is produced, and the piston is pressed down; and so
the process is continued.
The plugs A B, and the rod which connects them, are moved up and down
by proper levers, which receive their motion from the eccentric.
This contrivance is frequently modified, by conducting the steam from
above the piston to the condenser, through a tube in the plugs A B,
and their connecting rod. In figs. 26, 27. a tube passes through the
plugs A B and the rod which joins them. In the position fig. 26. steam
entering at S passes through the tube to the cylinder above the
piston, while the steam below the piston passes through C into the
condenser. A vacuum being thus made below the piston, and steam
pressing above it, it descends; and when it has arrived at the bottom
of the cylinder, the position of the plugs A B and the tube is changed
to that represented at fig. 27. The steam now entering at S passes to
the cylinder below the piston, while the steam above the piston passes
through C into the condenser. A vacuum is thus produced above the
piston, and steam pressure introduced below it, so that it ascends.
When it has arrived at the top of the cylinder, the plugs are moved
into the position represented in fig. 26., and similar effects being
produced, the piston again descends; and so the motion is continued.
The motion of the sliding tube may be produced as in the former
contrivances, by the action of the eccentric. It is also sometimes
done by a bracket fastened on the piston-rod of the air-pump. This
bracket, in the descent of the piston, strikes a projection on the
valve-rod, and drives it down; and in the ascent meets a similar
projection, and raises it.
(64.) Another method, worthy of notice for its elegance and
simplicity, is the _four-way cock_. A section of this contrivance is
given in figs. 28, 29.: C T S B are four passages or tubes; S leads
from the boiler, and introduces steam; C, opposite to it, leads to the
condenser; T is a tube which communicates with the top of the
cylinder; and B one which communicates with the bottom of the
cylinder. These four tubes communicate with a cock, which is furnished
with two curved passages, as represented in the figures; and these
passages are so formed, that, according to the position given to the
cock, they may be made to open a communication between any two
adjacent tubes of the four just mentioned. When the cock is placed as
in fig. 28. communication is opened between the steam-pipe and the top
of the cylinder by one of the curved passages, and between the
condenser and the bottom of the cylinder by the other curved passage.
In this case the steam passes from below the piston to the condenser,
leaving a vacuum under it, and steam is introduced from the boiler
above the piston. The piston therefore descends; and when it has
arrived at the bottom of the cylinder, the position of the cock is
changed to that represented in fig. 29. This change is made by turning
the cock through one fourth of an entire revolution, which may be done
by a lever moved by the eccentric, or by various other means. One of
the curved passages in the cock now opens a communication between the
steam-pipe and the bottom of the cylinder; while the other opens a
communication between the condenser and the top of the cylinder. By
these means, the steam from the boiler is introduced below the piston,
while the steam above the piston is drawn off to the condenser. A
vacuum being thus made above the piston, and steam introduced below
it, it ascends; and when it has arrived at the top of the cylinder,
the cock being moved back, it resumes the position in fig. 28., and
the same consequences ensue, the piston descends; and so the process
is continued. In figs. 30, 31. the four-way cock with the passages to
the top and bottom of the cylinder is represented on a larger scale.
This beautiful contrivance is not of late invention. It was used by
Papin, and is also described by Leupold in his _Theatrum Machinarum_,
a work published about the year 1720, in which an engine is described
acting with steam of high pressure, on a principle which we shall
describe in a subsequent chapter.
The four-way cock is liable to some practical objections. The quantity
of steam which fills the tubes between the cock and the cylinder, is
wasted every stroke. This objection, however, also applies to the
sliding valve (figs. 22, 23.), and to the sliding tube or D valves
(figs. 24, 25, 26, 27.). In fact, it is applicable to every
contrivance in which means of shutting off the steam are not placed at
both top and bottom of the cylinder. Besides this, however, the
various passages and tubes cannot be conveniently made large enough to
supply steam in sufficient abundance; and consequently it becomes
necessary to produce steam in the boiler of a more than ordinary
strength to bear the attenuation which it suffers in its passage
through so many narrow tubes.
[Illustration: Pl. VII.]
One of the greatest objections, however, to the use of the four-way
cock, particularly in large engines, is its unequal wear. The parts of
it near the passages having smaller surfaces, become more affected
by the friction, and in a short time the steam leaks between the cock
and its case, and becomes wasted, and tends to vitiate the vacuum.
These cocks are seldom used in condensing engines, except they be
small engines, but are frequently adopted in high-pressure
steam-engines; for in these the leakage is not of so much consequence,
as will appear hereafter.
CHAPTER VIII.
BOILER AND ITS APPENDAGES.--FURNACE.
The Boiler and its Appendages. -- Level Gauges. -- Feeding
Apparatus. -- Steam Gauge. -- Barometer Gauge. -- Safety Valves.
-- Self-regulating Damper. -- Edelcrantz's Valve. -- Furnace. --
Self-consuming Furnace. -- Brunton's Self-regulating Furnace. --
Oldham's Modification.
(65.) The regular action of a steam engine, as well as the economy of
fuel, depends in a great degree on the construction of the boiler or
apparatus for generating the steam. The boiler may be conceived as a
great magazine of steam for the use of the engine; and care must be
taken not only that a sufficient quantity be always ready for the
supply of the machine, but also that it shall be of the proper
quality; that is, that its pressure shall not exceed that which is
required, nor fall short of it. Precautions should, therefore, be
taken that the production of steam should be exactly proportioned to
the work to be done, and that the steam so produced shall be admitted
to the cylinder in the same proportion.
To accomplish this, various contrivances, eminently remarkable for
their ingenuity, have been resorted to, and which we shall now proceed
to describe.
(_d_) It may be premised that boilers have been made of various
figures, each having its own peculiar advantages and defects. That
which possesses the greatest degree of strength, is one of the shape
of a cylinder. This form was originally introduced by Oliver Evans, in
the United States. It will have its entire superiority when the fire
is made beneath it in a furnace of masonry. But this method is not
applicable to steam boats or locomotive engines. In these instances
the weight of the separate furnace is an objection; when, therefore,
this form is applied in them, the fire is usually made in a chamber of
cylindric shape, within the boiler, and the smoke is conveyed to the
chimney by flues, which pass through the water. These flues have, in
some cases, been reduced to the size of small tubes, and of this
method an example will be found in a subsequent part of this work.--A.
E.
(66.) Different methods have been, from time to time, suggested for
indicating the level of the water in the boiler. We have already
mentioned the two gauge-pipes used in the earlier steam engines (31.);
and which are still generally continued. There are, however, some
other methods which merit our attention.
A weight, F (fig. 32.), half immersed in the water in the boiler, is
supported by a wire, which, passing steam-tight through a small hole
in the top, is connected by a flexible string or chain passing over a
wheel, W, with a counterpoise, A, which is just sufficient to balance
F, when half immersed. If F be raised above the water, A being
lighter, will no longer balance it, and F will descend, pulling up A,
and turning the wheel W. If, on the other hand, F be plunged deeper in
the water, A, will more than balance it, and will pull it up. So that
the only position in which F and A will balance each other is when F
is half immersed. The wheel W is so adjusted, that when two pins,
placed on its rim, are in the horizontal position, as in fig. 32., the
water is at its proper level. Consequently it follows, that if the
water rise above this level, the weight F is lifted, and A falls, so
that the pins P P´ come into the position in fig. 33. If, on the other
hand, the level of the water falls, F falls and A rises, so that the
pins P P´ assume the position in fig. 34. Thus, in general, the
position of the pins P P´ becomes an indication of the quantity of
water in the boiler.
Another method is to place a glass tube (fig. 35.) with one end, T,
entering the boiler above the proper level, and the other end, T´,
entering it below the proper level. It must be evident that the water
in the tube will always stand at the same level as the water in the
boiler; since the lower part has a free communication with that water,
while the surface is submitted to the pressure of the same steam as
the water in the boiler. This, and the last-mentioned gauge, have the
advantage of addressing the eye of the engineer at once, without any
adjustment; whereas the gauge cocks must be both opened, whenever the
depth is to be ascertained.
These gauges, however, require the frequent attention of the
engine-man; and it becomes desirable either to find some more
effectual means of awakening that attention, or to render the supply
of the boiler independent of any attention. In order to enforce the
attention of the engine-man to replenish the boiler when partially
exhausted by evaporation, a tube was sometimes inserted at the lowest
level to which it was intended that the water should be permitted to
fall. This tube was conducted from the boiler into the engine-house,
where it terminated in a mouth-piece or whistle, so that whenever the
water fell below the level at which this tube was inserted in the
boiler, the steam would rush through it, and, issuing with great
velocity at the mouth-piece, would summon the engineer to his duty
with a call that would rouse him even from sleep.
(67.) In the most effectual of these methods, the task of replenishing
the boiler should still be executed by the engineer; and the utmost
that the boiler itself was made to do was to give due notice of the
necessity for the supply of water. The consequence was, among other
inconveniences, that the level of the water was subject to constant
variation.
To remedy this, a method has been invented by which the engine is made
to feed its own boiler. The pipe G´ (fig. 15.), which leads from the
hot water pump H´, terminates in a small cistern, C (fig. 36.), in
which the water is received. In the bottom of this cistern a valve, V,
is placed, which opens upwards, and communicates with a feed-pipe,
which descends into the boiler below the level of the water in it. The
stem of the valve V is connected with a lever turning on the centre D,
and loaded with a weight, F, dipped in the water in the boiler in a
manner similar to that described in fig. 32., and balanced by a
counterpoise, A, in exactly the same way. When the level of the water
in the boiler falls, the float F falls with it, and, pulling down the
arm E of the lever, raises the valve V, and lets the water descend
into the boiler from the cistern C. When the boiler has thus been
replenished, and the level raised to its former place, F will again be
raised, and the valve V closed by the weight A. In practice, however,
the valve V adjusts itself by means of the effect of the water on the
weight F, so as to permit the water from the feeding cistern C to flow
in a continued stream, just sufficient in quantity to supply the
consumption from evaporation, and to maintain the level of the water
in the boiler constantly the same.
By this singularly felicitous arrangement, the boiler is made to
replenish itself, or, more properly speaking, it is made to receive
such a supply as that it never wants replenishing, an effect which no
effort of attention on the part of an engine-man could produce. But
this is not the only good effect produced by this contrivance. A part
of the steam which originally left the boiler, and having discharged
its duty in moving the piston, was condensed and reconverted into
water, and lodged by the air-pump in the hot-well (47.), is here
again restored to the source from which it came, bringing back all the
unconsumed portion of its heat preparatory to being once more put in
circulation through the machine.
The entire quantity of hot water pumped into the cistern C is not
always required for the boiler. A waste-pipe may be provided for
carrying off the surplus, which may be turned to any purpose for which
it may be required; or it may be discharged into a cistern to cool,
preparatory to being restored to the cold cistern (fig. 12.), in case
water for the supply of that cistern be not sufficiently abundant.
In cities and places in which it becomes an object to prevent the
waste of water, the waste-pipes proceeding from the feed-cistern C
(fig. 36.) and from the cold cistern containing the condenser and
air-pump, may be conducted to a cistern A B (fig. 37.). Let C be the
pipe from the feeding cistern, and D that from the cold cistern; by
these pipes the waste water, from both these cisterns is deposited in
A B. In the bottom of A B is a valve V, opening upwards, connected
with a float F. When the quantity of water collected in the cistern A
B is such that the level rises considerably, the float F is raised,
and lifts the valve V, and the water flows into the main pipe, which
supplies water for working the engine: G is the cold water-pump for
the supply of the cold cistern.
This arrangement for saving the water discharged from the feeding and
condensing cisterns has been adopted in the printing office of the
Bank of Ireland, and a very considerable waste of water is thereby
prevented.
(68.) It is necessary to have a ready method of ascertaining at all
times the strength of the steam which is used in working the engine.
For this purpose a bent tube containing mercury is inserted into some
part of the apparatus which has free communication with the steam. It
is usually inserted in the _jacket_ of the cylinder (44.). Let A B C
(fig. 38.) be such a tube. The pressure of the steam forces the
mercury down in the leg A B, and up in the leg B C. If the mercury in
both legs be at exactly the same level, the pressure of the steam must
be exactly equal to that of the atmosphere; because the steam pressure
on the mercury in A B balances the atmospheric pressure on the mercury
in B C. If, however, the level of the mercury in B C be above the
level of the mercury in B A, the pressure of the steam will exceed
that of the atmosphere. The excess of its pressure above that of the
atmosphere may be found by observing the difference of the levels of
the mercury in the tubes B C and B A; allowing a pressure of one pound
on each square inch for every two inches in the difference of the
levels.