application of any standard of engine-power: the magnitude of
furnace, and the extent of heating surface necessary to produce
any required rate of evaporation in the boiler, are unascertained;
each engine-maker has his own rule in these matters, and all the
rules are equally unsupported by any experimental test entitled to
respect. Thus the circumstances that govern the rate of
evaporation in the boiler may be regarded as almost wholly
unknown. But supposing the rate of evaporation to be ascertained,
the amount of power absorbed by the condensation of steam on its
passage to the cylinder, the imperfect condensation of the same
steam after it has worked the piston, the friction of the various
moving parts of the machinery, and, above all, the difference of
effect of these losses of power in engines constructed on
different scales of magnitude, are absolutely unknown. We are,
therefore, not placed in a condition to assign any thing more than
a general account of what has been the practice of engine-makers
in constructing engines which are nominally of a certain power.
In common low-pressure engines of the larger kind, to which class
alone we at present refer, it has been usual, with the same fuel
and under like circumstances, to allow from 10 to 18 square feet
of heating surface in the boiler for every nominal horse-power of
the engine. Within these wide limits the practice of engine-makers
has varied. It is not, however, to be supposed, that the boiler
with 18 square feet of surface per horse-power has the same
evaporating power as that which has but 10. This difference,
therefore, amounts to nothing more than different manufacturers of
steam-engines putting into circulation boilers having powers
_really_ different while they are _nominally_ the same. The
magnitude of the cylinder is regulated by the nominal power of the
engine, and it is usual so to regulate the evaporating power of
the boiler, that the piston shall move at the average rate of 200
feet per minute. This being assumed, it is customary to allow
about 22 square inches of piston [Pg293] surface for every
nominal horse-power of the engine. If this power were in
conformity to the standard already defined, this amount of surface
moved at 200 feet per minute would be impelled by a pressure
amounting to 7-1/2 lbs. per square inch. The safety-valve of the
boiler of such engines is usually loaded at from 4 to 5 lbs. per
square inch, and consequently the steam in the boiler will have a
pressure of from 19 to 20 lbs. per square inch. If, therefore, the
effective pressure on the piston be really only 7-1/2 lbs. per
square inch, the pressure expended in overcoming the friction of
the engine, and the loss consequent on the partial condensation of
steam on one side and its imperfect condensation on the other,
would amount to from 12 to 13 lbs. per square inch, or nearly
double the assumed useful effect of the engine.
Messrs. Maudslay and Field are accustomed to allow an evaporation
of ten gallons, or 1·6 cubic feet of water per hour, for each
nominal horse-power of the engine. They also allow about 22 square
inches of piston surface per nominal horse-power, the piston being
supposed to move at the rate of 200 feet per second.[24]
The quantity of grate surface necessary in proportion to the power
of the engine, has been equally unascertained, and engine-makers
vary in their practice from half a square foot to one square foot
per nominal horse-power.
The proportion which the magnitude of the heating surface of the
boiler, and the fire surface of the grate bears to the evaporating
power of the boiler, has not been determined by experiment, nor,
so far as we are informed, by any well-ascertained practical
results.
The estimates or rather conjectures of engine-makers, of the
evaporation necessary to produce one horse-power, vary from one to
two cubic feet of water per hour. It has been [Pg294] already
shown that the evaporation of 900 cubic inches, or little more
than half a cubic foot per hour, evolves a gross mechanical effect
representing one horse-power; from which it appears, that if the
evaporation of the boilers of steam engines were what engineers
suppose them to be, the gross mechanical power produced in them
for every nominal horse-power of the engine varies in actual
amount from the power of two to that of four horses.
The above estimates must be understood as referring to
double-acting steam engines above thirty-horse power. The
circumstances attending the performance of single-acting engines
applied to the drainage of mines, have been ascertained with much
greater precision. This has been mainly owing to a spirited system
of general inspection, which has been established in Cornwall, to
which we shall hereafter more particularly advert.
(175.) In expressing the duty of engines, it would have been
desirable that the duty of the boiler should have been separated
from that of the engine.
The duty of a boiler is estimated by the volume of water
evaporated by a given quantity of fuel, independently of the time
which such evaporation may take. The duty, therefore, will be
expressed by the number of cubic feet of water evaporated, divided
by the number of bushels of coal necessary for that evaporation,
supposing the bushel of coal to be the unit of fuel. It will be
observed that the _duty_ of an engine or boiler is entirely
distinct from, and independent of, its _power_. One boiler may be
greater than another in power to any extent, while it may be equal
to or less than it in duty. A bushel of coals may evaporate the
same number of cubic feet of water under two boilers, but may take
twice as great a time to produce such evaporation under one than
under the other. In such a case the power of one boiler will be
double that of the other, while their duty will be the same.
In like manner, a bushel of coals consumed in working two engines
may produce the same useful effect, but it may produce that useful
effect in the one in half the time it takes to produce it in the
other. In that case the _duty_ of the engines will be the same,
but the _power_ of the one will be double that of the other.
[Pg295]
In fine, _power_ has reference to _time_,—_duty_, to _fuel_. The
more rapidly the engine produces its mechanical effect, the
greater its power will be, whatever may be the fuel consumed in
working it. And, on the other hand, the greater the useful effect
produced by a given weight of fuel, the greater will be the duty,
however long the time may be which the fuel may take to produce
the useful effect.
(176.) The proportion of the stroke to the diameter of the
cylinder must be determined by the velocity intended to be given
to the piston. With the same capacity of cylinder, and the same
evaporation in the boiler, the velocity of the piston will augment
as the magnitude of its diameter is diminished.
The proportion of the diameter to the stroke of the cylinder is
very various. In engines used for steam-vessels the length of the
cylinder very little exceeds its diameter. In land engines,
however, the proportion of the length to the diameter is greater.
It is maintained by some that the proportion of the diameter and
length of the cylinder should be such as to render its surface
exposed to the cooling of the external air, the smallest possible.
Tredgold has maintained that since, during the stroke, the steam
is gradually exposed to contact with the surface of the cylinder
from the top to the bottom, the mean surface exposed in contact
with steam being half that of the entire cylinder, the proportion
of the diameter to the stroke should be such that the surface of
half the length of the cylinder, added to the magnitude of the top
and bottom, shall be a minimum. If this principle be admitted,
then the best proportion of the diameter to the stroke would be
that of one to two, the length of the stroke being twice the
diameter of the cylinder; but since the whole surface of the
cylinder is constantly exposed to the cooling effects of the air,
and since in the intervals of the stroke there is no sensible
change of the temperature of the surface, the loss of heat by
cooling will in effect be the same, especially in double-acting
engines, as if the cylinder were constantly filled with steam. If
this be admitted, then the object should be to give the cylinder
such a proportion, that its entire surface, including the top and
bottom, shall be a minimum. [Pg296] The proportion given by this
condition would be very nearly that which is observed in the
cylinders of marine engines, viz. that the length of the cylinder
should be equal to its diameter.
If in a low-pressure engine the pressure of steam in the cylinder
be taken at 17 lbs. per square inch, then the volume of steam will
be about fifteen hundred times that of the water which produces
it. For every cubic foot of water, therefore, in the effective
evaporation of the boiler, 1500 cubic feet of steam will be passed
through the cylinder. If it be intended that the motion of the
piston shall be at the rate of 25 strokes per minute, or 1500
strokes per hour, then the capacity of that portion of the
cylinder between the steam-valve and the piston at the end of the
stroke, must consist of half as many cubic feet as there are cubic
feet per hour evaporated in the boiler. If the steam, therefore,
be cut off at half stroke, the number of cubic feet of space in
the cylinder will be equal to the number of cubic feet of water
effectively evaporated by the boiler; and if a cubic foot of water
effectively evaporated be taken as the measure of a horse-power,
then there would be as many cubic feet in the capacity of the
cylinder as is equal to the nominal power of the engine.
(177.) The duty of engines varies according to their form and
magnitude, the circumstances under which they are worked, and the
purposes to which they are applied. In double-acting engines
working without expansion, the coal consumed per nominal
horse-power per hour varies from 7 to 12 lbs. An examination of
the steam-logs of several government steamers made by me a few
years since, gave, as the average of consumption of fuel at that
time of the best class of marine engines, about 8 lbs. per nominal
horse-power per hour. Since, however, no account could be obtained
of the actual evaporation of water in the boiler, nor, with the
necessary degree of precision, of the quantity and pressure of the
steam which passed through the cylinders, this estimate must be
regarded as an approximation subject to several causes of error.
The question of the duty of boilers and engines applied to the
[Pg297] general purposes of manufactures and navigation, is one
which has not yet been satisfactorily investigated; and it were
much to be desired that the proprietors of such engines should
combine to establish a strict analysis of their performance in
reference to their consumption of fuel, their evaporation of
water, and their useful effects. The results of such an
investigation, if properly conducted, would perhaps tend more to
the improvement of the steam engine than any discoveries in
science, or inventions in mechanical detail likely to be made in
the present stage of the progress of that machine.
(178.) A strict investigation of this kind has been for many years
carried on respecting the performance of the steam engines used
for the drainage of the mines in Cornwall; and it has been
attended with effects the most beneficial to the interests of
those concerned in them. The engines to which this important
inquiry has been applied being used for the purpose of pumping,
are generally single-acting engines, in which steam is used
expansively to a great extent. The steam is produced under a very
high pressure in the boiler, and being admitted to the cylinder is
cut off after a small portion of the entire stroke has been made,
the remainder of the stroke being produced by the expansion of the
steam.
About the year 1811, a number of the proprietors of the principal
Cornish mines agreed to establish this system of inspection, under
the management and direction of Captain Joel Lean, and to publish
monthly reports. In these reports were stated the following
particulars:—1. The load per square inch on the piston; 2. The
consumption of coal in bushels; 3. The number of strokes made by
the engine; 4. The length of the strokes in the pumps; 5. The load
in pounds; 6. The duty of the engine, expressed by the number of
pounds raised one foot high by the consumption of a bushel of
coals; 7. The number of strokes per minute; 8. The diameter and
stroke of the cylinder, and a general description of the engine.
When these reports were commenced, the number of engines brought
under inspection was twenty-one. In the year 1813 it increased
to twenty-nine; in 1814 to thirty-two; in 1820 the number
reported upon increased [Pg298] to forty; in 1828 the number was
fifty-seven; and in 1836 it was sixty-one. This gradual increase
in the number of engines brought under this system of inspection,
was produced by the good effects which attended it. These
beneficial consequences were manifested, not only in the improved
performance of the same engines, but in the gradually improved
efficiency of those which were afterwards constructed.
The following table taken from the statement of the duty of
Cornish engines by Thomas Lean and brother, lately published by
the British Association, will show in a striking manner the
improvement of the Cornish engines, from the commencement of this
system of inspection to the present time. The duty is expressed by
the number of pounds raised one foot high by the consumption of a
bushel of coals.
———————————————————————————————————————————————————————————————
| No. of | Average Duty of the | Average Duty of the best
Years.|Engines.| Whole. | Engine.
——————+————————+————————————————————-+———————————————————————-—
1812 | 21 | 19,300,000 |
1813 | 29 | 19,500,000 | 26,400,000
1814 | 32 | 20,600,000 | 32,000,000
1815 | 35 | 20,500,000 | 28,700,000
1816 | 35 | 23,000,000 | 32,400,000
1817 | 35 | 26,500,000 | 41,600,000
1818 | 36 | 25,400,000 | 39,300,000
1819 | 40 | 26,300,000 | 40,000,000
1820 | 46 | 28,700,000 | 41,300,000
1821 | 45 | 28,200,000 | 42,800,000
1822 | 52 | 28,900,000 | 42,500,000
1823 | 52 | 28,200,000 | 42,100,000
1824 | 49 | 28,300,000 | 43,500,000
1825 | 56 | 32,000,000 | 45,400,000
1826 | 51 | 30,500,000 | 45,200,000
1827 | 51 | 32,100,000 | 59,700,000
1828 | 57 | 37,100,000 | 76,800,000
1829 | 53 | 41,700,000 | 77,000,000
1830 | 56 | 43,300,000 | 78,000,000
1831 | 58 | 43,400,000 | 71,100,000
1832 | 59 | 45,000,000 | 85,000,000
1833 | 56 | 46,600,000 | 84,300,000
1834 | 52 | 47,800,000 | 90,900,000
1835 | 51 | 47,800,000 | 91,700,000
1836 | 61 | 46,600,000 | 85,400,000
1837 | 58 | 47,000,000 | 87,200,000
1838 | 61 | 48,700,000 | 84,200,000
——————+————————+————————————————————-+———————————————————————-—
[Pg299] As an example of the beneficial effects produced upon the
efficiency of an individual engine by the first application of
this system of inspection, the case of the Stray Park engine may
be mentioned. This engine, constructed by Boulton and Watt, had a
sixty inch cylinder, and when first reported in 1811, its duty
amounted to 16,000,000 pounds. After having been reported on for
three years, its duty was found to have increased to 32,000,000;
this estimate being taken from the average result of twelve
months' performance. Its duty was doubled in less than three
years.
It will appear, by inspection of the duties registered in the
preceding table, that the augmentation of the efficiency of the
engines has not been the effect of any great or sudden improvement,
but has rather resulted from the combination of a great number of
small improvements in the details of the operation of these
machines. In these improvements more is due to the successful
application of practical experience than to any new principles
developed by scientific research. Mr. John Taylor, in his "Records
of Mining," has traced the successive improvements on which the
increased duty of engines depends, and has connected these
improvements with their causes in the order of their dates. The
following results, abridged from his estimates, may not be
uninteresting:—
In 1769, soon after the date of the earliest discoveries of Mr.
Watt, but before they had come into practical application, Smeaton
computed that the average duty of fifteen atmospheric engines,
working at Newcastle-on-Tyne, was 5,590,000. The duty of the best
of these engines was 7,440,000, and that of the worst 3,220,000.
In 1772, Smeaton commenced his improvements on the atmospheric
engine, and raised the duty to 9,450,000.
In 1776, Watt obtained a duty of 21,600,000.
At this time Smeaton acknowledged that Watt's engines gave a duty
amounting to double that of his own.
In 1778-79, Watt reported a duty of 23,400,000.
From 1779 to 1788, Watt introduced the application of expansion,
and raised the duty to 26,600,000. [Pg300]
In 1798, an engine by Boulton and Watt, erected at Herland, was
reported as giving a duty of 27,000,000.
This engine, which was probably the best which at that time had
ever been erected, attracted the particular attention of Mr. Watt,
who, on visiting Cornwall, went to see it, and had many
experiments tried with it. It was under the care of Mr. Murdock,
the agent of Messrs. Boulton and Watt in Cornwall. When Mr. Watt
inspected it he pronounced it perfect, and that further
improvement could not be expected. How singular an instance this
of the impossibility, even of the most sagacious, to foresee the
results of mechanical improvement! In twenty years afterwards the
average duty of the best engine was nearly 40,000,000, and in
forty years it was above 84,000,000.
[Illustration: BOILER MANUFACTORY.]
FOOTNOTES:
[24] If 22 square inches of piston surface be allowed to
represent a horse-power, the power of an engine may always be
computed by dividing the square of the diameter of the piston
expressed in inches by 28. And, on the other hand, to find the
diameter of piston which would correspond to any given power,
multiply the number of horses' power by 28, and take the
square root of the product. These rules, however, cannot be
applied if the piston be supposed to move with any other
velocity; since, in that case, the same amount of piston
surface would cease to represent a horse-power, unless the
effective pressure on the piston were at the same time
changed.
[Pg301]
[Illustration: WATT'S CHAPEL IN HANDSWORTH CHURCH.]
CHAP. X.
NOTICE OF THE LIFE OF MR. WATT. — HIS FRIENDS AND ASSOCIATES
AT BIRMINGHAM. — INVENTION OF THE COPYING PRESS. — HEATING BY
STEAM. — DRYING LINEN BY STEAM. — THEORY OF THE COMPOSITION OF
WATER. — FIRST MARRIAGE OF WATT. — DEATH OF HIS FIRST WIFE. —
HIS SECOND MARRIAGE. — DEATH OF HIS YOUNGER SON. — EXTRACTS
FROM HIS LETTERS. — CHARACTER OF WATT BY LORD BROUGHAM. — BY
SIR WALTER SCOTT. — BY LORD JEFFREY. — OCCUPATION OF HIS OLD
AGE. — INVENTION OF MACHINE FOR COPYING SCULPTURE. — HIS LAST
DAYS. — MONUMENTS.
(179.) Having brought this historical analysis of the invention
and application of the steam engine to the date of the decease of
the illustrious man, to the powers of whose mind the world stands
indebted for the benefits conferred upon [Pg302] mankind by that
machine, it will perhaps not be deemed an improper digression in
this work, to devote some pages to a notice of the principal
labours of the same mind in other departments of art and science,
and to circumstances connected with his personal history and the
close of his life, which cannot fail to possess general interest.
At the period when Watt, having connected himself in partnership
with Boulton, went to reside at Soho, near Birmingham, a number of
persons, some of whom have since attained great celebrity by their
discoveries and their works, and all of whom were devoted to
inquiries connected with the arts and sciences, resided in that
neighbourhood. Among these may be mentioned PRIESTLEY, whose
discoveries in physical science have rendered his name immortal;
DARWIN, the philosopher and poet; WITHERING, a distinguished
physician and botanist; KEIR, a chemist, who published a translation
of Macquer, with annotations; GALTON, the ornithologist; and
EDGEWORTH, whose investigations respecting wheeled carriages and
other subjects, have rendered him well known. A society was formed
by these and other individuals, of which Boulton and Watt were
leading members, the meetings of which were held monthly on the
evening of full moon, and which was thence called the _Lunar
Society_. At the meetings of this society, subjects connected with
the arts and sciences were discussed, and out of those discussions
occasionally arose suggestions not unattended with important and
advantageous consequences. At one of these meetings, Darwin stated
that he had discovered a pen formed with two quills, by means of
which, at a single operation, an original and a copy of a letter
might be produced. Watt almost instantly observed that he thought he
could find a better expedient, and that he would turn it in his mind
that night. By the next morning the COPYING PRESS was invented, for
which he afterwards obtained a patent.
This machine, which is now so generally used in counting-houses,
consists of a rolling-press, by which a leaf of thin paper,
previously damped, is pressed upon the letter to be copied. The
writing, of which the ink is not yet quite dry, leaves its
impression upon the thin paper thus pressed upon [Pg303] it, and
the copy taken in this manner is read through the semi-transparent
paper. If a letter be written with ink suitable for this purpose,
a copy may be taken at any time within several hours after the
letter is written.
The method of heating apartments and buildings by steam, which has
since been improved and brought into extensive use, was likewise
brought forward by Watt. Although this contrivance had been
previously pointed out by Sir Hugh Platt about the middle of the
seventeenth century, and by Colonel Cooke in 1745, yet these
suggestions remained barren. Mr. Watt gave detailed methods of
heating buildings by steam[25]; and also invented a machine for
drying linen by steam, a description of which he communicated to
Dr. Brewster, which was read in December, 1824, before the Society
for promoting Useful Arts in Scotland.[26]
But the circumstance, exclusive of those connected with the
invention of the steam engine, which is by far the most memorable
in the career of Watt, is the share which he had in the discovery
of the composition of water. As this circumstance has recently
excited much interest, and led to some controversy, we shall here
state, as distinctly as possible, the leading facts connected with
it.
Water, which was so long held to be a simple element, has, in
modern times, been proved to be a substance consisting of two
aeriform bodies or gases chemically combined. These two gases are
those called in chemistry _oxygen_ and _hydrogen_. If eight grains
weight of oxygen be mixed with one grain weight of hydrogen, and
the mixture be submitted to such effects as would cause the
chemical combination of these two airs, it would be converted into
nine grains weight of pure water.
If, on the other hand, nine grains weight of pure water be
submitted to any conditions which would separate its constituent
parts, the result would be eight grains weight of oxygen gas, and
one grain weight of hydrogen gas. There are a variety of methods
in physics by which these effects would be [Pg304] produced. It
will be sufficient here to state one method of producing each of
the above changes.
If eight grains weight of oxygen be inclosed in a strong vessel
with one grain weight of hydrogen, all other substances being
excluded, and the mixture be inflamed, an explosion will take
place, the gases will disappear, and a small quantity of water
will be the only substance remaining in the vessel. If this water
be weighed, it will be found to weigh exactly nine grains.