brought in line, the space between the column base and the stone may be filled in with molten lead, or sulphur, or Portland cement, or it may be caulked with iron borings. It is very important that the columns be made to bear evenly over the entire surface of their bases. Many cases have been known where this has not been attended to, where the base of the column (or the stone itself) has been cracked (see Art. 153).

When the columns have been fixed and before they are finally bolted down, the connecting girders, when there are any, for carrying the principals should be lifted into their place by means of a derrick and bolted to the columns. There is little difficulty in lifting girders in this way, as they are stiff and not liable to buckle. It is different, however, in lifting principals on account of their want of lateral stiffness. If the principals are small the derrick chain may be attached to a single point, namely, the crown. It is preferable, however, to have the chain-sling attached to two points. In larger trusses two derricks may be necessary, and there should be two or more points of attachment. When a roof truss is suspended in this way it is exposed to stresses it was never designed to bear, the main tension members being subjected to compressive and the rafters to tensile stresses. In order to prevent the ties from buckling they should be stiffened by lashing light timber poles to the truss.

When the first principal is lifted, it should be securely fixed to the ground or some fixed object by means of ropes or chains, before it is released from the derrick. As each successive principal is lifted, it is lashed to the preceding one until the purlins are fixed. It is also advisable to attach the wind-ties as the work proceeds. Many accidents have been known to occur by neglecting these precautions. A gale of wind suddenly springing up has blown down many a partially-erected roof when the principals were not properly stayed or braced together.

Sometimes the covering is not put on until the whole of the framework is fixed, but the more usual plan with large roofs is to proceed in laying the covering as each bay of ironwork is completed.

Arched ribs may be lifted in one or more pieces according to the span. When in three pieces, the abutment ends are first lifted and fixed, and then the centre pieces dropped in and the connections made good. This may, as a rule, be done without any elaborate scaffolding, except in the case of very large spans, when it becomes necessary to build a stage from the ground and erect the ribs upon it. It is not necessary for the stage in such cases to extend the whole length of the roof, it being usually

made about the width of two bays, and arranged so as to travel longitudinally with the roof by running on rails laid on the ground. The same staging is used for fixing the purlins and the covering.

After the framing and covering have been fixed, the last process is the painting of the roof. The number of coats which the iron and timber work receives is usually three, though four coats are sometimes specified. The first coat is usually put on before the work leaves the contractor's yard, the remaining coats after the work is fixed.

CHAPTER XXXI.

ENGINEERS' SPECIFICATIONS OF IRON AND STEEL BRIDGes.

THE two following are examples of engineers' specifications, so far as they apply to the iron and steel work of bridges and roofs. They are selected with care, and are examples of the most recent work:

MANCHESTER, SHEFFIELD, AND LINCOLNSHIRE RAILWAY.

Extracts from Specification for Bridge Work, October, 1889.

CAST-IRON WORK.

1. Cast Iron. The cast iron must be of the best grey metal, free from cinder or graphite; the castings must be free from air-blows, honeycomb, sand, or other flaws, and the quality shall be such that a bar, 4 feet long by 1 inch wide and 2 inches deep, placed on solid supports, giving a clear span of 3 feet 6 inches, shall carry, without showing any signs of fracture, not less than 3000 lbs. placed in the centre of the bar between the supports.

2. Castings.-The castings must be clean and neat, not buckled or in any way defective, and must be in exact accordance with the drawings. All patterns must be approved before being cast from, and special care must be taken with those for the parapets. All castings when complete shall be submitted to any test that the engineer may think it necessary to apply, and any casting which may prove defective shall be replaced by another of approved quality, at the contractor's cost.

3. All bolt-holes must be drilled true to a template, and, except when otherwise specified, no cast holes will be allowed. The diameter of the

holes must not exceed that of the bolts by more than one-sixteenth of an inch.

4. All joints not specially shown on the drawings to be faced shall be provided with chipping pieces, and accurately fitted. The joints and bearing surfaces of cylinders for bridge or other foundations shall be accurately faced in the lathe.

5. No casting shall be painted until it has been examined and approved by the engineer or his inspector, after which it shall be thoroughly scraped clean, and well painted with four coats of Colley's Torbay or other approved paint. Any casting in which plugging or other attempts to conceal defects may be discovered will be at once rejected.

WROUGHT-IRON WORK.

6. Wrought Iron.-All iron shall be of best Staffordshire or other equally approved British make, and samples, selected from the bulk by the engineer or his inspector, shall be capable of withstanding in University College, London, or other approved machine, the following tests :

7. All rivets must be of best Yorkshire iron, capable of being bent double cold without fracture, and must be made out of the solid. Except where countersunk, the rivet heads are to be cupped, and they shall be free from cracks or other defects.

8. General. All plates shall be of uniform thickness thoughout, and carefully curved or bent to the required forms. All bars, plates, angles, and tees shall be of the sizes shown on the drawings, and placed in the work

so that the fibres of the iron may run in the direction of the greatest strain. The edges of all plates shall be planed, and all joints shall be true and close butts. The ends of all angle- or tee-iron stiffeners shall be cut square, and neatly dressed in a machine-saw or otherwise. The work must be prepared and the holes marked from proper templates, so as to insure a uniform width of plates in the flanges, and accurate correspondence of rivet holes through the several thicknesses. The rivet-holes shall be drilled. The rivets must be made to well fill the holes, and rivetted by machine power whenever practicable, and must be countersunk when ordered. All angles and cranks must be neatly formed with easy curve, and the work must be free from cracks, scales, ragged edges, and flaws of every kind. All girders must be built with a proper camber on the under side, which will generally be 1 inch to every 40 feet, and cover-plates and packing-strips must be fixed when necessary.

Asphalted felt is to be placed under all bearings of main girders.

9. Painting, &c.-The iron and steel work is not to be painted nor dipped in oil at the maker's works, except when the girders are put together, and then the whole of the surfaces in contact shall, before they are placed together, receive one coat of Colley's Torbay, or other approved paint. When the bridges are complete, and sufficient time has elapsed to allow the mill scale to drop off, the girders are to be well scraped, and brushed with wire brushes; and when clean and free from rust and dust, the whole of the surfaces shall receive four coats of the same paint, each coat to vary in colour, and to be left of an approved colour. Any painting damaged during the term of maintenance shall receive one additional coat or more if ordered by the engineer.

10 Hobson's Patent Flooring.-The platform plates shall be neatly fitted and rivetted into their places, and buckled where so shown on the drawings. The flooring is to be that known as Hobson's patent flooring.

11. The parapets and hand-railing shall be very neatly fitted together, felt being inserted where necessary to prevent rattling.

12. All bolts and nuts shall have a clean-cut Whitworth's thread to hold tightly, and shall be provided with washers where required. The heads of all bolts shall be formed out of the solid, and not welded on. The ends of rods and bolts shall, before securing, be swelled out so as to maintain the full sectional area of the iron at the bottom of the thread. All nuts and heads shall be of Whitworth's proportions. All bolts shall project at least one-half diameter beyond the nuts when screwed up.

13. The whole of the iron work is to be fitted together at the maker's yard, and properly marked before being taken to pieces, and when delivered on the works is to be carefully stocked, and protected from injury.

14. The contractor must provide at his own cost all necessary gantries or staging required for fixing and painting the iron work, and remove the same on completion of the contract.

15. Wherever holes are provided in floor plates, troughs, &c., for drainage they shall be fitted with tubes projecting I inch above the plate, in order to prevent the coating of the asphalte from running into and choking the holes.

STEEL.

16. All rolled steel must be homogeneous in character, free from surface defects, and capable of resisting a tensile strain of not less than 30 tons per square inch of section. It must show a contraction of area at the point of fracture of not less than 40 per cent. of the original area. The elongation before fracture shall be from 15 to 20 per cent. in a length of 10 inches. All steel must be annealed before leaving the maker's works. The tests for steel are to be conducted in the same manner as provided for wrought iron.

MAINTENANCE.

17. Maintenance. The whole of the bridge work is to be maintained by the contractor at his own cost for twelve calendar months after the opening of the railway, in good order and condition, and to the satisfaction of the engineer, without any interruption to the traffic on the railway; and all materials, workmanship, and labour of any kind whatever, which are requisite for such maintenance of works and permanent way, are to be furnished by the contractor at his own cost; and the work is to be delivered up by the contractor, in such good order and condition, as shall be satisfactory to the engineer; and should it happen that any work of repair or renewal is in course of execution at the expiration of the said twelve months, such work is, nevertheless, to be completed by the contractor at his own cost.

LANCASHIRE AND YORKSHIRE RAILWAY COMPANY.
Specification of Ironwork.

1. Wrought Iron.-The plates, bars, tees, angle-irons, &c., are to be of the best quality and all of British manufacture, capable of bearing the following strains:

That for plates lengthwise, 21 tons per square inch; contraction of area at point of fracture 8 per cent.

That for plates crosswise, 17 tons per square inch; contraction of area at point of fracture 4 per cent.

That for angle- and tee-irons, 22 tons per square inch; contraction of area at point of fracture 15 per cent.

That for flat bars, 23 tons per square inch; contraction of area at point of fracture 20 per cent.

That for round bars up to 14 inches diameter, 21 tons per square inch; contraction of area at point of fracture 17 per cent.

That for round bars above 14 inches diameter, 20 tons per square inch; contraction of area at point of fracture 15 per cent.

Should any of the tests be lower than 1 per cent. of the above strains they will be rejected. Each test shall be taken independently, and no average of results taken.

2. Testing-All the ironwork will be subjected to such tests as the engineer may direct, at the contractor's expense; and should any parts fail or be broken in the testing, or be objected to on account of bad workmanship or materials, the contractor must replace the same at his own cost.

The engineer must be advised in writing when any considerable quantity of iron is in the contractor's works.