MODES OF ESTIMATING THE POWER AND PERFORMANCE OF ENGINES AND BOILERS.
HORSES POWER.
209. _Q._--What do you understand by a horse power?
_A._--An amount of mechanical force that will raise 33,000 lbs. one foot
high in a minute. This standard was adopted by Mr. Watt, as the average
force exerted by the strongest London horses; the object of his
investigation being to enable him to determine the relation between the
power of a certain size of engine and the power of a horse, so that when it
was desired to supersede the use of horses by the erection of an engine, he
might, from the number of horses employed, determine the size of engine
that would be suitable for the work.
210. _Q._--Then when we talk of an engine of 200 horse power, it is meant
that the impelling efficacy is equal to that of 200 horses, each lifting
33,000 lbs. one foot high in a minute?
_A._--No, not now; such was the case in Watt's engines, but the capacity of
cylinder answerable to a horse power has been increased by most engineers
since his time, and the pressure on the piston has been increased also, so
that what is now called a 200 horse power engine exerts, almost in every
case, a greater power than was exerted in Watt's time, and a horse power,
in the popular sense of the term, has become a mere conventional unit for
expressing a certain size of engine, without reference to the power
exerted.
211. _Q._--Then, each nominal horse power of a modern engine may raise much
more than 33,000 lbs. one foot high in a minute?
_A._--Yes; some raise 52,000 lbs., others 60,000 lbs., and others 66,000
lbs., one foot high in a minute by each nominal horse power. Some engines
indeed work as high as five times above the nominal power, and therefore no
comparison can be made between the performances of different engines,
unless the power actually exerted be first discovered.
212. _Q._--How is the power actually exerted by engines ascertained?
_A._--By means of an instrument called the indicator, which is a miniature
cylinder and piston attached to the cylinder cover of the main engine, and
which indicates, by the pressure exerted on a spring, the amount of
pressure or vacuum existing within the cylinder. From this pressure,
expressed in pounds per square inch, deduct a pound and a half of pressure
for friction, the loss of power in working the air pump, &c.; multiply the
area of the piston in square inches by this residual pressure, and by the
motion of the piston, in feet per minute, and divide by 33,000; the
quotient is the actual number of horses power of the engine. The same
result is attained by squaring the diameter of the cylinder, multiplying by
the pressure per square inch, as shown by the indicator, less a pound and a
half, and by the motion of the piston, in feet per minute, and dividing by
42,017.
213. _Q._ How is the nominal power of an engine ascertained?
_A._--Since the nominal power is a mere conventional expression, it is
clear that it must be determined by a merely conventional process. The
nominal power of ordinary condensing engines may be ascertained by the
following rule: multiply the square of the diameter of the cylinder in
inches, by the velocity of the piston in feet per minute, and divide the
product by 6,000; the quotient is the number of nominal horses power. In
using this rule, however, it is necessary to adopt the speed of piston
prescribed by Mr. Watt, which varies with the length of the stroke. The
speed of piston with a 2 feet stroke is, according to his system, 160 per
minute; with a 2 ft. 6 in. stroke, 170; 3 ft., 180; 3 ft. 6 in., 189; 4
ft., 200; 5 ft., 215; 6 ft., 228; 7 ft., 245; 8 ft., 256 ft.
214. _Q._--Does not the speed of the piston increase with the length of the
stroke?
_A._--It does: the speed of the piston varies nearly as the cube root of
the length of the stroke.
215. _Q._--And may not therefore some multiple of the cube root of the
length of the stroke be substituted for the velocity of the piston in
determining the nominal power?
_A._--The substitution is quite practicable, and will accomplish some
simplification, as the speed of piston proper for the different lengths of
stroke cannot always be remembered. The rule for the nominal power of
condensing engines when thus arranged, will be as follows: multiply the
square of the diameter of the cylinder in inches by the cube root of the
stroke in feet, and divide the product by 47; the quotient is the number of
nominal horses power of the engine, supposing it to be of the ordinary
condensing description. This rule assumes the existence of a uniform
effective pressure upon the piston of 7 lbs. per square inch; Mr. Watt
estimated the effective pressure upon the piston of his 4 horse power
engines at 6-8 lbs. per square inch, and the pressure increased slightly
with the power, and became 6.94 lbs. per square inch in engines of 100
horse power; but it appears to be more convenient to take a uniform
pressure of 7 lbs. for all powers. Small engines, indeed, are somewhat less
effective in proportion than large ones, but the difference can be made up
by slightly increasing the pressure in the boiler; and small boilers will
bear such an increase without inconvenience.
216. _Q._--How do you ascertain the power of high pressure engines?
_A._--The actual power is readily ascertained by the indicator, by the same
process by which the actual power of low pressure engines is ascertained.
The friction of a locomotive engine when unloaded is found by experiment to
be about 1 lb. per square inch on the surface of the pistons, and the
additional friction caused by any additional resistance is estimated at
about .14 of that resistance; but it will be a sufficiently near
approximation to the power consumed by friction in high pressure engines,
if we make a deduction of a pound and a half from the pressure on that
account, as in the case of low pressure engines. High pressure engines, it
is true, have no air pump to work; but the deduction of a pound and a half
of pressure is relatively a much smaller one where the pressure is high,
than where it does not much exceed the pressure of the atmosphere. The
rule, therefore, for the actual horse power of a high pressure engine will
stand thus: square the diameter of the cylinder in inches, multiply by the
pressure of the steam in the cylinder per square inch less 1-1/2 lb., and
by the speed of the piston in feet per minute, and divide by 42,017; the
quotient is the actual horse power.
217. _Q._--But how do you ascertain the nominal horse power of high
pressure engines?
_A._--The nominal horse power of a high pressure engine has never been
defined; but it should obviously hold the same relation to the actual power
as that which obtains in the case of condensing engines, so that an engine
of a given nominal power may be capable of performing the same work,
whether high pressure or condensing. This relation is maintained in the
following rule, which expresses the nominal horse power of high pressure
engines: multiply the square of the diameter of the cylinder in inches by
the cube root of the length of stroke in feet, and divide the product by
15.6. This rule gives the nominal power of a high pressure engine three
times greater than that of a low pressure engine of the same dimensions;
the average effective pressure being taken at 21 lbs. per square inch
instead of 7 lbs., and the speed of the piston in feet per minute being in
both rules 128 times the cube root of the length of stroke.[1]
218. _Q._--Is 128 times the cube root of the stroke in feet per minute the
ordinary speed of all engines?
_A._--Locomotive engines travel at a quicker speed--an innovation brought
about not by any process of scientific deduction, but by the accidents and
exigencies of railway transit. Most other engines, however, travel at about
the speed of 128 times the cube root of the stroke in feet; but some marine
condensing engines of recent construction travel at as high a rate as 700
feet per minute. To mitigate the shock of the air pump valves in cases in
which a high speed has been desirable, as in the case of marine engines
employed to drive the screw propeller without intermediate gearing, India
rubber discs, resting on a perforated metal plate, are now generally
adopted; but the India rubber should be very thick, and the guards employed
to keep the discs down should be of the same diameter as the discs
themselves.
219. _Q._--Can you suggest any eligible method of enabling condensing
engines to work satisfactorily at a high rate of speed?
_A._--The most feasible way of enabling condensing engines to work
satisfactorily at a high speed, appears to lie in the application of
balance weights to the engine, so as to balance the momentum of its moving
parts, and the engine must also be made very strong and rigid. It appears
to be advisable to perform the condensation partly in the air pump, instead
of altogether in the condenser, as a better vacuum and a superior action of
the air pump valves will thus be obtained. Engines constructed upon this
plan may be driven at four times the speed of common engines, whereby an
engine of large power may be purchased for a very moderate price, and be
capable of being put into a very small compass; while the motion, from
being more equable, will be better adapted for most purposes for which a
rotary motion is required. Even for pumping mines and blowing iron
furnaces, engines of this kind appear likely to come into use, for they are
more suitable than other engines for driving the centrifugal pump, which in
many cases appears likely to supersede other kinds of pumps for lifting
water; and they are also conveniently applicable to the driving of fans,
which, when so arranged that the air condensed by one fan is employed to
feed another, and so on through a series of 4 or 5, have succeeded in
forcing air into a furnace with a pressure of 2-1/2 lbs. on the square
inch, and with a far steadier flow than can be obtained by a blast engine
with any conceivable kind of compensating apparatus. They are equally
applicable if blast cylinders be employed.
220. _Q._--Then, if by this modification of the engine you enable it to
work at four times the speed, you also enable it to exert four times the
power?
_A._--Yes; always supposing it to be fully supplied with steam. The nominal
power of this new species of engine can readily be ascertained by taking
into account the speed of the piston, and this is taken into account by the
Admiralty rule for power.
221. _Q._--What is the Admiralty rule for determining the power of an
engine?
_A._--Square the diameter of the cylinder in inches, which multiply by the
speed of the piston in feet per minute, and divide by 6,000; the quotient
is the power of the engine by the Admiralty rule.[2]
222. _Q._--The high speed engine does not require so heavy a fly wheel as
common engines?
_A._--No; the fly wheel will be lighter, both by virtue of its greater
velocity of rotation, and because the impulse communicated by the piston is
less in amount and more frequently repeated, so as to approach more nearly
to the condition of a uniform pressure.
223. _Q._--Can nominal be transformed into actual horse power?
_A._--No; that is not possible in the case of common condensing engines.
The actual power exerted by an engine cannot be deduced from its nominal
power, neither can the nominal power be deduced from the power actually
exerted, or from anything else than the dimensions of the cylinder. The
actual horse power being a dynamical unit, and the nominal horse power a
measure of capacity of the cylinder, are obviously incomparable things.
224. _Q._--That is, the _nominal_ power is a commercial unit by which
engines are bought and sold, and the _actual_ power a scientific unit by
which the quality of their performance is determined?
_A._--Yes; the nominal power is as much a commercial measure as a yard or a
bushel, and is not a thing to be ascertained by any process of science, but
to be fixed by authority in the same manner as other measures. The actual
power, on the contrary, is a mechanical force or dynamical effort capable
of raising a given weight through a given distance in a given time, and of
which the amount is ascertainable by scientific investigation.
225. _Q._--Is there any other measure of an actual horse power than 33,000
lbs. raised one foot high in the minute?
_A._--There cannot be any _different_ measure, but there are several
equivalent measures. Thus the evaporation of a cubic foot of water in the
hour, or the expenditure of 33 cubic feet of low pressure steam per minute,
is reckoned equivalent to an actual horse power, or 528 cubic feet of water
raised one foot high in the minute involves the same result.
[1] Tables of the horse power of both high and low pressure
engines are given in the Key.
[2] Example.--What is the power of an engine of 42 inches
diameter, 3-1/2 feet stroke, and making 85 strokes per minute? The
speed of the piston will be 7 (the length of a double stroke) x 85 =
595 feet per minute. Now 42 x 42 = 1,764 x 595 = 1,049,580 / 6,000 =
175 horses power.
DUTY OF ENGINES AND BOILERS.
226. _Q._--What is meant by the duty of a engine?
_A._--The work done in relation to the fuel consumed.
227. _Q._--And how is the duty ascertained?
_A._--In ordinary mill or marine engines it can only be ascertained by the
indicator, as the load upon such engines is variable, and cannot readily be
determined; but in the case of engines pumping water, where the load is
constant, the number of strokes performed by the engine will represent the
work done, and the amount of work done by a given quantity of coal
represents the duty. In Cornwall the duty of an engine is expressed by the
number of millions of pounds raised one foot high by a bushel, or 94 lbs.
of Welsh coal. A bushel of Newcastle coal will only weigh 84 Lbs.; and in
comparing the duty of a Cornish engine with the performance of an engine in
some locality where a different kind of coal is used, it is necessary to
pay regard to such variations.
228. _Q._--Can you tell the duty of an engine when you know its consumption
of coal per horse power per hour?
_A._--Yes, if the power given be the actual, and not the nominal, power.
Divide 166.32 by the number of pounds of coal consumed per actual horse
power per hour; the quotient is the duty in millions of pounds. If you
already have the duty in millions of pounds, and wish to know the
equivalent consumption in pounds per actual horse power per hour, divide
166.32 by the duty in millions of pounds; the quotient is the consumption
per actual horse power per hour. The duty of a locomotive engine is
expressed by the weight of coke it consumes in transporting a ton through
the distance of one mile upon a railway; but this is a very imperfect
method of representing the duty, as the tractive efficacy of a pound of
coke becomes less as the speed of the locomotive becomes greater; and the
law of variation is not accurately known.
229. _Q._--What amount of power is generated in good engines of the
ordinary kind by a given weight of coal?
_A._--The duty of different kinds of engines varies very much, and there
are also great differences in the performance of different engines of the
same class. In ordinary rotative condensing engines of good construction,
10 lbs. of coal per nominal horse power per hour is a common consumption;
but such engines exert nearly twice their nominal power, so that the
consumption per actual horse power per hour may be taken at from 5 to 6
lbs. Engines working very expansively, however, attain an economy much
superior to this. The average duty of the pumping engines in Cornwall is
about 60,000,000 lbs. raised 1 ft. high by a bushel of Welsh coals, which
weighs 94 lbs. This is equivalent to a consumption of 3.1 lbs. of coal per
actual horse power per hour; but some engines reach a duty of above
100,000,000, or 1.74 lbs. of coal per actual horse power per hour.
Locomotives consume from 8 to 10 lbs. of coke in evaporating a cubic foot
of water, and the evaporation of a cubic foot of water per hour may be set
down as representing an actual horse power in locomotives as well as in
condensing engines, if expansion be not employed. When the locomotive is
worked expansively, however, there is of course a less consumption of water
and fuel per horse power, or per ton per mile, than when the full pressure
is used throughout the stroke; and most locomotives now operate with as
much expansion as can be conveniently given by the slide valves.
230. _Q._--But is not the evaporative power of locomotives affected
materially by the proportions of the boiler?
_A._--Yes, but this may be said of all boilers; but in locomotive boilers,
perhaps, the effect of any misproportion becomes more speedily manifest. A
high temperature of the fire box is found to be conducive to economy of
fuel; and this condition, in its turn, involves a small area of grate bars.
The heating surface of locomotive boilers should be about 80 square feet
for each square foot of grate bars, and upon each foot of grate bars about
1 cwt. of coke should be burnt in the hour.
231. _Q._--Probably the heat is more rapidly absorbed when the temperature
of the furnace is high?
_A._--That seems to be the explanation. The rapidity with which a hot body
imparts heat to a colder, varies as the square of the difference of
temperature; so that if the temperature of the furnace be very high, the
larger part of the heat passes into the water at the furnace, thereby
leaving little to be transmitted by the tubes. If, on the contrary, the
temperature of the furnace be low, a large part of the heat will pass into
the tubes, and more tube surface will be required to absorb it. About 16
cubic feet of water should be evaporated by a locomotive boiler for each,
square foot of fire grate, which, with the proportion of heating surface
already mentioned, leaves 5 square feet of heating surface to evaporate a
cubic foot of water in the hour. This is only about half the amount of
surface usual in land and marine boilers per cubic foot evaporated, and its
small amount is due altogether to the high temperature of the furnace,
which, by the rapidity of transmission it causes, is tantamount to an
additional amount of heating surface.
232. _Q._--You have stated that the steam and vacuum gauges are generally
glass tubes, up which mercury is forced by the steam or sucked by the
vacuum?
_A._--Vacuum gauges are very often of this construction, but steam
gauges more frequently consist of a small iron tube, bent like the letter
U, and into which mercury is poured. The one end of this tube communicates
with the boiler, and the other end with the atmosphere; and when the
pressure of the steam rises in the boiler, the mercury is forced down in
the leg communicating with the boiler and rises in the other leg, and the
difference of level in the legs denotes the pressure of the steam. In this
gauge a rise of the mercury one inch in the one leg involves a difference
of the level between the two legs of two inches, and an inch of rise is,
therefore, equivalent to two inches of mercury, or a pound of pressure. A
small float of wood is placed in the open leg to show the rise or fall of
the mercury, and this leg is surmounted by a brass scale, graduated in
inches, to the marks of which the float points.
233. _Q._--What other kinds of steam and vacuum gauges are there?
_A._--There are many other kinds; but probably Bourdon's gauges are now in
more extended use than, any other, and their operation has been found to be
satisfactory in practice. The principle of their action may be explained to
be, that a thin elliptical metal tube, if bent into a ring, will seek to
coil or uncoil itself if subjected to external or internal pressure, and to
an extent proportional to the pressure applied. The end of the tube is
sharpened into an index, and moves to an extent corresponding to the
pressure applied to the tube; but in the more recent forms of this
apparatus, a dial and a hand, like those of a clock, are employed, and the
hand is moved round by a toothed sector connected to the tube, and which
sector acts on a pinion attached to the hand. Mr. Shank, of Paisley, has
lately introduced a form of steam gauge like a thermometer, with a
flattened bulb; and the pressure of the steam, by compressing the bulb,
causes the mercury to rise to a point proportional to the pressure applied.
THE INDICATOR.
234. _Q._--You have already stated that the actual power of an engine is
ascertained by an instrument called the indicator, which consists of a
small cylinder with a piston moving against a spring, and compressing it to
an extent answerable to the pressure of the steam. Will you explain further
the structure and mode of using that instrument?
[Illustration: Fig. 36]
_A._--The structure of the common form of indicator will be most readily
apprehended by a reference to fig. 36, which is a McNaught's indicator.
Upon a movable barrel A, a piece of paper is wound, the ends of which are
secured by the slight brass clamps shown in the drawing. The barrel is
supported by the bracket _b_, proceeding from the body of the indicator,
and at the bottom of the barrel a watch spring is coiled with one end
attached to the barrel and the other end to the bracket, so that when the
barrel is drawn round by a string wound upon its lower end like a roller
blind, the spring returns the barrel to its original position, when the
string is relaxed. The string is attached to some suitable part of the
engine, and at every stroke the string is drawn out, turning round the
barrel, and the barrel is returned again by the spring on the return
stroke.
235. _Q_--But in what way can these reciprocations of the barrel determine
the power of the engine?
_A._--They do not determine it of themselves, but are only part of the
operation. In the inside of the cylinder _c_ there is a small piston moving
steam tight in a cylinder of which _d_ is the piston rod, and _e_ a spiral
spring of steel, which the piston, when forced upwards by the steam or
sucked downwards by the vacuum, either compresses or extends; _f_ is a cock
attached to the cylinder of the indicator, and which is screwed into the
cylinder cover. It is obvious that, so soon as this cock is opened, the
piston will be forced up when the space above the piston of the engine is
opened to the boiler, and sucked down when that space is opened to the
condenser--in each case to an extent proportionate to the pressure of the
steam or the perfection of the vacuum, the top of the piston _c_ being open
to the atmosphere. A pencil, _p_, with a knife hinge, is inserted into the
piston rod, at _e_, and the point of the pencil bears upon the surface of
the paper wound upon the drum A. If the drum A did not revolve, this pencil
would merely trace on the paper a vertical line; but as the drum A moves
round and back again every stroke of the engine, and as the pencil moves up
and down again every stroke of the engine, the combined movements trace
upon the paper a species of rectangle, which is called an indicator
diagram; and the nature of this diagram determines the nature of the
engine's performance.
236. _Q._--How does it do this?
_A._--It is clear that if the pencil was moved up instantaneously to the
top of its stroke, and was also moved down instantaneously to the bottom of
its stroke, and if it remained without fluctuation while at the top and
bottom, the figure described by the pencil would be a perfect rectangle, of
which the vertical height would represent the total pressure of the steam
and vacuum, and therefore the total pressure urging the piston of the
engine. But in practice the pencil will neither rise nor fall
instantaneously, nor will it remain at a uniform height throughout the
stroke. If the steam be worked expansively the pressure will begin to fall
so soon as the steam is cut off; and at the end of the stroke, when the
steam comes to be discharged, the subsidence of pressure will not be
instantaneous, but will occupy an appreciable time. It is clear, therefore,
that in no engine can the diagram described by an indicator be a complete
rectangle; but the more nearly it approaches to a rectangle, the larger
will be the power produced at every stroke with any given pressure, and the
area of the space included within the diagram will in every case accurately
represent the power exerted by the engine during that stroke.
237. _Q._--And how is this area ascertained?
_A._--It may be ascertained in various ways; but the usual mode is to take
the vertical height of the diagram at a number of equidistant points on a
base line, and then to take the mean of these several heights as
representative of the mean pressure actually urging the piston. Now if you
have the pressure on the piston per square inch, and if you know the number
of square inches in its area, and the velocity with which it moves in feet
per minute, you have obviously the dynamical effort of the engine, or, in
other words, its actual power.
238. _Q._--How is the base line you have referred to obtained?
_A._--In proceeding to take an indicator diagram, the first thing to be
done is to allow the barrel to make two or three reciprocations with the
pencil resting against it, before opening the cock attached to the
cylinder. There will thus be traced a horizontal line, which is called the
_atmospheric line_, and in condensing engines, a part of the diagram will
be above and a part of it below this line; whereas, in high pressure
engines the whole of the diagram will be above this line. Upon this line
the vertical ordinates may be set off at equal distances, or upon any base
line parallel to it; but the usual course is to erect the ordinates on the
atmospheric line.
239. _Q._--Will you give an example of an indicator diagram?
[Illustration: Fig. 37]
_A._--Fig. 37 is an indicator diagram taken from a low pressure engine, and
the waving line _a b c_, forming a sort of irregular parallelogram, is that
which is described by the pencil. The atmospheric line is represented by
the line o o. The scale at the side shows the pressure of the steam, which
in this engine rose to about 9 lbs. per square inch, and the vacuum fell to
11 lbs. The steam begins to be cut off when, about one-fourth of the stroke
has been performed, and the pressure consequently falls.
240. _Q._--Is this species of indicator which you have just described
applicable to locomotive engines?
_A._--It is no doubt applicable under suitable conditions; but another
species of indicator has been applied by Mr. Gooch to locomotive engines,
which presents several features of superiority for such a purpose.
This indicator has its cylinder placed horizontally; and its piston
compresses two elliptical springs; a slide valve is substituted for a cock,
to open or close the communication with the engine. The top of the piston
rod of this indicator is connected to the short arm of a smaller lever, to
the longer arm of which the pencil is attached, and the pencil has thus a
considerably larger amount of motion than the piston; but it moves in the
arc of a circle instead of in a straight line. The pencil marks on a web of
paper, which is unwound from one drum and wound on to another, so that a
succession of diagrams are taken without the necessity of any intermediate
manipulation.
241. _Q._--These diagrams being taken with a pencil moving in an arc, will
be of a distorted form?
_A._--They will not be of the usual form, but they may be easily translated
into the usual form. It is undoubtedly preferable that the indicator should
act immediately in the production of the final form of diagram.
DYNAMOMETER, GAUGES, AND CATARACT.
242. _Q._--What other gauges or instruments are there for telling the
state, or regulating the power of an engine?
_A._--There is the counter for telling the number of strokes the engine
makes, and the dynamometer for ascertaining the tractive power of steam
vessels or locomotives; then there are the gauge cocks, and glass tubes, or
floats, for telling the height of water in the boiler; and in pumping
engines there is the cataract for regulating the speed of the engine.
243. _Q._--Will you describe the mechanism of the counter?
_A._--The counter consists of a train of wheel work, so contrived that by
every stroke of the engine an index hand is moved forward a certain space,
whereby the number of strokes made by the engine in any given time is
accurately recorded. In most cases the motion is communicated by means of a
detent,--attached to some reciprocating part of the engine,--to a ratchet
wheel which gives motion to the other wheels in its slow revolution; but it
is preferable to derive the motion from some revolving part of the engine
by means of an endless screw, as where the ratchet is used the detent will
sometimes fail to carry it round the proper distance. In the counter
contrived by Mr. Adie, an endless screw works into the rim of two small
wheels situated on the same axis, but one wheel having a tooth more than
the other, whereby a differential motion is obtained; and the difference in
the velocity of the two wheels, or their motion upon one another, expresses
the number of strokes performed. The endless screw is attached to some
revolving part of the engine, whereby a rotatory motion is imparted to it;
and the wheels into which the screws work hang down from it like a
pendulum, and are kept stationary by the action of gravity.
244. _Q._--What is the nature of the dynamometer?
_A._--The dynamometer employed for ascertaining the traction upon railways
consists of two flat springs joined together at the ends by links, and the
amount of separation of the springs at the centre indicates, by means of a
suitable hand and dial, the force of traction. A cylinder of oil, with a
small hole through its piston, is sometimes added to this instrument to
prevent sudden fluctuations. In screw vessels the forward thrust of the
screw is measured by a dynamometer constructed on the principle of a
weighing machine, in which a small spring pressure at the index will
balance a very great pressure where the thrust is employed; and in each
case the variations of pressure are recorded by a pencil on a sheet of
paper, carried forward by suitable mechanism, whereby the mean thrust is
easily ascertained. The tractive force of paddle wheel steamers is
ascertained by a dynamometer fixed on shore, to which the floating vessel
is attached by a rope. Sometimes the power of an engine is ascertained by a
friction break dynamometer applied to the shaft.
345. _Q._--What will determine the amount of thrust shown by the
dynamometer?
_A._--In locomotives and in paddle steamers it will be determined by the
force turning the wheels, and by the smallness of the diameter of the
wheels; for with small wheels the thrust will be greater than with large
wheels. In screw vessels the thrust will be determined by the force turning
round the screw, and by the smallness of the screw's pitch; for with any
given force of torsion a fine pitch of screw will give a greater thrust
than a coarse pitch of screw, just as is the case when a screw works in a
solid nut.
246. _Q._--Will you explain the use of the glass gauges affixed to the
boiler?
_A._--The glass gauges are tubes affixed to the fronts of boilers, by the
aid of which the height of the water within the boilers is readily
ascertainable, for the water will stand at the same height in the tube as
in the boiler, with which there is a communication maintained both at the
top and bottom of the tube by suitable stopcocks. The cocks connecting the
glass tube with the boiler should always be so constructed that the tube
may be blown through with the steam, to clear it of any internal concretion
that may impair its transparency; and the construction of the sockets in
which the tube is inserted should be such, that, even when there is steam
in the boiler, a broken tube may be replaced with facility.
247. _Q._--What then are the gauge cocks?
_A._--The gauge cocks are cocks penetrating the boiler at different
heights, and which, when opened, tell whether it is water or steam that
exists at the level at which they are respectively inserted. It is unsafe
to trust to the glass gauges altogether as a means of ascertaining the
water level, as sometimes they become choked, and it is necessary,
therefore, to have gauge cocks in addition; but if the boiler be short of
steam, and a partial vacuum be produced within it, the glass gauges become
of essential service, as the gauge cocks will not operate in such a case,
for though opened, instead of steam and water escaping from them, the air
will rush into the boiler. It is expedient to carry a pipe from the lower
end of the glass tube downward into the water of the boiler, and a pipe
from the upper end upward into the steam in the boiler, so as to prevent
the water from boiling down through the tube, as it might otherwise do, and
prevent the level of the water from being ascertainable. The average level
of water in the boiler should be above the centre of the tube; and the
lowest of the gauge cocks should always run water, and the highest should
always blow steam.
248. _Q._--Is not a float sometimes employed to indicate the level of the
water in the boiler?
_A._--A float for telling the height of water in the boiler is employed
only in the case of land boilers, and its action is like that of a buoy
floating on the surface, which, by means of a light rod passing vertically
through the boiler, shows at what height the water stands. The float is
usually formed of stone or iron, and is so counterbalanced as to make its
operation the same as if it were a buoy of timber; and it is generally put
in connection with the feed valve, so that in proportion as the float
rises, the supply of feed water is diminished. The feed water in land
boilers is admitted from a small open cistern, situated at the top of an
upright or stand pipe set upon the boiler, and in which there is a column
of water sufficiently high to balance the pressure of the steam.
249. _Q._--What is the cataract which is employed to regulate the speed of
pumping engines?
[Illustration: Fig. 38. ]
_A._--The cataract consists of a small pump-plunger _b_ and barrel, set in
a cistern of water, the barrel being furnished on the one side with a
valve, _c_, opening inwards, through which the water obtains admission to
the pump chamber from the cistern, and on the other by a plug, _d_, through
which, if the plunger be forced down, the water must pass out of the pump
chamber. The engine in the upward stroke of the piston, which is
accomplished by the preponderance of weight at the pump end of the beam,
raises up the plunger of the cataract by means of a small rod,--the water
entering readily through the valve already referred to; and when the engine
reaches the top of the stroke, it liberates the rod by which the plunger
has been drawn up, and the plunger then descends by gravity, forcing out
the water through the cock, the orifice of which has previously been
adjusted, and the plunger in its descent opens the injection valve, which
causes the engine to make a stroke.
250. _Q._--Suppose the cock of the cataract be shut?
_A._--If the cock of the cataract be shut, it is clear that the plunger
cannot descend at all, and as in that case the injection valve cannot be
opened, the engine must stand still; but if the cock be slightly opened,
the plunger will descend slowly, the injection valve will slowly open, and
the engine will make a gradual stroke as it obtains the water necessary for
condensation. The extent to which the cock is open, therefore, will
regulate the speed with which the engine works; so that, by the use of the
cataract, the speed of the engine may be varied to suit the variations in
the quantity of water requiring to be lifted from the mine. In some cases
an air cylinder, and in other cases an oil cylinder, is employed instead of
the apparatus just described; but the principle on which the whole of these
contrivances operate is identical, and the only difference is in the
detail.
251. _Q._--You have now shown that the performance of an engine is
determinable by the indicator; but how do you determine the power of the
boiler?
_A._--By the quantity of water it evaporates. There is, however, no very
convenient instrument for determining the quantity of water supplied to a
boiler, and the consequence is that this element is seldom ascertained.