constructing the piston and cylinder with greater precision than
had been usual in the old engines. To fit the cover to the
cylinder so as to be steam-tight; to construct the piston rod so
as to move through it without allowing the escape of steam, and
yet at the same time without injurious friction; to connect the
piston rod with the piston, so as to drive the [Pg147] latter
through the cylinder with a perfectly straight and parallel
motion; to make such connection perfectly centrical and firm, and
yet to allow the piston in its ascent to come nearly into contact
with the cover of the cylinder—were all difficulties peculiar to
the new engine. In the atmospheric engine the shank of the piston
rod was rough and square, and the rod was secured to the piston by
two or four branches or stays, as represented in _fig._ 25. It is
evident that such a construction would be inadmissible in an
engine in which the piston in its ascent must be brought nearly
into contact with the close cover of the cylinder. Besides this
the piston rod of an atmospheric engine might throughout its whole
length have any form which was most convenient, and required no
other property than the strength necessary to work the beam. In
the new engine, on the contrary, it was necessary that it should
be accurately turned and finely polished, so as to pass through
the hole in the top of the cylinder, and be maintained in it
steam-tight. This was effected by a contrivance called a
_stuffing-box_ B, represented in _fig._ 26. A hole is made in the
cover of the cylinder very little greater in magnitude than the
diameter of the piston rod. Above this hole is a cup in which,
around the piston, is placed a stuffing of hemp or tow, which is
saturated with oil or melted tallow. This collar of hemp is
pressed down by another piece, also perforated with a hole through
which the piston rod plays, and which is screwed down on the said
collar of hemp.
(79.) Although the imperfect manner in which the interior of the
cylinders was then formed impaired the efficiency of the [Pg148]
new engines, yet such imperfections were not so injurious as in
the old atmospheric engines. Any imperfection of form of the inner
surface of the cylinder would necessarily cause more or less steam
or air to escape between the piston and cylinder. In the improved
engine this steam passing into the vacuum below the piston would
rush into the condenser, and be there condensed, so that its
effect in resisting the motion of the piston would necessarily be
trifling. But on the other hand, any escape of air between the
piston and cylinder of an atmospheric engine would introduce an
elastic fluid under the piston, which would injuriously affect the
action of the machine.
[Illustration: _Fig._ 27.]
To make the pistons move sufficiently steam-tight in these early
imperfect cylinders, Watt contrived a packing formed of a collar
of hemp, or tow, as represented in _fig._ 27. The bottom of the
piston was formed of a circular plate of a diameter nearly, but
not altogether equal to the interior diameter of the cylinder. The
part of the piston above this was considerably less in diameter,
so that the piston was surrounded by a circular groove or channel
two inches wide, into which hemp or soft rope, called _gasket_,
was run, so as to form the packing. The top of the piston was
placed over this, having a rim or projecting part, which entered
the circular groove and pressed upon the packing, the cover being
pressed downwards by screws passing through the piston. The lower
part of the groove round the piston was rounded with a curve, so
that the pressure on the packing might force the latter against
the inner surface of the cylinder. This packing was kept supplied
with melted tallow, as already described, from the funnel, screwed
into the top of the cylinder. The metallic edges of the piston
were by this means prevented from coming into contact with the
surface of the cylinder, which was only pressed upon by the
stuffing or packing projecting beyond these.
(80.) Improved methods of boring soon, however, relieved [Pg149]
the engine from a part of these imperfections, and Watt writes to
Mr. Smeaton in the letter above quoted as follows:—
"Mr. Wilkinson has improved the art of boring cylinders; so that I
promise, upon a 72 inch cylinder, being not further distant from
absolute truth than the thickness of a thin sixpence in the worst
part. I am labouring to improve the regulators; my scheme is to
make them acute conical valves, shut by a weight, and opened by
the force of the steam. They bid fair for success, and will be
tried in a few days."
The person here alluded to was Mr. John Wilkinson, of Bersham near
Chester, who, about the year 1775, contrived a new machine for
accurately boring the insides of cylinders. The cylinder being
first obtained from the foundery with a surface as accurate as the
process of casting would admit, had its inner surface reduced to
still greater accuracy by this machine, which consists of a
straight central bar extended along the axis of the cylinder,
which was made to revolve slowly round it. During the operation of
boring, the borer or cutter was fitted to slide along this bar,
which being perfectly straight, served as a sort of ruler to guide
the borer or cutter in its progress through the cylinder. In this
manner the interior surface of the cylinder was rendered not only
true and straight in its longitudinal direction, but also
perfectly circular in its cross section.
The grease found to be most eligible for lubrication was the
tallow of beef or mutton; but in the earlier cylinders this was
soon consumed by reason of the imperfection of the boring, and the
piston being left dry ceased to be steam-tight. To prevent this,
Watt sought for some substance, which while it would thicken the
tallow, and detain it around the piston, would not be subject to
decomposition by heat. Black lead dust was used for this purpose,
but was soon found to wear the cylinder. In the mean while,
however, the improved method of boring supplied cylinders which
rendered this expedient unnecessary.
When the inner surface of the cylinder is perfectly true and
smooth, the packing of the piston is soon rendered solid and hard,
being moulded to the cylinder by working, so as to fit it
perfectly. When by wear it became loose, it was [Pg150] only
necessary to tighten the screws by which the top and bottom of the
piston were held together. The packing being compressed by those
means, was forced outwards towards the surface of the cylinder, so
as to be rendered steam-tight.
(81.) It was not until about the year 1778, nine years after the
date of the patent, and thirteen after the invention of separate
condensation, that any impression was produced on the mining
interests by the advantages which were presented to them by these
vast improvements. This long interval, however, had not elapsed
without considerable advantage; for although all the great leading
principles of the contrivance were invented so early as the year
1765, yet the details of construction had been in a state of
progressive and continued improvement from the time Watt joined
Dr. Roebuck, in 1769, to the period now adverted to.
The advantages which the engine offered in the form in which it
has been just described, were numerous and important, as compared
even with the most improved form of the atmospheric engine; and it
should be remembered, that that machine had also gone on
progressively improving, and was probably indebted for some of its
ameliorations to hints derived from the labours of Watt, and to
the adoption of such of his expedients as were applicable to this
imperfect machine, and could be adopted without an infraction of
his patent.
In the most improved forms to which the atmospheric engine had
then attained, the quantity of steam wasted at each stroke of the
piston was equal to the contents of the cylinder. Such engines,
therefore, consumed twice the fuel which would be requisite, if
all sources of waste could have been removed. In Watt's engines,
the steam consumed at each stroke of the piston amounted only to
1-1/4 times the contents of the cylinder. The waste steam,
therefore, per stroke, was only a quarter of what was usefully
employed. The absolute waste, therefore, of the best atmospheric
engines was four times that of the improved engine, and
consequently the saving of fuel in the improved engines amounted
to about three eighths of all the fuel consumed in atmospheric
engines of the same power. [Pg151]
(82.) But independently of this saving of steam, which would
otherwise be wasted, the power of Watt's engine, as compared with
the atmospheric engine, was so much augmented that the former
would work against a resistance of ten pounds on the square inch
under the same circumstances in which the latter would not move
against more than seven pounds. The cause of this augmentation of
power is easily explained. In the atmospheric engine the
temperature of the condensed steam could not be reduced below 152°
without incurring a greater loss than would be compensated by the
advantage to be obtained from any higher degree of condensation.
Now steam raised from water at 152° has a pressure of nearly four
pounds per square inch. This pressure, therefore, acted below the
piston resisting the atmospheric pressure above. In Watt's engine,
however, the condenser was kept at a temperature of about 100°, at
which temperature steam has a pressure of less than one pound per
square inch. A resisting force upon the piston of three pounds per
square inch was therefore saved in Watt's engine as compared with
the atmospheric engine.
(83.) Besides these direct sources of economy, there were other
advantages incidental to Watt's engine. An atmospheric engine
possessed very limited power of adaptation to a varying load. The
moving power being the atmospheric pressure, was not under
control, and, on the other hand, was subject to variations from
day to day and from hour to hour, according to the changes of the
barometer. In the first construction of such an engine, therefore,
its power being necessarily adapted to the greatest load which it
would have to move, whenever the load upon its pumps was
diminished, the motion of the piston in descending would be
rapidly accelerated in consequence of the moving power exceeding
the resistance. By this the machinery would be subject to sudden
shocks, which were productive of rapid wear, and exposed the
machinery to the danger of fracture. To remedy this inconvenience,
the following expedient was provided in the atmospheric engine:
whenever the load on the engine was materially diminished, the
quantity of water admitted through the injection valve to condense
the steam was proportionally [Pg152] diminished. An imperfect
condensation being therefore produced, vapour remained in the
cylinder under the piston, the pressure of which resisted the
atmosphere, and mitigated the force of the machine. Besides this,
a cock was provided in the bottom of the cylinder, called an _air
cock_, by which atmospheric air could be admitted to resist the
piston whenever the motion was too rapid.
These expedients, however, were all attended with a waste of fuel
in relation to the work done by the engine; for it is evident that
the consumption of steam was necessarily the same, whether the
engine was working against its full load or against a reduced
resistance.
On the other hand, in the improved engine of Watt, when the load, to
work against which the engine exerted its full power, was
diminished, a cock or valve was provided in the steam pipe leading
from the boiler, which was called a _throttle valve_, by adjusting
which the passage in that pipe could be more or less contracted. By
regulating this cock the supply of steam from the boiler was
checked, and the quantity transmitted to the cylinder diminished, so
that its effect upon the piston might be rendered equal to the
amount of the diminished resistance. By this means the quantity of
steam transmitted to the cylinder was rendered exactly proportional
to the work which the engine had to perform. If, under such
circumstances, the boiler was worked to its full power, so as to
produce steam as fast as it would when the engine was working at
full power, then no saving of fuel would be effected, since the
surplus steam produced in the boiler would necessarily escape at the
safety valve. But in such case the fireman was directed to limit the
fuel of the furnace until the discharge at the safety valve ceased.
By these expedients, the actual consumption of fuel in one of
these improved engines was always in the exact proportion of the
work which it performed, whether it worked at full power or at any
degree under its regular power.
(84.) Notwithstanding these and other advantages attending the new
engines, Boulton and Watt experienced difficulties all but
insurmountable in getting them into use. No manufactory existed in
the country possessing machinery capable of [Pg153] executing
with the necessary precision the valves and other parts which
required exact execution, and the patentees were compelled to
construct machinery at Soho for this purpose; and even after they
succeeded in getting the cylinders properly bored, the piston rods
exactly turned and polished, the spindle valves constructed so as
to be steam-tight, and every other arrangement completed which was
necessary for the efficiency of the machine, the novelty of the
engine, and the difficulty which was supposed to attend its
maintenance in good working order, formed strong objections to its
adoption.
To remove such objections, great sacrifices were necessary on the
part of Boulton and Watt; and they accordingly resolved to
undertake the construction of the new engines without any profit,
giving them to the parties requiring their use at first cost, on
the condition of being remunerated by a small share of what they
would save in fuel.
"We have no objection," writes Mr. Boulton, "to contract with the
Carron Company to direct the making of an engine to return the
water for their mills. * * * * We do not aim at profits in engine
building, but shall take our profits out of the saving of fuel; so
that if we save nothing, we shall take nothing. Our terms are as
follows: we will make all the necessary plans, sections, and
elevations for the building, and for the engine with its
appurtenances, specifying all cast and forged iron work, and every
other particular relative to the engine. We will give all
necessary directions to your workmen, which they must implicitly
obey. We will execute, for a stipulated price, the valves, and all
other parts which may·require exact execution, at Soho; we will
see that all the parts are put together, and set to work,
properly; we will keep our own work in repair for one year, and we
have no other objection to seven years than the inconvenience of
the distance. We will guarantee that the engine so constructed
shall raise at least 20,000 cubic feet of water twenty-four feet
high with each hundred weight of coals burnt.
"When all this is done, a fair and candid comparison shall be made
between it, and your own engine, or any other engine in Scotland,
from which comparison the amount of savings in fuel shall be
estimated, and that amount being [Pg154] divided into three
parts, we shall be entitled to one of those parts, in recompense
for our patent licence, our drawings, &c. &c. Our own share of
savings shall be estimated in money, according to the value of
your coals delivered under the boiler, and you shall annually pay
us that sum, during twenty-five years from the day you begin to
work; provided you continue the use of the engine so long. And in
case you sell the engine, or remove it to any other place, you
must previously give us notice, for we shall then be entitled to
our third of the savings of fuel, according to the value of coals
at such new place. This is a necessary condition, otherwise the
engine which we make for you at an expense of two thousand pounds
may be sold in Cornwall for ten thousand pounds.
"Such parts of the engine as we execute at Soho we will be paid
for at a fair price; I conclude, from all the observations I have
had an opportunity of making, that our engines are four times
better than the common engines. In boilers, which are a very
expensive article, the savings will be in proportion to the
savings of coal. If you compare our engine with the common engine
(not in size, but in power), you will find the original expense of
erecting one to be nearly the same.
"Mr. Wilkinson has bored us several cylinders, almost without
error; that of fifty inches diameter, which we put up at Tipton,
does not err the thickness of an old shilling in any part; so that
you must either improve your method of boring, or we must furnish
the cylinder to you."
The reluctance of mining companies to relinquish the old engines,
even on these terms, led them to propose to Mr. Watt to grant them a
licence for the use of his condenser, to be applied to the
atmospheric engine, without the introduction of other improvements.
Such a proposition was made to him by Mr. Smeaton, in the year 1778,
to which he returned the following answer:—
"I have several times considered the propriety of the application
of my condensers to common engines, and have made experiments with
that view upon our engine at Soho, but have never found such
results as would induce me to try [Pg155] it any where else; and,
in consequence, we refused to make that application to Wheal
Virgin engines in Cornwall, and to some others; our reasons were,
that though it might have enabled them to have gone deeper with
their present engines, yet, the savings of fuel would not have
been great, in comparison to the complete machine. By adding
condensers to engines that were not in good order, our engine
would have been introduced into that country (which we look upon
as our richest mine) in an unfavourable point of view, and without
such profits as would have been satisfactory either to us or to
the adventurers; and if we had granted the use of condensers to
one, we must have done so to all, and thereby have curtailed our
profits, and perhaps injured our reputation. Besides, where a new
engine is to be erected, and to be equally well executed in point
of workmanship and materials, an engine of the same power cannot
be constructed materially cheaper on the old plan than on ours;
for our boiler and cylinder are much smaller, and the building,
the lever, the chains, together with all the pump and pit work,
are only the same. * * * *
"We charge our profits in proportion to the saving made in fuel by
our engine, when compared with a common one which burns the same
kind of coals; we ask one third of these savings to be paid us
annually, or half yearly; the payment being redeemable in the
option of our employer, at ten years' purchase; and when the coals
are low priced, we should also make some charge as engineers. In
all these comparisons our own interest has made us except your
(Mr. Smeaton) improved engines, unless we were allowed a greater
proportion of the savings."
Their exertions to improve the manufacture of engines at Soho is
shown by the following letter from Mr. Boulton, in the same
correspondence to Mr. Smeaton:—
"We are systematising the business of engine making, as we have
done before in the button manufactory; we are training up workmen,
and making tools and machines to form the different parts of Mr.
Watt's engines with more accuracy, and at a cheaper rate than can
possibly be done by the ordinary methods of working. Our workshop
and apparatus will be of [Pg156] sufficient extent to execute all
the engines which are likely to be soon wanted in this country;
and it will not be worth the expense for any other engineers to
erect similar works, for that would be like building a mill to
grind a bushel of corn.
"I can assure you from experience, that our small engine at Soho
is capable of raising 500,000 cubic feet of water 1 foot high with
every 112 lbs. of coals, and we are in hopes of doing much more.
Mr. Watt's engine has a very great advantage in mines, which are
continually working deeper: suppose, for instance, that a mine is
50 fathoms deep, you may have an engine which will be equal to
draining the water when the mine is worked, to 100 fathoms deep,
and yet you can constantly adapt the engine to its load, whether
it be 50 or 100 fathoms, or any intermediate depth; and the
consumption of coals will be less in proportion when working at
the lesser than at the greater depths; supposing it works, as our
engines generally do, at 11 lbs. per square inch, when the mine
becomes 100 fathoms deep."
(85.) The great improvement which has been introduced within the
last half century, in the details of Watt's steam engine, will be
rendered manifest by comparing the effects of a given weight of
fuel here supplied by Mr. Boulton with the effects which the same
weight of fuel is now known to produce in the best pumping engines
worked in Cornwall. One of these engines, in good working order,
has been known to raise 125,000,000 lbs. 1 foot high, by the
combustion of a bushel of coals. But the average performance of
even the best engines is below this amount. If we take it at
90,000,000, this will be equivalent to the weight of about 1-1/2
million cubic feet of water, a bushel of coals being 3/4 cwt. It
will therefore follow that, with the present engines, one hundred
weight of coals is capable of raising about two million cubic feet
of water one foot high, being a duty four times that assigned to
the early engines by Mr. Boulton.
(86.) At the time that Watt, in conjunction with Dr. Roebuck,
obtained the patent for his improved engine, the idea occurred to
him, that the steam which had impelled the piston in its descent
rushed from the cylinder with a mechanical force much more than
sufficient to overcome any resistance [Pg157] which it had to
encounter in its passage to the condenser; and that such force
might be rendered available as a moving power, in addition to that
already obtained from the steam during the stroke of the piston.
This notion involved the whole principle of the expansive action
of steam, which subsequently proved to be of such importance in
the performance of steam engines. Watt was, however, so much
engrossed at that time, and subsequently, by the difficulties he
had to encounter in the construction of his engines, that he did
not attempt to bring this principle into operation. It was not
until after he had organised that part of the establishment at
Soho which was appropriated to the manufacture of steam engines,
that he proceeded to apply the expansive principle. Since the date
of the patent which he took out for this (1782), was subsequent to
the application of the same principle by another engineer, named
Hornblower, it is right to state, that the claim of Mr. Watt to
this important step in the improvement of the steam engine, is
established by a letter addressed by him to Dr. Small, of
Birmingham, dated Glasgow, May, 1769:—
"I mentioned to you a method of still doubling the effect of the
steam, and that tolerably easy, by using the power of steam
rushing into a vacuum, at present lost. This would do little more
than double the effect, but it would too much enlarge the vessels
to use it all: it is peculiarly applicable to wheel engines, and
may supply the want of a condenser, where the force of steam only
is used; for open one of the steam valves, and admit steam until
one fourth of the distance between it and the next valve is filled
with steam, then shut the valve, and the steam will continue to
expand, and to press round the wheel, with a diminishing power,
ending in one fourth of its first exertion. The sum of the series
you will find greater than one half, though only one fourth of
steam was used. The power will indeed be unequal, but this can be
remedied by a fly, or by several other means."
In 1776 the engine, which had been then recently erected at Soho,
was adapted to act upon the principle of expansion. When the
piston had been pressed down in the cylinder for a certain portion
of the stroke, the further supply of steam [Pg158] from the
boiler was cut off, by closing the upper steam valve, and the
remainder of the stroke was accomplished by the expansive power of
the steam which had already been introduced into the cylinder.
(87.) To make this method of applying the force of steam
intelligible, some previous explanation of mechanical principles
will be necessary.
If a body which offers a certain resistance be urged by a certain
moving force, the motion which it will receive will depend on the
relation between the energy of the moving force and the amount of
the resistance opposed to it. If the moving force be precisely
equal to the resistance, the motion which the body will receive
will be perfectly uniform.
If the energy of the moving force be greater than the resistance,
then its surplus or excess above the amount of resistance will be
expended in imparting momentum to the mass of the body moved, and
the latter will, consequently, continually acquire augmented
speed. The motion of the body will, therefore, be in this case
accelerated.