Frederick in experimental attempts, continued over a period of ten
years, at the construction of the regenerative gas furnace. At length,
in 1861, he proposed the application of his furnace to an “open hearth,”
and during the next few years some partial attempts to carry out his
process were made, and he himself had established experimental works at
Birmingham in order to mature his processes, while Messrs. Martin of
Sireuil, in France, having obtained licences under Siemens’ patents,
gave their attention to a modification of his process, by which they
succeeded in producing excellent steel. Siemens having in 1868 proved
the practicability of his plans by converting at his Birmingham works
some old phosphorized iron rails into serviceable steel, a company was
formed, and in 1869 the Landore Siemens’ Steel Works were established at
Landore in Glamorganshire, and a few years after, these had sixteen
Siemens open hearth melting furnaces at work, giving a total output of
1,200 tons of steel per week. The number of furnaces was subsequently
increased. Extensive works specially designed for carrying out the
Siemens and the Siemens-Martin process were shortly afterwards erected
at other places, as at Newtown, near Glasgow, Panteg in Wales, etc. In
Great Britain the open hearth process gradually gained upon the
Bessemer, until in 1893, when the total output of both kinds amounted to
nearly 3,000,000 tons, this was almost equally divided between them, and
since that period the steel made by the former has greatly surpassed in
amount that made by the latter.
How the regenerative stove, or heat accumulator, works, and how it is
applied in the open hearth process, the reader may learn by aid of the
diagram Fig. 26, in which however no representation of the disposition
of the parts in any actual furnace is given, nor any details of
construction beyond what is necessary to make the principle clear. On
the right and on the left of the diagram will be seen a pair of similar
chambers which are shown as partly below the level of the ground S S´,
such being a usual disposition. The outer walls of these chambers are
thick and the interior is entirely lined with the most refractory
fire-bricks, of which also is formed the partition in between each pair
of compartments, as well as the passages from the top of each opening on
the furnace H. Each chamber or compartment is filled with rows of
fire-bricks, laid chequerwise so as to leave a multitude of channels
between. At the bottom of the chamber on the left let us suppose
atmospheric air to be admitted by the channels A, A, A, and a
combustible gas which we may take to be a mixture of carbonic oxide with
some hydrogen is admitted in the same way to the second compartment on
the left through the passages G, G, G. Supposing the apparatus quite
cold in the first instance, the gas would ascend into the furnace H as
shown by the arrows, because it might be drawn by an up-draught in a
chimney connected with the six chambers shown at the bottom of the
right, and it would also tend to rise up into the space H by its lighter
specific gravity, and there it could be set on fire, when a volume of
flame would pass across to the right, a plentiful supply of air rushing
in through the air chamber from A, A, A, and the products of the
combustion, mainly hot carbonic acid gas and hot nitrogen gas, in
passing through the right-hand chambers, would make the bricks in both
compartments very hot after a time, for the current would divide itself
between the two passages, as indicated by the divided arrow. We have not
shown the valves by which the workman is able, by merely pulling a
lever, to shut off the air supply from A, A, A, and of gas from G, G, G,
and put these channels into direct communication with the up-draught
chimney, at the same time supplying gas at G´, G´, G´, and air at A´,
A´, A´. These rise up among the now heated bricks each in its own
compartment, but mix where they enter the furnace H, now hot enough to
set them on fire, and the gaseous products of combustion, hotter now
than before, descend among the fire-bricks of the left-hand
compartments, heating them in turn. After another period, say half an
hour, the valves are again reversed, and again gas and air both heated
burn in the space H, and their products supply still more heat to the
right-hand compartments. And so the action may be continued with a great
temperature each time produced by the combustion of the combining bodies
at increasingly higher temperatures. Thus, if cold gas and air by
combination give rise to 500° of heat, when the same combine, at say the
initial temperature of 400°, the result would be a temperature of 900°;
if burnt at this latter degree, then 900° + 500° would be reached, and
so on. It would seem as if there were no limit to the temperatures
obtainable in this way. But the nature of the materials of which the
furnace is constructed imposes a limit, for even the most refractory
matters yield at length, and the working would come to an end by the
fusing of the brickwork. The diagram is a section through the length of
the hearth (for it is usually oblong in plan), and the low arch above H
being exposed to the fiercest heat, is formed of the most refractory
“silica bricks,” that is, bricks made of coarsely ground silica held
together with a little lime; yet this extremely resisting material is
acted upon, and the arch has to be renewed every few months or sometimes
weeks. The hearth itself is supported by massive iron plates, shown in
the diagram by the thick lines, above which is laid a deep bed L, of
quartz sand or ganister, or where required a _basic_ lining, beaten hard
down, and forming a kind of basin with sides sloping down in all
directions to a point immediately below the centre of the fire-brick
door D, where is the aperture for tapping, stopped by a mixture of sand
and clay until the metal is ready for drawing off, when it runs outside
into an iron spout lined with sand and is received into the ingot
moulds. B in the figure represents the “bath,” as it is called, of
molten metal, which, in the larger furnaces, where 20 tons of metal is
operated on at once, may occupy an area of 150 square ft.
It need hardly be mentioned that there has to be a certain adjustment
between the volumes of air and of gas that pass into the regenerative
stoves, in order that the best effect may be obtained. Besides the limit
of temperature occasioned by the nature of the materials, there is a
chemical reason why the regenerative stoves cannot increase the
temperature indefinitely. It is noticed that when the temperature of the
furnace has become very high indeed, the flame over the hearth assumes a
peculiar appearance, being interrupted by dark spaces. These are
attributable to what is called in chemistry “dissociation,”—in this case
the dissociation of carbonic acid gas, which by the heat alone separates
into carbonic oxide and oxygen gases. In the same way these gases refuse
to combine if brought together heated beyond a certain temperature. This
phenomenon of dissociation is a general one, for it is found that for
any pair of substances there is a characteristic range of temperature
above or below which they refuse to combine. The gas used in these
stoves is either unpurified coal gas, or that produced by passing steam
over red-hot coal or coke.
We have spoken of the Siemens and the Siemens-Martin open hearth
processes. In the latter a charge of pig iron, say 1½ tons, is first
melted on the hearth, then about 2 tons of wrought iron is added in
successive portions, and in like manner nearly as much scrap steel
(_i.e._ turnings, etc.), the final addition being half a ton of
spiegeleisen containing 12 per cent. of manganese. A furnace of
corresponding dimensions will allow of three charges every twenty-four
hours. In the Siemens process it is not wrought iron or steel scrap that
is mainly used to decarbonize the pig, but a pure oxide ore. This is
thrown into the bath of molten metal in quantities of a few cwts. at a
time, when a violent ebullition occurs. When samples of the metal and of
the slag are found to be satisfactory, spiegeleisen or ferro-manganese
is added, and the charge is cast. This process takes a rather longer
time than the former, but gives steel of more uniform character. In both
processes, phosphorus is oxidized at the high temperature attained and
passes into the slag, which last floats of course on the molten metal
and is from time to time tapped off as the action proceeds.
[Illustration:
FIG. 26_a_.—_Rolling Mill._
]
Fig. 26_a_ shows a rolling mill with what is called a “two-high” train
for finishing bars by passing them between the grooves cut in the rolls
to give the required section. The rolls in the illustration turn in one
direction only, and therefore the bars after emerging from the larger
grooves have to be drawn back over the machine and set into a smaller
pair from the same side. This inconvenience is avoided in the
“three-high train,” on which three rolls revolve, and the bars can be
passed through them from one side to the other alternately. The celerity
with which a glowing steel ingot is without re-heating converted into a
straight steel rail 60 or 100 feet long, by passing a few times
backwards and forwards between the rolls, is very striking. These rolls
are made of solid steel, and in some cases have a diameter of 26 inches
or more.
_IRON IN ARCHITECTURE._
Everyone knows how much iron is used in those great engineering
structures that mark the present age, and of which a few examples will
be described in succeeding articles. One other feature of the nineteenth
century is the use of iron in architecture. Some have, indeed, protested
against the use of iron for this purpose, and would even deny the name
of architecture to any structure obviously or chiefly formed of that
material. Stone and wood, they say, are the only proper materials,
because each part must be wrought by hand, and cannot be cast or
moulded; and further, iron being liable to rust, suggests decay and want
of permanence, and these are characters incompatible with noble
building. All this can rest only on a relative degree of truth—as, for
instance, machinery is used to dress and shape both wood and stone, and
the permanence of even the latter is as much dependent on conditions as
that of iron. Iron used in architecture is hideous when applied in
shapes appropriate only to stone; but when it is disposed in the way
suggested by its own properties, and receives ornament suitable to its
own nature, the result is harmonious and graceful, and the structure may
display beauties that could be attained by no other materials. Be that
as it may, the great and lofty covered spaces that are required for our
railway stations and for other purposes could have been obtained only by
the free use of iron, and everyone can recall to mind instances of such
structure not devoid of elegance, in spite of the absence—the proper
absence—of the Classic “orders” or Gothic “styles.” The first notable
instance of the application of iron on a large scale was the erection of
the “Crystal Palace,” in Hyde Park, for the great Exhibition of 1851. It
was taken down and re-erected at Sydenham, and there it has become so
well known to everyone that any description of it is quite unnecessary
in this place.
As another conspicuous example of what may be done with iron, the Eiffel
Tower at Paris may be briefly described.
The idea of erecting a tower 1,000 feet high was not of itself new. It
had been entertained in England as early as 1833, in America in 1874,
and in Paris itself in 1881. It has been reserved for M. Gustave Eiffel,
a native of Dijon, who commenced to practise as an engineer in 1855, to
realize this ambitious project. He has long been occupied in the
construction of great railway bridges and viaducts, and in these he has
adopted a system peculiar to himself of braced wrought-iron piers
without masonry or cast-iron columns. He also was the first French
engineer to erect bridges of great span without scaffolding. In the
Garabit viaduct he planned an arch of 541 feet, crossing the Truyère at
a height of nearly 400 feet above it. One result of M. Eiffel’s studies
in connection with these lofty piers was his proposal to erect the tower
for the Paris Exhibition of 1889. This proposal met with great
opposition on the part of many influential people in Paris—authors,
painters, architects, and others protesting with great energy against
the modern Tower of Babel, which was, as they said, to disfigure and
profane the noble stone buildings of Paris by the monstrosities of a
machine maker, etc. etc. The Eiffel Tower is now constructed, and no one
has heard that it has dishonoured the monuments of Paris, for it has
been instead a triumph of French skill, the glory of its designer, and
the wonder of the Exhibition.
[Illustration:
FIG. 26_b_.—_The Eiffel Tower in course of construction._
]
The tower rests on four independent foundations, each at the angle of a
square of about 330 feet in the side, and it may be noted that the two
foundations near the Seine had to be differently treated from the other
two, where a bed of gravel 18 feet thick was found at 23 feet below the
surface, and where a bed of concrete, 7 feet thick, gave a good
foundation. The foundations next the river had to be sunk 50 feet below
the surface to obtain perfectly good foundations. Underlying the whole
is a deep stratum of clay; but this is separated from the foundations by
a layer of gravel of sufficient thickness. Above this are beds of
concrete, covering an area of 60 square metres, and on the concrete
rests a pile of masonry. Each of the four piles is bound together by two
great iron bars, 25 feet long and 4 inches diameter, uniting the masonry
by means of iron cramps, and anchoring the support of the structure,
although its stability is already secured by its mere weight. The tower
is of curved pyramidal form, so designed that it shall be capable of
resisting wind pressure, without requiring the four corner structures to
be connected by diagonal bracing. The four curved supports are, in fact,
connected with each other only by girders at the platforms on the
several stages, until at a considerable length they are sufficiently
near to each other to admit the use of the ordinary diagonals. The work
was begun at the end of January, 1887, and M. Eiffel notes how the
imagination of the workmen was impressed by the notion of the vast
height of the intended structure. Not steel, but iron is the material
used throughout, and the weight of it is about 7,300 tons, without
reckoning what is used in the foundations, and in the machinery
connected with the lifts, etc. It has long ago been found that stone
would be an unsuitable material for a structure of this kind, and it is
obvious that only iron could possibly have been used to build a tower of
so vast a height and within so short a space of time, for it was
completed in April, 1889. A comparison of heights with the loftiest
stone edifices may not be without interest. The highest building in
Paris is the dome of the Invalides, 344 feet; Strasburg Cathedral rises
to 466 feet; the Great Pyramid to 479 feet; the apex of the spire in the
recently completed Cathedral at Cologne to 522 feet. These are
overtopped by the lofty stone obelisk the Americans have erected at
Washington, which attains a height of more than 550 feet. Such spires
and towers have been erected only at the cost of immense labour. But
iron, which can be so readily joined by riveting, lends itself
invitingly to the skill of the constructor, more particularly by reason
of the wonderful tensile strength it possesses. It is scarcely possible
to convey any adequate idea of the great complicated network of bracings
by which in the Eiffel Tower each standard of the columns is united to
the rest to form one rigid pile. The horizontal girders unite the four
piers in forming the supports of the first storey some 170 feet above
the base. The arches which spring from the ground and rise nearly to the
level of these girders are not so much intended to add to the strength
of the structure as to increase its architectural effect. The first
storey stands about 180 feet above the ground, and is provided with
arcades, from which fine views of Paris may be obtained. Here there are
spacious restaurants of four different nationalities. And in the centre
of the second storey (380 feet high) is a station where passengers
change from the inclined lifts to enter other elevators that ascend
vertically to the higher stages of the tower. On the third storey, 900
feet above the ground, there is a saloon more than 50 feet square,
completely shut in by glass, whence a vast panorama may be contemplated.
Above this again are laboratories and scientific observatories, and,
crowning all, is the lighthouse, provided with a system of optical
apparatus for projecting the rays from a powerful electric light. This
light has been seen from the Cathedral at Orléans, a distance of about
70 miles.
[Illustration:
FIG. 26_c_.—_The Eiffel Tower._
]
The buildings of the Paris Exhibition of 1889 are themselves splendid
examples, not only of engineering skill, but of good taste and elegant
design in iron structures and their decorations. The vast _Salle des
Machines_ (machinery hall) exceeds in dimensions anything of the kind in
existence, for it is nearly a quarter of a mile long, and its roof
covers at one span its width of 380 feet, rising to a height of 150 feet
in the centre. This great hall is to remain permanently, as well as the
other principal galleries with their graceful domes.
The Eiffel Tower having proved one of the most striking features of the
great Paris Exhibition, and of itself a novelty sufficient to attract
visitors to the spot, and having, long before the Exhibition closed,
completely defrayed the expense of its construction, with a handsome
profit besides, its success has naturally provoked similar
enterprises,—as, for instance, at Blackpool, a seaside resort in
Lancashire, there has been erected an openwork metal tower, resembling
the Paris structure, but of far less altitude.
_Tall Buildings in American Cities._
In several of the great cities of the United States, the last few years
have witnessed a novel and characteristic development of the use of iron
in architecture. In many structures on the older continent, this
material has been frankly and effectively employed, forming the obvious
framework of the erection, even when the leading motive was quite other
than a display of engineering skill. The Crystal Palace at Sydenham and
other erections have been referred to, in which iron has taken its place
as the main component of structures designed more or less to fulfil
æsthetic requirements: the guiding principle in “tall office buildings”
in the cities of the Western continent is, on the contrary, avowedly
utilitarian. Iron has, of course, long been used in the form of pillars,
beams, etc., in ordinary buildings, and it is only the extraordinary
extension of this employment of it, after the lift or elevator had been
perfected, and the ground-space in great commercial centres was daily
becoming more valuable, that has led to the erection of structures of
the “sky-scraper” class in American cities. For a given plot at a stated
rent, a building of many stories, let throughout as offices, will
obviously bring to its owner a greater return than one of few stories.
The elevators now make a tenth story practically as accessible as a
third storey, and the tall building readily fills with tenants. No claim
for artistic beauty has been advanced for these structures, which aim
simply at being places of business, and if provision be made for
sufficient floor-space and daylight, and for artificial lighting,
heating, and ventilation, together with the ordinary conveniences of
modern life, and ready elevator service, nothing more is required by the
utilitarian spirit, that seeks only facilities for money-getting. These
tall buildings are usually erected on plots disproportionately small,
and the architectural effect is apt to be bizarre and incongruous,
especially when the structure shoots up skyward in some comparatively
narrow street amid more modest surroundings. They are really engineering
structures, but invested with features belonging to edifices of quite
another order of construction. If they are necessities of the place and
period, and are “come to stay,” it cannot be doubted but that decoration
of an appropriate and harmonious character will, in course of time, be
evolved along with them, when the conventionality that clings to
architecture shall be broken through, and a new style appear, as
consistent, and therefore as beautiful, in relation to the “tall office
building,” as were those of the Greek temple and the Gothic minster in
their free and natural adaptation.
[Illustration:
PLATE IV.
THE AMERICAN TRACT SOCIETY BUILDING.
]
[Illustration:
FIG. 26_d_.—_St. Paul Building, N. Y._
]
Here, apparently, is the opportunity for the advent of a new and
characteristic style. There is great ingenuity displayed in the
arrangement and internal finish of these buildings. But besides the
somewhat novel application of iron, the most notable circumstances
regarding them are the tendency to make them of greater and greater
height, and the wonderfully short time in which, upon occasion, they can
be run up. Chicago has recently been noted for its tall edifices, among
which may be named _The Reliance Building_, erected upon a site only 55
feet in breadth, but rising in fourteen stories to the height of 200
feet, and presenting the appearance of a tower. There are no cast iron
pillars, but the whole metal framework is of rolled steel, the columns
consisting of eight angle-sections, bolted together in two-story
lengths, adjoining columns breaking joint at each floor, and braced
together with plate girders, 24 inches deep, bolted to the face of the
columns, with which they form a rigid connection. Externally, the
edifice shows nothing but white enamelled terra-cotta and plate glass.
This building was originally a strongly-built structure of five stories,
the lower one being occupied as a bank. The foundations and the first
story were taken out, and prepared for the lofty edifice, the
superstructure being the while supported on screws. Then the three upper
stories were taken down, and the building was continued from the second
story, which was filled with tenants while the building was in course of
erection above.
[Illustration:
FIG. 26_e_.—_Manhattan Insurance Co.’s Building, in course of
erection._
]