placed at that part of the boiler which is nearest the end of the
flues which issue into the chimney. By such means the temperature
of the water in contact with those flues will be lowest at the
place where the temperature of the heated air intended to act upon
it is also lowest. The difference of the temperatures will
therefore be greater than it would be if the point of the boiler
containing water of a higher temperature was left in contact with
this part of the flue.
[Illustration: _Fig._ 79.]
(159.) It is necessary to have a ready method of ascertaining at
all times the pressure 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. Let A B C (_fig._ 79.) 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
level 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.
If, on the contrary, the level of the mercury in B C should fall
below its level in A B, the atmospheric pressure will [Pg271]
exceed that of the steam, and the quantity of the excess may be
ascertained exactly in the same way.
If the tube be glass, the difference of levels of the mercury
would be visible; but it is most commonly made of iron; and in
order to ascertain the level, a thin wooden rod with a float is
inserted in the open end of B C, so that the portion of the stick
within the tube indicates the distance of the level of the mercury
from its mouth. A bulb or cistern of mercury might be substituted
for the leg A B, as in the common barometer. This instrument is
called the _steam-gauge_.
If the steam-gauge be used as a measure of the strength of the
steam which presses on the piston, it ought to be on the same side
of the throttle-valve (which is regulated by the governor) as the
cylinder; for if it were on the same side of the throttle-valve
with the boiler, it would not be affected by the changes which the
steam may undergo in passing through the throttle-valve, when
partially closed by the agency of the governor.
For boilers in which steam of very high pressure is used, as in
those of locomotive engines, a steam-gauge, constructed on the above
principle, would have inconvenient or impracticable length. In such
boilers the pressure of the steam is equal to four or five times
that of the atmosphere, to indicate which the column of mercury in
the steam-gauge would be four or five feet in height. In such cases
a thermometer-gauge may be used with advantage. The principle of
this gauge is founded on the fact, that between the pressure and
temperature of steam produced in contact with water there is a fixed
relation, the same temperature always corresponding to the same
pressure. If, therefore, a thermometer be immersed in the boiler
which shall show the temperature of the steam, a scale may be
attached to it, on which shall be engraved the corresponding
pressures. Such gauges are now very generally used on locomotive
engines.
[Illustration: _Fig._ 80.]
(160.) The force with which the piston is pressed depends on two
things, 1st, the actual strength of the steam which presses on it;
and, 2dly, on the actual strength of the vapour which resists it.
For although the vacuum produced by the method of separate
condensation be much more perfect than [Pg272] what had been
produced in the atmospheric engines, yet still some vapour of a
small degree of elasticity is found to be raised from the hot
water in the bottom of the condenser before it can be extracted by
the air-pump. One of these pressures is indicated by the
steam-gauge already described; but still, before we can estimate
the force with which the piston descends, it is necessary to
ascertain the force of the vapour which remains uncondensed, and
resists the motion of the piston. Another gauge, called the
barometer-gauge, is provided for this purpose. A glass tube A B
(_fig._ 80.), more than thirty inches long and open at both ends,
is placed in an upright or vertical position, having the lower end
B immersed in a cistern of mercury C. To the upper end is attached
a metal tube, which communicates with the condenser, in which a
constant vacuum, or rather high degree of rarefaction, is
sustained. The same vacuum must therefore exist in the tube A B,
above the level of the mercury, and the atmospheric pressure on
the surface of the mercury in the cistern C will force the mercury
up in the tube A B, until the column which is suspended in it is
equal to the difference between the atmospheric pressure and the
pressure of the uncondensed steam. The difference between the
column of mercury sustained in this instrument and in the common
barometer, will determine the strength of the uncondensed steam,
allowing a force proportional to one pound per square inch for
every two inches of mercury in the difference of the two columns.
In a well-constructed engine which is in good order, there is very
little difference between the altitude in the barometer-gauge and
the common barometer.
To compute the force with which the piston descends, thus becomes a
very simple arithmetical process. First, ascertain the difference of
the levels of the mercury in the steam-gauge; this gives the excess
of the steam pressure above the atmospheric pressure. Then find the
height of the mercury in the barometer-gauge; this gives the excess
of the atmospheric pressure above the uncondensed steam. Hence, if
these two heights be added together, we shall obtain the [Pg273]
excess of the impelling force of the steam from the boiler, on the
one side of the piston, above the resistance of the uncondensed
steam on the other side: this will give the effective impelling
force. Now, if one pound be allowed for every two inches of mercury
in the two columns just mentioned, we shall have the number of
pounds of impelling pressure on every square inch of the piston.
Then, if the number of square inches in the section of the piston be
found, and multiplied by the number of pounds on each square inch,
the force with which it moves will be obtained.
From what we have stated it appears that, in order to estimate the
force with which the piston is urged, it is necessary to refer to
both the barometer and the steam-gauge. This double computation
may be obviated by making one gauge serve both purposes. If the
end C of the steam-gauge (_fig._ 79.), instead of communicating
with the atmosphere were continued to the condenser, we should
have the pressure of the steam acting upon the mercury in the tube
B A, and the pressure of the uncondensed vapour which resists the
piston acting on the mercury in the tube B C. Hence the difference
of the levels of the mercury in the tubes would at once indicate
the difference between the force of the steam and that of the
uncondensed vapour, which is the effective force with which the
piston is urged.
(161.) But these methods of determining the effective force by
which the piston is urged, can only be regarded as approximations,
and not very perfect ones. If the condensation of steam on one
side of the piston were instantaneously effected, or the
uncondensed vapour were of the same tension during the whole
stroke; and if, besides this, the pressure of steam on the piston
were of uniform intensity from the beginning to the end of the
stroke, then the steam and barometer gauges taken together would
become an accurate index of the effective force of steam on the
piston: but such is not the case. When the steam is first admitted
through the steam-valve it acts on the piston with a pressure
which is first slightly diminished, and afterwards a little
increased, until it arrives at that part of the stroke at which
the steam-valve is closed, after which the pressure is diminished.
The [Pg274] pressure, therefore, urging the piston is subject to
variation; but the pressure of the uncondensed vapour on the other
side of the piston is subject to still greater change. At the
moment the exhausting-valve is opened, the piston is relieved from
the pressure upon it by the commencement of the condensation; but
this process during the descent of the piston is gradual, and the
vacuum is rendered more and more perfect, until the piston has
nearly attained the limit of its play. These variations, both as
well of the force urging the piston as of the force resisting it,
are such as not to be capable of being accurately measured by a
mercurial column, since they would produce oscillations in such a
column, which would render any observations of its mean height
impracticable.
To measure the mean efficient force of the piston, taking into
account these circumstances, Mr. Watt invented an instrument,
which, like all his mechanical inventions, has answered its
purpose perfectly, and is still in general use. This instrument,
called an _indicator_, consists of a cylinder of about 1-3/4 inch
in diameter, and 8 inches in length. It is bored with great
accuracy, and fitted with a solid piston moving steam-tight in it
with very little friction. The rod of this piston is guided in the
direction of the axis of the cylinder through a collar in the top,
so as not to be subject to friction in any part of its play. At
the bottom of the cylinder is a pipe governed by a stop-cock and
turned in a screw, by which the instrument may be screwed on the
top of the steam-cylinder of the engine. In this position, if the
stop-cock of the indicator be opened, a free communication will be
made between the cylinder of the indicator and that of the engine.
The piston-rod of the indicator is attached to a spiral spring,
which is capable of extension and compression, and which by its
elasticity is capable of measuring the force which extends or
compresses it in the same manner as a spring steel-yard or
balance. If a scale be attached to the instrument at any point on
the piston-rod to which an index might be attached, then the
position of that index upon the scale would be governed by the
position of the indicator-piston in its cylinder. If any force
pressed the indicator-piston upwards, so as to compress the
spring, [Pg275] the index would rise upon the scale; and if, on
the other hand, a force pressed the indicator-piston downwards,
then the spiral spring would be extended, and the index on the
piston-rod descend upon the scale. In each case the force of the
spring, whether compressed or extended, would be equal to the
force urging the indicator-piston, and the scale might be so
divided as to show the amount of this force.
Now, let the instrument be supposed to be screwed upon the top of
the cylinder of a steam-engine, and the stop-cock opened so as to
leave a free communication between the cylinder of the indicator
below its piston and the cylinder of the steam-engine above the
steam-piston. At the moment the upper steam-valve is opened, the
steam rushing in upon the steam-piston will also pass into the
indicator, and press the indicator-piston upwards: the index upon
its piston-rod will point upon the scale to the amount of pressure
thus exerted. As the steam-piston descends, the indicator-piston
will vary its position with the varying pressure of the steam in
the cylinder, and the index on the piston-rod will play upon the
scale, so as to show the pressure of the steam at each point
during the descent of the piston.
If it were possible to observe and record the varying position of
the index on the piston-rod of the indicator, and to refer each of
these varying positions to the corresponding point of the descending
stroke, we should then be able to declare the actual pressure of the
steam at every point of the stroke. But it is evident that such an
observation would not be practicable. A method, however, was
contrived by Mr. Southern, an assistant of Messrs. Boulton and Watt,
by which this is perfectly effected. A square piece of paper, or
card, is stretched upon a board, which slides in grooves formed in a
frame. This frame is placed in a vertical position near the
indicator, so that the paper may be moved in a horizontal direction
backwards and forwards, through a space of fourteen or fifteen
inches. Instead of an index a pencil is attached to the indicator of
the piston-rod: this pencil is lightly pressed by a spring against
the paper above mentioned, and as the paper is moved in a horizontal
direction [Pg276] under the pencil, would trace upon the paper a
line. If the pencil were stationary this line would be straight and
horizontal, but if the pencil were subject to a vertical motion, the
line traced on the paper moved under the pencil horizontally would
be a curve, the form of which would depend on the vertical motion of
the pencil. The board thus supporting the paper is put into
connexion by a light cord carried over pulleys with some part of the
parallel motion, by which it is alternately moved to the right and
to the left. As the piston ascends or descends, the whole play of
the board in the horizontal direction will therefore represent the
length of the stroke, and every fractional part of that play will
correspond to a proportional part of the stroke of the steam-piston.
[Illustration: _Fig._ 81.]
The apparatus being thus arranged, let us suppose the steam-piston
at the top of the cylinder commencing its descent. As it descends,
the pencil attached to the indicator piston-rod varies its height
according to the varying pressure of the steam in the cylinder. At
the same time the paper is moved uniformly under the pencil, and a
curved line is traced upon it from right to left. When the piston
has reached the bottom of the cylinder, the upper exhausting-valve
is opened, and the steam drawn off to the condenser. The
indicator-piston being immediately relieved from a part of the
pressure acting upon it descends, and with it the pencil also
descends; but at the same time the steam-piston has begun to ascend,
and the paper to return from left to right under the pencil. While
the steam-piston continues to ascend, the condensation becomes more
and more perfect, and the vacuum in the cylinder, and therefore also
in the indicator, being gradually increased in power, the
atmospheric pressure above the indicator-piston presses it downwards
and stretches the spring. The pencil meanwhile, with the paper
moving under it from right to left, traces a second curve. As the
former curve showed the actual pressure of the steam impelling the
piston in its descent, this latter will show the pressure of the
uncondensed steam raising the piston in its ascent, and a comparison
of the two will exhibit the effective force on the piston. _Fig._
81. represents such a diagram as would be [Pg277] produced by this
instrument. A B C is the curve traced by the pencil during the
descent of the piston, and C D E that during its ascent. A is the
position of the pencil at the moment the piston commences its
descent, B is its position at the middle of the stroke, and C at the
termination of the stroke. On closing the upper steam-valve and
closing the exhausting-valve, the indicator-piston being gradually
relieved from the pressure of the steam the pencil descends, and at
the same time the paper moving from left to right, the pencil traces
the curve C D E, the gradual descent of this curve showing the
progressive increase of the vacuum. As the atmospheric pressure
constantly acts above the piston of the indicator, its position will
be determined by the difference between the atmospheric pressure and
the pressure of the steam below it; and therefore the difference
between the heights of the pencil at corresponding points in the
ascending and descending stroke, will express the difference between
the pressure of the steam impelling the piston in the ascent and
resisting it in the descent at these points. Thus at the middle of
the stroke, the line B D will express the extent to which the spring
governing the indicator-piston would be stretched by the difference
between the force of steam impelling the piston at the middle of the
descending stroke, and the force of steam resisting it at the middle
of the ascending stroke. The force therefore measured by the line B
D will be the effective force on the piston at that point; and the
same may be said of every part of the diagram produced by the
indicator.
The whole mechanical effect produced by the stroke of the piston
being composed of the aggregate of all its varying effects
throughout the stroke, the determination of its amount [Pg278] is
a matter of easy calculation by the measurement of the diagram
supplied by the indicator. Let the horizontal play of the pencil
from A to C be divided into any proposed number of equal parts,
say ten: at the middle of the stroke, B D expresses the effective
force on the piston, and if this be considered to be uniform
through the tenth part of the stroke, as from _f_ to _g_, then the
number of pounds expressed by B D multiplied by the tenth part of
the stroke expressed in parts of a foot, will be the mechanical
effect through that part of the stroke expressed in pounds' weight
raised one foot. In like manner _m n_ will express the effective
force on the piston after three fourths of the stroke have been
performed, and if this be multiplied by a tenth part of the stroke
as before, the mechanical effect similarly expressed will be
obtained; and the same process being applied to any successive
tenth part of the stroke, and the numerical results thus obtained
being added together, the whole effect of the stroke will be
obtained, expressed in pounds' weight raised one foot.
(162.) By means of the indicator, the actual mechanical effect
produced by each stroke of the engine can be obtained, and if the
actual number of strokes made in any given time be known, the
whole effect of the moving power would be determined. An
instrument called a _counter_ was also contrived by Watt, to be
attached either to the working beam or to any other reciprocating
part of the engine. This instrument consisted of a train of
wheel-work with governing hands or indices moved upon divided
dials, like the hand of a clock. A record of the strokes was
preserved by means precisely similar to those by which the hands
of a clock or time-piece indicated and recorded the number of
vibrations of the pendulum or balance-wheel.
(163.) To secure the boiler from accidents arising from the steam
contained in it acquiring an undue pressure, a safety-valve is
used, similar in principle to those adopted in the early engines.
This valve is represented in _fig._ 71. at N. It is a conical
valve, kept down by a weight sliding on a rod upon it. When the
pressure of the steam overcomes the force of this weight, it
raises the valve and escapes, being carried off through the tube.
[Pg279]
With a view to the economy of heat, this waste steam tube is
sometimes conducted into the feeding cistern, where the steam
carried off by it is condensed, and heats the feeding water.
The magnitude of the safety-valve should be such that, when open,
steam should be capable of passing through it as rapidly as it is
generated in the boiler. The superficial magnitude, therefore, of
such valves must be proportional to the evaporating power of the
boiler. In low pressure boilers the steam is generally limited to
five or six pounds' pressure per square inch, and consequently the
load over the safety-valve in pounds would be found by multiplying
the superficial magnitude of its smallest part by these numbers.
In boilers in which the steam is maintained at a higher pressure,
it would be inconvenient to place upon the safety-valve the
necessary weight. In such cases a lever is used, the shorter arm
of which presses down the valve, and the longer arm is held down
by a weight capable of adjustment, so that the pressure on the
valve may be regulated at discretion. Two safety-valves should be
provided on all boilers, one of which should be locked up, so that
the persons in care of the engine should have no power to increase
the load upon it. In such case, however, it is necessary that a
handle connected with the valve should project outside the box
containing it, so that it may always be possible for the engineer
to ascertain that the valve is not locked in its seat, a
circumstance which is liable to happen.
Sometimes also two safety-valves are provided, one loaded a little
heavier than the other. The escape of steam from the lighter valve
in this case gives notice to the engine-man of the growing
increase of pressure, and warns him to check the production of
steam. The lever by which the safety-valve is held down is
sometimes acted on by a spiral spring, capable of being so
adjusted as to produce any required pressure on the valve. This
arrangement is adopted in locomotive engines, where steam of very
high pressure is used; and in such cases also there are always
provided two such valves, one of which cannot be increased in its
pressure.
The pipe by which the boiler is fed with water will [Pg280]
necessarily act as a safety-valve, for when the pressure of the
steam increases in an undue degree, it will press the water in the
boiler up through the feed-pipe, so as to discharge it into the
feed-cistern, a circumstance which would immediately give notice
of the internal state of the boiler. The steam-gauge, already
described (_fig._ 79.), would also act as a safety-valve; for if
the pressure of steam in the boiler should be so augmented as to
blow the mercury out of the steam-gauge, the steam would then
issue through the gauge, and the pressure of the boiler be
reduced, provided that the magnitude of the tube forming the
steam-gauge were sufficient for this purpose.
(164.) In high pressure boilers which are exposed to extreme
temperatures and pressures, and which are therefore subject to
danger of explosion, a plug of metal is sometimes inserted, which is
capable of being fused at a temperature above which the boiler
should not be permitted to be raised. If the pressure of steam
increase beyond the proper limit, the temperature of the water and
steam will undergo a corresponding increase; and if the metal of the
plug be capable of being fused at such a temperature, the plug will
fall out of the boiler, and the steam and water will issue from it.
Various alloys of metal are fusible at temperatures sufficiently low
for this purpose. An alloy composed of one part of lead, three of
tin, and five of bismuth, will fuse at the common temperature of
boiling water; and alloys of the same metals, in various
proportions, will fuse at different temperatures from 200° to 400°.
Although fusible plugs may be used, in addition to other means of
insuring safety, they ought not to be exclusively relied on at the
ordinary working pressure of the boiler. The fusible plug ought to
be capable of more than resisting the pressure; but if it be so,
its point of fusion would be one at which the steam would have a
pressure of at least two atmospheres above its working pressure.
The plug would therefore be capable of being fused only as soon as
the steam would acquire a pressure of 30 lbs. per inch above its
regular working pressure.
When a boiler ceases to be worked, and the furnace has been
extinguished, the space within it appropriated to steam [Pg281]
will be left a vacuum by the condensation of the steam with which
it was previously filled. The external pressure of the atmosphere
acting on the boiler would, under such circumstances, have a
tendency to crush it inwards. To prevent this, a safety-valve is
provided, opening inwards, and balanced by a weight sufficient to
keep it closed until it be relieved from the pressure of the steam
below.