structed on this principle, with both girders and joists of rolled iron, and with a row of cast-iron columns down the centre of the room, supporting a line of rolled-iron girders, which pass through a hole in the top of the column, as in Fig. 9.

The joists are carried over the backs of the girders, and are rolled of a length to span the full width of the building, including the bearing on each of the side-walls. It is an important advantage (peculiar to the use of wrought iron), to have the joists rolled in long lengths, and fixed with an intermediate support on the main girders, as an efficient tie is thus given to the building, and the strength of the floor is greatly increased. The ends of the joists are either pierced and secured to the

slightest vibration in the floors; whilst in the Staffordshire potteries, where the floors are formed with cast-iron girders and brick arches, and nearly double the thickness, with the same works going on, the vibration is so great as to impart a disagreeable tremor to the voice of any one speaking, while standing upon them."

walls by a rod, or S piece, or they may be bolted to a plate of iron built into the wall. The extreme ends of the girders being also secured to the wall, a tie in each direction is given, and the whole forms a framework of wrought iron, possessing immense strength, each part affording efficient aid to the rest. In buildings of great width, the additional rows of columns would enable the same arrangement to be adopted; the joists being, in that case, rolled in long and short lengths, placed alternately, so as to break joint, and bolted, or riveted together, at the points where they take a bearing on the girders.

Many trials have been made of the transverse strength of the rolled-iron joists, with the object of ascertaining to what extent they may be loaded, without affecting their elasticity; and, comparing the results obtained, with the strength of similar joists of cast iron, and taking 450 lbs. on the centre, as the breaking weight of an inch-square bar of the latter material 4 feet 6 inches long, and assuming, for the sake of comparison, that the wrought, or rolled iron may be safely loaded to its elastic limit, and the cast iron to one-third of its breaking weight, the relative strength of the rolled, and of the cast iron, of the same section, would be in round numbers, as 2 to 1.

These experiments have, however, been carried, in some cases, much beyond the elastic limit of the iron, and it has been found, that an increase of the load, or strain, of 25 to 60 per cent., has caused a permanent set of only of an inch, in a length of joist of 16 feet, between the points of support. These results serve to show how little the elasticity is injured, by a load far exceeding that which is practically applied.

to

It will be seen, that the great object, in this system, is to have the building so constructed as, in the event of fire, to confine it to the locality where it originates; an object of great importance in all cases, but particularly in mills and buildings of that description, where fires not unfrequently happen, from causes which the utmost care will scarcely prevent. That fires should extend, in these buildings, with such extraordinary rapidity is quite intelligible, when it is considered, that the floors themselves afford the fuel, and that they are so constructed as, by an unlimited supply of air, to aid, rather than to

arrest, the work of destruction; and that they are moreover not unfrequently completely saturated with oil.1

The question of cost is of much importance in connexion with this system of construction: with every desire, however, to place this point in its true light, yet, differing as estimates for buildings general do, it is far more difficult than would at first appear, to state absolutely, either the actual, or the relative expense. In instituting a comparison, something will necessarily depend upon locality; the relative prices of timber and iron varying considerably in different places. Much will also depend upon the nature of the finished surface adopted, and on the kind of timber floor with which the comparison is made. Instances have occurred, in which the adoption of the system having been made to turn wholly on the question of cost, the contractor has given the proprietor the option, either of employing timber construction of the commonest kind, or the fire-proof principle, with a surface of cement, or gypsum, at the same expense in either case. There can be little doubt, that in the cases, where a surface of Portland cement is adopted, the fire-proof construction can generally be executed at about the same cost as well-constructed and substantial timber-floors; but when a surface of flooring-boards is required to be laid upon the fire-proof foundation, some additional cost may attend the adop

' The practical value of concrete, in resisting the effects of intense heat, in a building on fire, was shown at the destruction of Alderman Humphery's premises, near London-bridge, in 1851. The following is extracted from "The Times" report of Feb. 20:-" As floor after floor gave way, dropping their blazing contents into the stories below, the volume of flame increased. The immense mass of burning material gradually sank, until it rested upon the ceiling of the ground-floor. It now became a matter of surprise, that this floor, which contained very large stores of provisions, such as butter, cheese, bacon, &c., did not ignite. The arrival of Alderman Humphery solved the problem. He explained, that after the disastrous fire, by which Fenning's wharf and the adjacent premises were destroyed, he had determined to render the basement story fire-proof. Accordingly he had formed the ceiling of the basement floor of a thick layer of concrete." "The engines continued to play upon the enormous mass of burning material, which lay piled to a considerable height above the basement, and the weight and heat of which rendered it very doubtful how long the concrete flooring would withstand the severe test to which it was put. Fortunately the fear proved groundless, and during the whole of the day no appearance of any increase of the mischief was manifested."

tion of the principle, so long as the present exceptional high price of iron continues. The difference, however, is trifling, when compared with the many advantages thus secured.

An example, affording a close approximation, is given at page 263, of the comparative cost of a floor for a room of average size, in a dwelling-house, whether constructed of timber, or on the fire-proof principle, the calculations being based upon average prices; and it will be seen, that the differences are very inconsiderable. In order to establish an accurate comparison of cost, the details of the prices, which are there given, should be compared with those of any particular locality, and be corrected accordingly. The estimated comparative cost of the floor of a mill, or factory, whether constructed with timber,-with wrought-iron girders and joists, concrete, &c.,—or with cast-iron girders and brick arches,— is also given. For the floors of warehouses, no general statement can be given, as the cost will vary with the required degree of strength, and other circumstances. The same remark will also apply to the cost of floors, where the bearings are such as to require the use of main girders and minor joists; the expense depending greatly upon the distances between the girders, which the requirements of the building may render necessary.

The cost of roofs differs little from that of floors, and is governed very much by the nature of the external, or finishing surface adopted.

The introduction of any new process, in the ordinary arts of life, is always attended with difficulty, and often meets with great opposition; and any departure from a mode of construction in general use, and which has been sanctioned by so many successive generations, must necessarily be a work of considerable time. The prejudices of some and the doubts of others have to be removed; and the opposition of parties interested in resisting any change has frequently to be encountered: to which, perhaps, may be added the excess of caution, which induces a certain degree of suspicion of the soundness of the theories propounded. Hence the mere substitution of hoopiron for the timber bond, almostly universally employed, until

a very recent period, was regarded with a considerable degree of distrust and suspicion, and for years after the introduction of this material, its adoption was opposed by those whose prejudices, or interests, were stronger than their desire for improvement. If, however, these remarks are true, in reference to a minor alteration, such as that just referred to, it might readily be supposed, that they would apply, with much greater force, to so radical a change as the one now submitted to the Institution. The advantages, however, of the substitution of hoop-iron for timber bond are now universally acknowledged,—and thus it is with almost every real improvement,-its progress, though slow, is steady and certain, and, like truth, it will ultimately prevail.

The desirableness of fire-proof construction will not, it is presumed, be denied; the simple fact of nearly half a million sterling being annually paid, in London alone, for fire-insurance, exclusive of the duty, placing this point in the strongest light, merely as a commercial question. Upwards of one thousand fires now occur annually in the metropolis, and the official inquiries as to the supposed causes of these fires show, that more than one-half of those which reach the structure of a building are considered to have originated in defective, or overheated chimney-flues and stoves, acting upon the timbers; it is therefore manifest, that the substitution of iron for timber joists would have prevented many of these fires. There is reason to fear that some of them are the work of the incendiary; and while the general adoption of a system of fire-proof construction would prove a remedy for a fruitful source of crime, there is no doubt that Insurance Companies themselves would ultimately be considerable gainers.

It is, however, important, that any system of fire-proof construction, claiming universal applicability, should, besides being free from those objections which have been referred to, as applying to timber, be also simple and economical; and these points, it is hoped, have been rendered apparent as belonging to the system described.

Although in the course of these remarks reference has necessarily been made, and an objection stated, to the iron-girder and