430 
NATURE 
ae 
[ Sept. 3, 1885 
in constructive engineering can be overcome, which would 
otherwise have been most formidable. 
Mr. William Jones read a paper on processes for the recovery 
of tar and ammonia from blast-furnaces fed with raw coal. The 
coal generally employed is what is known as Scotch splint coal ; 
it contains on an average 40 per cent. of volatile matter and 50 
to 55 per cent. of fixed carbon, Theaverage amount of nitrogen 
in the coal is 1°35 per cent. ; if all this nitrogen was evolved as 
ammonia and this again converted into sulphate, it would 
amount to 1424 lbs. of pure sulphate, equal to 152°8 Ibs. of com- 
mercial sulphate containing 24 per cent. of real ammonia ; in 
blast-furnace practice only 17 to 20 per cent. of the theoretical 
quantity, or 25 to 28 Ibs. per ton, is recovered, whilst in gas- 
works 144 per cent. is evolved. Two methods are mainly em- 
ployed: the one depending on the condensation or cooling of 
the gas from the blast-furnace, and the other on the treatment 
of the hot gas with either dilute sulphuric acid or sulphurous 
acid, which absorbs the ammonia in the gas ; towers or scrubbers 
have to be used for washing the gas in both methods of treat- 
ment. The paper contains a detailed description of the various 
processes employed in carrying out these two methods for the 
recovery of by-products. The make of sulphate of ammonia 
from blast furnaces in Scotland has been greatly exaggerated. 
Even a year hence, when the whole plant being laid down will 
be available, the make will not exceed 4000 tons per annum. If 
the gases of the whole of the blast-furnaces in Scotland at 
present in blast were being treated for ammonia, the turn-out of 
sulphate of ammonia would be some 18,000 tons per annum, 
equal to about 22 per cent. of the present British production. 
With the discussion of this paper proceedings on Wednesday 
began ; after which Mr. J. Riley read a paper descriptive of an ex- 
perimental cupola-furnace, which it is proposed to employ in con- 
nection with the open-hearth process, with the object of shorten- 
ing the time employed. Many years ago Mr. Hackney tried at 
Landore the experiment of pre-melting the pig-iron in a cupola, 
whence the fluid charge was quickly and readily transferred to 
the melting-furnace. Instead, however, of saving three or four 
hours by charging fluid metal, it was found, on repeating the 
experiment at Hallside, that there was only a saving of about a 
quarter of an hour in time, obtained at the expense of the coke 
and labour expended at the cupola. This is due to the circum- 
stance that during the melting of a charge in the open-hearth 
furnace a large proportion of the silicon and carbon is removed, 
leaving little more than half the carbon to be eliminated in sub- 
sequent operations. Now in the case of the fluid charges, which 
had been pre-melted with coke in the cupola, these changes have 
not taken place, and the time required to remove the impurities 
from the fluid metal, after being charged on the open-hearth 
furnace, is almost as long as that required to melt and purify the 
solid charge. The idea occurred to the author to substitute 
gaseous for solid fuel in the cupola. The gas generator has a 
closed grate and is dependent upon forced blast, and the air for 
supporting combustion in the body of the furnace is also obtained 
from the blower, and is heated. 
The experiments made with this cupola prove that not only is 
there a saving in time and fuel, but that the percentage of silicon 
and carbon in the pig-iron and steel scraps are very much re- 
duced, so that it is anticipated that when the fluid metal can be 
charged direct into the open-hearth furnace, the time for its 
conversion into mild steel will be greatly shortened. 
This paper caused a very lively discussion, with which the 
proceedings on Wednesday terminated. 
On Thursday amongst other papers was one descriptive of the 
Forth Bridge by Mr. Baker, which we print below 2 extenso. 
In the afternoon of each day the members visited various steel 
and iron works in the neighbourhood. 
THE FORTH BRIDGE? 
AS the members of the Iron and Steel Institute purpose paying 
a visit to the Forth Bridge works, I have been requested 
by the Secretary to prepare a short paper on the subject for the 
information of the members, and do so with pleasure. 
The North British Railway Company for many years have 
striven hard to obtain a physical connection of their lines north 
and south of the Forth by means of a bridge. Twenty years 
ago they were authorised by Act of Parliament to build a bridge 
across the Forth at a point five miles above the site of that now 
« Paper read at the Glasgow meeting'of the Iron and Steel Institute by 
Mr. Benjamin Baker, M. Inst.C.E. 
under construction, but borings 120 feet in depth showed nothing 
but soft silt and mud, and the bridge, which was to have been 
two miles in length, inclusive of four spans of 500 feet each, was 
luckily abandoned, as the difficulties with the foundations would 
have proved practically insuperable. In 1873 another Act was 
passed for a bridge across a narrower and deeper part of the 
Forth at Queensferry. At low water the width of the channel 
there is about 4000 feet ; and the island of Inchgarvie affording 
| a foundation for a central pier, it was possible to cross the 200 
feet deep portion of the sea-way by a couple of spans from 1600 
feet to 1700 feet each in the clear. Sir Thomas Bouch prepared 
a design for this bridge on the suspension principle, with towers 
665 feet in height from base to summit, and the contract for its 
construction was let to Mr. Arrol. Owing to the subsequent 
fall of the Tay Bridge, public confidence in Sir Thomas Bouch’s 
design was shaken, and in session 1881 a bill for the abandon- 
ment of the Forth Bridge was proceeded with. Whilst in Com- 
mittee, the different companies interested, namely, the North 
British, Great Northern, North-Eastern, and Midland Railway 
Companies, ordered a final reference of the whole question to 
their respective consulting engineers, with the result that the 
abandonment bill was dropped, and the design for a cantilever 
or continuous girder bridge prepared by Mr. Fowler and myself, 
in consultation with Mr. Harrison and Mr. Barlow, was substi- 
tuted for the original suspension bridge. In 1882 the necessary 
Parliamentary powers were obtained, and in January 1883 the 
works were commenced by Messrs. Tancred, Arrol, and Co., 
the contractors. 
The total length of viaduct included in the contract sum of 
1,600,000/, is about 14 miles, and there are— 
2 spans of 1710 feet each. 
2 ” 675 ” 
T5te |e 168 ” 
5 29 25 9 
Including piers, there is thus one mile of main spans, and half a 
mile of viaduct approach. The clear headway is 1§0 feet above 
high water, and the tops of the great cantilevers are more than 
200 feet higher still. There will be about 45,000 tons of steel 
in the superstructure of the bridge, and 120,000 cubic yards of 
masonry in the piers. 
Piers.—The South Queensferry main pier consists of a group 
of four cylindrical piers of masonry and concrete, founded by 
means of pneumatic caissons on the strong boulder clay consti- 
tuting the bed of the Forth at this point. Owing to the slope 
of the clay, the caissons required to be sunk to depths varying 
from about 70 feet to 90 feet below high water. The diameter 
ranges from 70 feet at the base to 60 feet at low-water level, 
above which the iron skin of the caisson is replaced by a facing 
of Aberdeen granite. At the base of the caissons is a working 
chamber 7 feet in height supplied with compressed air, and elec- 
trically lighted, for the men excavating the material. This 
chamber was kept clear of water by a pressure of air considerably 
less, as a rule, than that due to the head of water outside. For 
example, at 90 feet below high water, when the north-east 
caisson had been sunk through a considerable thickness of silt, 
the air-pressure required to be maintained at 18lbs. per square 
inch only, although at the reduced depth of 57 feet it was found 
convenient to work at 30 lbs. air-pressure. Three shafts and 
air-locks were provided for each |caisson, two for the excavated 
material, and one for the men. The former had two horizontal 
sliding doors actuated by small hydraulic rams, and the skip 
containing the clay and boulders was hoisted up the 9go-feet 
shaft by a steam-engine mounted on the side of the air-lock. As 
a rule, from 200 to 300 skips of excavated material were raised 
per day of 24 hours bya force of from 20 to 3o men. ‘The 
maximum number of skip-loads was 363, and of men 33. The 
size of the skips was 3 feet diameter by 4 feet 3 inches high. 
Owing to the extreme hardness of the clay it was necessary to 
provide a certain number of spades having hydraulic rams in the 
handles, which, abutting against the roof of the working 
chamber, sliced the clay readily. 
At the present time three of the South Queensferry caissons 
have been sunk successfully to the full depth, and the fourth and 
last would also have been completed but for an unfortunate 
accident which happened to it at the beginning of the year. By 
some means the caisson, which had been floated into position for 
some weeks, accidentally filled with water, and sank and slid 
forward on the mud. It is now being carefully cased in timber 
to admit of the water being pumped out and the caisson floated 
again into position. 
