ordinary Greek moldings with their French names. However necessary it
must be for the architect, and however admirable it may be for the art
student, to know the names of all moldings by heart and to be able
to describe each one accurately, such proficiency is not required at
present and is not necessary for the understanding of the present
theme. Some moldings have square edges, some round. The curved edges
of some are simple, of others complex. Each has its name, and of some
the name is descriptive. The term molding would seem to indicate that
moldings were made apart and subsequently applied to the main object.
Whatever be the origin of moldings, the same rules apply to them which
apply to other profiles, with the additional rule that moldings must
always be kept subordinate to the principal object. For instance, in
the view (Fig. 6) the pedestal marked _bon_ is good, because the body
of the pedestal is the principal object and it is clearly seen that
the moldings at the base and at the top are subordinate and merely
ornamental, while the pedestal marked _mauvais_ is decidedly bad,
because more vertical space is given to the moldings than to the shaft,
confusing outline, weakening the shaft, and destroying the sense of
strong and steady support.
Readers may at once make use of the information already acquired by
seeing how these rules apply to their own lamps, candlesticks, pieces
of furniture, etc.
The next view (Fig. 7) shows incidentally how much better it is under
all circumstances to mark with fillets and lines the changes from one
curve to another, for you certainly see how much more substantial
character and beauty has _B_ than _A_.
[Illustration: FIG. 11.]
Finally, let it be said, and said emphatically, that though there
are profiles which require the use of the compass to draw them, and
though all architectural details must be worked out with mathematical
accuracy, those profiles and outlines are the most beautiful where it
is evident that artistic skill governing a free hand has controlled and
where mechanical assistance is so subordinate as to be overlooked.
There is very little to be said about surfaces or forms of two
dimensions. The principal requirement is that outlines should be
agreeable and must be well defined. In fact, the two qualities are
inseparable, for a well-defined outline is agreeable and a badly
defined one is sure to be disagreeable. By well-defined is meant
that its particular shape should easily appear and be clearly
distinguishable. For instance, a square should appear with sides
distinctly equal; a circle should have but one center. In an
architectural opening either arch or entablature should prevail, and
the character of the arch should be evident. In the examples presented
(Fig. 8) in the view these principles are violated. The first figure
is so clearly a square that at first, and before you have examined
it closely, you think it is a square. It leaves an indefinite and
consequently disagreeable impression. The same criticism applies to
the second object, apparently a mirror. The glass is round, but the
frame is so irregular that the impress of the circle is destroyed, and
there is left an undecided and therefore uncomfortable sensation. In
the third example the arch is so poorly defined and so weak, while the
entablature above it is so strong and so prominent, that the result
is a composition that fails to give pleasure, because no distinct
idea is conveyed. In the last example the outlines of the arch are so
indefinite that its character is indistinguishable. You can not see
which prevails, the round arch or the pointed arch.
[Illustration: FIG. 12.]
The same principles apply to smaller objects and to details, as seen
in the next view (Fig. 9). To the left the date plate on top is bad
in comparison with the one beneath it, because its direction is not
so well marked and its corner projections are too large. In the
lambrequins on the right, those are good in which the general direction
is properly marked, and in which subdivisions are kept properly
subordinated. Lambrequins have so entirely gone out of use nowadays
that it is difficult to recall the time when they were regarded as
indispensable parts of furniture.
There is one other point to which your attention should be called--that
is, stability. If an object be intended to stand, its center of
gravity should be so well within its base that there will be no danger
of its being upset by the ordinary uses to which it is exposed. Pots
and pans, pitchers, lamps, and candlesticks, of general and daily
household use, should have bases so broad and weight so low that the
accidental bump of the inexperienced “help” will not be inevitably
fatal.
When utensils are made more for show than for use, as those in Fig.
10, and are to occupy places of comparative security, beauty more than
utility may be considered in the proportions of their supports. Where
utility has disappeared altogether and the suggested outline of a vase,
for instance, is used for purely ornamental purpose, supports may be
done away with altogether, as appears in these drawings of Italian
tapestries of the seventeenth century (Fig. 11).
The stability of pendant objects must also be considered. It is
evident that the perpendicular line of suspension must be the line of
equilibrium, and that these two must correspond with the design (Fig.
12). Whether any objects should under any circumstances be exposed to
the real and apparent danger of falling is a question. We have got so
into the habit of hanging pictures, engravings, and other works of art
in our houses, and of seeing them hung in galleries, that we have lost
sight of the incongruity of the custom. Pictures should be impaneled,
and be permanent parts of the walls on which they appear. But, then,
how could they be moved when owners tire of them, or tire of their
houses, or how could they be gathered together in museums for purposes
of study and public enjoyment? Picture frames are of comparatively
modern invention. The idea of buying a picture for the purpose of
selling it again was not entertained before the fifteenth century.
Pictures were as substantial parts of churches and houses as were
shrines and fireplaces.
Having very cursively reviewed the elements of form, we are in a
position to understand decoration, which is simply the application to
form of ornament.
* * * * *
The highest authenticated points at which flowering plants have
heretofore been found growing upon the Andes are at about 17,000
feet, although the Kew Herbarium contains several specimens labeled
as having been found at altitudes of from 17,000 to 18,000 feet.
Sir Martin Conway has brought back from his recent explorations in
the Bolivian mountains at least half a dozen species from 18,000
feet and upward, the highest being from about 18,500 feet. They
include a saxifrage, a mallow, a valerian, and several _Compositæ_.
_Compositæ_ likewise attain the upper limit of phanerogamous
vegetation in Thibet, where, in latitudes from 30° to 34°, one was
found by Dr. Thorold at 19,000 feet.
STEAM TURBINES AND HIGH-SPEED VESSELS.[F]
BY THE HON. CHARLES A. PARSONS, F. R. S.
[F] Abstract of the Presidential Address to the Institution of
Junior Engineers, November 3, 1899.
All heat engines at present in use take in heat from a source at a
high temperature and discharge most of it at a lower temperature, the
disappearance of heat in the process being the equivalent of the work
done by the engine. In all cases at the present time the source of heat
is from fuel of some kind, and after working the engine the residue
is discharged in the case of the steam engine either to the condenser
or in the exhaust steam when non-condensing. In the gas engine it is
discharged in the waste gases and into the water jacket around the
cylinder.
The earliest records of heat engines are found in the Pneumatics of
Hero of Alexandria, about 200 B. C. He describes a reaction steam
turbine, a spherical vessel mounted on axes supplied with steam through
one of the trunnions from a boiler beneath; the steam escaping through
two nozzles diametrically opposite to each other and tangential to the
sphere, causing the sphere to rotate by the reaction or momentum of the
issuing steam, and analogous to a Barker’s water wheel.
Thus, the first engine deriving its motive power from fuel was a
crude form of steam turbine, and though it could have been applied to
useful work, and could easily have been made sufficiently economical
to replace manual and horse power in many instances, yet it lay
dormant till 1629 A. D., when Bianca suggested the same principle in
a different form. Bianca’s steam turbine consisted simply of a steam
jet fed from a boiler impinging against vanes or paddles attached to
the rim of a wheel which was blown round by the momentum of the steam
issuing from the jet.
The piston engine is, however, of comparatively modern origin, and
dates from about the year 1700 A. D. Engines of this class are so well
known that it suffices to say that they have been practically the sole
motive-power engines from fuel in use from 1700 up to 1845, and have
constituted one of the most important factors in the development of
modern engineering enterprise.
Air engines were introduced about the year 1845, and although the
larger engines of the Stirling type were very economical in fuel, yet,
on account of the inherent difficulty of heating large volumes of air
within metal chambers or pipes--a difficulty arising from the low
conductibility of air and consequently the overheating and burning
of the metal--they have only come into commercial use for very small
powers.
[Illustration: FIG. 1.--LONGITUDINAL DIAGRAM OF PARSONS’S STEAM
TURBINE, WITH SIDE OF CONTAINING CASE REMOVED. The construction of
the blades and guide vanes is more clearly shown in Fig. 2. The steam
enters at _J_ and exhausts after leaving the low-pressure cylinder _C_.]
During the last thirty-five years gas engines have been perfected, and
more recently oil engines, and in point of efficiency both convert
a somewhat larger percentage of the heat energy of the fuel into
mechanical energy than the best steam engines. All successful oil and
gas engines are at present internal-combustion engines, the fuel being
burned in a gaseous form inside the working cylinder.
Very numerous attempts have, however, been made to construct
internal-combustion engines to burn solid fuel instead of gas. Some
have been so far successful as to work with good economy in fuel,
but the bar to their commercial success has been the cutting of the
cylinder and valves by fine particles of fuel. This difficulty is not
present when the fuel is introduced in the gaseous or liquid form, and
hence the success of gas and oil engines; but could this difficulty be
overcome, the solid fuel would be the cheaper to use.
Internal-combustion engines, gas engines, oil engines, cannon, etc.,
owe their superior economy in fuel to the very high temperature at
which the heat is transferred from the fuel to the working substance
of the engine, and consequently the great range of temperature in the
working substance of the engine. In steam engines the temperature is
limited by the practical difficulties of deterioration of metal and
materials involved in the construction.
About fifteen years ago I was led by circumstances to investigate
the subject of improving the steam turbine. In recent times several
attempts had been made to apply steam turbine wheels of the Hero and
Bianca types to the driving of circular saws and fans. The velocity
of rotation with either of these types must necessarily be very high
in order to obtain a reasonable efficiency from the steam, a velocity
much in excess of that suitable for the direct driving of almost all
classes of machinery; gearing was considered objectionable, and it
therefore appeared desirable to adopt some form of turbine in which the
steam should be gradually expanded in small steps or drops in pressure
so as to keep the velocity of flow sufficiently low to allow of a
comparatively moderate speed of rotation of the turbine engine.
The method adopted was to gather a number of turbines of the parallel
flow type on to one shaft and contained in one case, the turbines
each consisting of a ring of guide and a ring of moving blades, the
successive rings of blades or turbines being graduated in size, those
nearer the exhaust end being larger than those near the steam inlet, so
as to allow a gradual expansion of the steam during its passage through
the turbines.
The form of the turbine was that of a rotating drum, with outwardly
projecting rings of blades which nearly touched the containing
cylindrical case, and on the case inwardly projecting rings of guide
blades which nearly touched the drum. In the first examples of the
engine there were two groups of turbines right-and left-handed on each
side of the steam inlet, the exhaust taking place at each end of the
turbine case, so as to completely balance end pressure from the steam.
More recently one series of turbines only has been used, those on
the other side of the steam inlet being replaced by packing rings or
rotating balance pistons which balance the end pressure and divert the
whole of the steam through the turbines on the other side.
[Illustration: FIG. 2 SHOWS THE ARRANGEMENT OF MOVING BLADES AND GUIDE
VANES IN A PARSONS’S TURBINE. The top outer cover has been removed.
The cylinder containing the revolving barrel has, as will be seen, a
greater internal diameter than the diameter of the drum. It is the
annular space thus formed through which the steam flows and which
contains the revolving blades and the fixed guide blades. Between each
two rings of moving blades there is a ring of guide blades, the latter
being keyed into the containing case. The vanes are set at an angle, so
that the steam acts on them as wind on the sails of a windmill.]
The steam entering the annular space between the shaft and the case
passes firstly through a ring of guide blades attached to the case,
and is given a rotational direction of flow; it then passes to the
succeeding ring of blades attached to the shaft, by which its direction
of rotation is reversed, thereby impressing the difference of its
rotational momentum in torque to the shaft. The steam then passes to
the second ring of guide blades, and the process is repeated, and so
on, gradually expanding by small increments at each ring of blades; the
succeeding rings of blades get longer and wider, and at intervals the
diameter of the turbine drums, cylinders, and rings are also increased.
In condensing turbine engines of the larger size an expansion ratio in
the turbines of one hundredfold and upward is attained before the steam
passes to the exhaust pipe and condenser.
The loss of power present in engines of the piston class, due to
cylinder condensation arising from the variation of steam pressure
in the cylinder, is not present in the steam turbine, as the steam
pressure remains constant at each turbine ring and each part of the
cylinder and barrel, and the numerous tests of steam consumption that
have been made have shown that compound steam turbine engines of
moderate sizes when working with a condenser are comparable in steam
consumption per effective horse power with the best compound or triple
condensing steam engines of the piston type. They have been constructed
in sizes up to about one thousand horse power for driving alternators
and dynamos, and several sets of about two thousand horse power are
nearing completion.
The application of the compound steam turbine to the propulsion of
vessels is a subject of considerable general interest, in view of the
possible and probable general adoption of this class of engine in fast
vessels.
In the turbine is found an engine of extremely light weight, with a
perfectly uniform turning moment, and very economical in steam in
proportion to the power developed, and, further, it can be perfectly
balanced so that no perceptible vibration is imparted to the ship. The
problem of proportioning the engine to the screw propellers and to the
ship to be driven has been the subject of costly experiments extending
over several years, with the result that a satisfactory solution has
been found, giving very economical results in regard to pounds of
steam consumed in the engines per effective horse power developed in
propelling the vessel, results which are equal or superior to those so
far obtained with triple-expansion engines of ordinary type in torpedo
boats or torpedo-boat destroyers. The arrangement adopted may be best
described by saying that instead of placing, as usual, one engine to
drive one screw shaft, the turbine engine is divided into two, three,
or sometimes more separate turbines, each driving a separate screw
shaft, the steam passing successively through these turbines; thus
when there are three turbines driving three shafts, the steam from the
boiler passes through the high-pressure turbine, thence through the
intermediate, and lastly through the low, and thence to the condenser.
[Illustration: FIG. 3.--A SEVENTY-FIVE KILOWATT TURBINE ENGINE DIRECTLY
CONNECTED TO A DYNAMO. The turbine engine is on the right.]
As to the propellers, these approach closely to the usual form. It has,
however, been found best to place two propellers of approximately the
same pitch on each shaft at some considerable distance apart, so that
the after one shall not be seriously affected by the wash of the one
in front. The advantage of this arrangement is that a sufficient blade
area is obtained to carry the thrust necessary to drive the vessel with
a lesser diameter of propeller, and so permitting of a higher speed of
revolution of the engines.
The problem was complicated by the question of cavitation, which,
though previously anticipated, was first practically found to exist
by Mr. Thornycroft and Mr. Barnaby in 1894, and by them it was
experimentally determined that cavitation, or the hollowing out of the
water into vacuous spaces and vortices by the blades of the propeller,
commences to take place when the mean thrust pressure on the projected
area of the blades exceeds eleven pounds and a quarter per square
inch. This limit has since been corroborated during the trials of the
Turbinia.
This phenomenon has also been further investigated in the case of
model propellers working in an oval tank of water, and to permit of
cavitation at more moderate speeds than would otherwise have been
necessary, the following arrangement was adopted: The tank was closed,
plate-glass windows being provided on each side, through which the
propeller could be observed, and the atmospheric pressure was removed
from the surface of the water by an air pump; under this condition the
only forces tending to prevent cavitation were the small head of water
above the propeller, and capillary attraction.
In the case of a propeller of two inches in diameter, cavitation
commenced at about twelve hundred revolutions, and became very
pronounced at fifteen hundred. Had the atmospheric pressure not been
removed, speeds of twelve thousand and fifteen thousand respectively
would have been necessary.
Photographs were taken with a camera made for the purpose, with a focal
plane shutter giving an exposure of about one thousandth of a second,
the illumination being by sunlight concentrated on the propeller from a
twenty-four-inch concave mirror.
Photographs were also taken by intermittent illumination of the
propeller from an arc lamp, the arrangement consisting of an ordinary
lantern condenser, which projected the beam on to a small concave
mirror, mounted on a prolongation of the propeller shaft, the reflected
beam being caught by a small stationary concave mirror at a definite
position in each revolution and reflected on to the propeller. By this
means the propeller was illuminated in a definite position at each
revolution, and to the eye it appeared as stationary. The cavities
about the blades could also be clearly seen and traced, the photographs
being taken with an ordinary camera and about ten seconds’ exposure.
A series of experiments was also made with model propellers in water
at and just below the boiling point, dynamometric measurements being
taken of power and thrust with various widths of propeller blade, the
conclusion arrived at being that wide and thin blades are essential for
fast speeds at sea, as well as a coarse pitch ratio of propeller.
The first vessel fitted with steam turbine machinery was the Turbinia.
She was commenced in 1894, and, after many alterations and preliminary
trials, was satisfactorily completed in the spring of 1897. Her
principal features are: Length, one hundred feet; beam, nine feet;
five-foot draught of water under the propellers; forty-four tons and
a half displacement on trial; she is fitted with a water-tube boiler
of eleven hundred feet total heating surface, and forty-two square
feet of grate area, with closed stoke-holds supplied with air from a
centrifugal fan mounted on a prolongation of the low-pressure turbine
shaft. The engines consist of three compound steam turbines, high
pressure, intermediate, and low pressure, each driving one screw shaft;
on each of the shafts are three propellers, making nine in all; the
condenser is of the usual type, and has four thousand square feet of
surface.
When officially tested by Professor Ewing, F. R. S., assisted by
Professor Dunkerley, she attained a mean speed on a measured mile of
thirty-two knots and three quarters, and the consumption of steam
for all purposes was computed to be fourteen pounds and a half per
indicated horse power of the main engines. Subsequently, after some
small alterations to the steam pipe, she was further pressed, and is
estimated to have reached the speed of thirty-four knots and a half.
She was, and still is, therefore, the fastest vessel afloat; she has
been out in very rough weather, is an excellent sea boat, and at all
speeds there is an almost complete absence of vibration.
In the Turbinia the exceptional speed results principally from two
causes: 1. The engines, screws, and shafting are exceptionally light.
2. The economy of steam in the main engines is greater than usual.
At full speed the steam pressure in the boiler is two hundred and ten
pounds; at the engines, one hundred and seventy-five; and the vacuum in
the condenser twenty-seven inches, representing an expansion ratio in
the turbines of about one hundred and ten after allowance has been made
for wire-drawing in the exhaust pipe.
The first vessels of larger size than the Turbinia to be fitted with
steam turbine machinery are the torpedo-boat destroyer Viper for
the British Government, and a similar vessel for Messrs. Sir W. G.
Armstrong, Whitworth & Company.
[Illustration: FIG. 4.--THE TURBINIA RUNNING ABOUT FORTY MILES AN HOUR.]
These vessels are of approximately the same dimensions as the
thirty-knot destroyers now in her Majesty’s service, but have slightly
more displacement. The boilers are about twelve per cent larger,
and it is estimated that upward of ten thousand horse power will be
realized under the usual conditions, as against sixty-five hundred with
reciprocating engines.
The engines of these vessels are in duplicate. Two screw shafts are
placed on each side of the vessel, driven respectively by a high- and
a low-pressure turbine; to each of the low-pressure turbine shafts
a small reversing turbine is permanently coupled for going astern,
the estimated speed astern being fifteen knots and a half, and ahead
thirty-five knots; two propellers are placed on each shaft.[G]
[G] On her second trial trip the Viper attained a mean speed of
34.8 knots, her fastest trial being over 35 knots, or about
41 statute miles per hour, with an indicated horse power of
11,000. This vessel is of about 350 tons displacement.