Principles are generally very soon determined, and progress ensues, not by additions to the principles, but by improvement in the methods of giving to those principles a practical shape, or by combining in one structure principles of construction which had been hitherto used apart. Therefore, to avoid the necessity of having a pause, in referring to a work, by finding that one is overstepping the boundary of principle, and trenching within the domain of construction, I think it will be well to treat these two heads together.
If my record had gone back to just before 1851 (the date of the great exhibition), I might have described much progress in the principles of girder construction; for shortly prior to that date, the plain cast-iron beam, with the greater part of the metal in the web, and with but little in the top and bottom flange, was in common use; and even in the preparation of the building for that exhibition, it is recorded that one of the engineers connected therewith had great difficulty in understanding how it was that the form of open work girder, with double diagonals introduced therein (a form which was for years afterward known as the exhibition girder), was any stronger than a girder with open panels separated by uprights, and without any diagonals. But, long before 1862, the Warren and other truss-girders had come into use, and I am inclined to say that, so far as novelty in the principle of girder-construction is concerned, I must confine myself to that combination of principles which is represented by the suspended cantilever, of which the Forth Bridge, only now in course of construction, affords the most notable instance. It is difficult to see how a rigid bridge, with 1,700 foot spans, and with the necessity for so much clear headway below, could have been constructed without the application of this principle.
BRIDGE CONSTRUCTION.
Pursuing this subject of bridge work, the St. Louis Bridge of Mr. Eads may, I think, be fairly said to embody a principle of construction novel since 1862, that of employing for the arch-ribs tubes composed of steel staves hooped together. Further, in suspension bridges there has been introduced that which I think is fairly entitled to rank among principles of construction, the light upper chain, from which are suspended the linked truss-rods, doing the actual work of supporting the load, the rods being maintained in straight lines, and without the flexure at the joints due to their weight. In the East River Bridge, New York, there was also introduced that which I believe was a novelty in the mode of applying the wire cables. These were not made as untwisted cables and then hoisted into place, thereby imposing severe strains upon many of the wires composing the cable through their flexure over the saddles and elsewhere, but the individual wires were led over from side to side, each one having the length appropriate to its position, and all, therefore, when the bridge was erected, having the same initial strain and the same fair play. Within the period we are considering, the employment of testing-machines has come into the daily practice of the engineer; by the use of these he is made experimentally acquainted with the various physical properties of the materials he employs, and is also enabled in the largest of these machines to test the strength and usefulness of these materials, when assembled into forms, to resist strains, as columns or as girders. I of course do not for one moment mean to say that experimental machines were unknown or unused prior to 1862—chain cable testing-machines are of old date, and were employed by our past President, Mr. Barlow, and by others, in their early experiments upon steel; but I speak of it as a matter of congratulation that, in lieu of such machines being used by the few, and at rare intervals upon small specimens, for experimental purposes, they are now employed in daily practice and on a large scale.
In harbor work we have had the principle of construction employed by Mr. Stoney at Dublin, where cement masonry is moulded into the form of the wall for its whole height and thickness, and for such a length forward as can be admitted, having regard to the practical limit of the weight of the block, and then, the block being carried to its place, is lowered on to the bottom, which has been prepared to receive it, and is secured to the work already executed by groove and tongue.
It would not be right, even in this brief notice of such a mode of construction, to omit mention of the very carefully thought out apparatus by which the blocks are raised off the seats whereon they have been made, and are transported to their destination. It is no simple undertaking (even in these days) to raise (otherwise than hydraulically) a weight of 350 tons, which is the weight of the blocks with which Mr. Stoney deals. But he does this by means of pulley-blocks attached to shears built on the vessel which is to transport the block, and he contrives to lift the weight without putting upon his chains the extra strain due to the friction of the numerous pulleys over which they pass. The height of the lift is only the few inches needed to raise the block clear of the quay on which it has been formed, and this is obtained by winding up the chain by steam gear quite taut, so as to take a considerable strain, but not that equal to the weight of the block, and then water is pumped into the opposite end of the vessel to that upon which the shears are carried, this latter end rises, and the block is raised off the seat on which it was formed, without the chains being put to work to do the actual lifting at all. The vessel, with the block suspended to the shear legs and over the bows, is then ready to be removed to the place where the block has to be laid. A word must here be said about an extremely ingenious mode of dealing with the slack chain, to prevent its becoming fouled, and not paying out properly, when the block is being lowered. This is accomplished by reeving the slack of each chain over two fixed sets of multiple sheaves.
A donkey-engine works a little crab having a large drum, the chain from which is connected with the main chain, and draws it round the multiple sheaves so as to take up the slack as fast as the main crab gives it out. The steam is always on the donkey, which is of such limited dimensions that it can do no injury to the chain even when its full power is in vain endeavoring to draw it any further; directly, however, the main crab gives more slack, and the chain between it and the two sets of sheaves falls into a deeper catenary, and one which therefore puts less opposition to the motion of the donkey-engine, that engine goes to work and makes a further haul upon the slack, and in this way, and automatically, the slack is kept clear.
PNEUMATIC FOUNDATIONS.
A noteworthy instance of the use of pneumatic appliances in cylinder sinking for foundations is that in progress at the Forth Bridge. The wrought-iron cylinders are 70 feet in diameter at the cutting-edge, and have a taper of about 1 in 46. They are, however, at a height of 1 foot above low water (that is, at the commencement of the masonry work of the pier) reduced to 60 feet in diameter; at their bottoms there is a roofed chamber, into which the air is pumped, and in which the men work when excavating, this roof being supported by ample main and cross lattice girders. Shafts with air-locks and pipes for admitting water and ejecting silt are provided. The air-locks are fitted with sliding doors, worked by hydraulic rams, or by hand, the doors being interlocked in a manner similar to that in which railway points and signals are interlocked, so that one door cannot be opened until the other is closed. The hoisting of the excavated material is done by a steam engine fixed outside the lock, this engine working a shaft on which there is a drum inside the lock, the shaft passing air-tight through a stuffing box. A separate air-lock, with doors, ladder, etc., complete, is provided to give ingress and egress for the workmen. I have already adverted to one Scotch bridge; I now have to mention another, viz., the Tay Bridge, also now in course of construction. Here the cylinders are sunk, while being guided, through wrought-iron pontoons, which are floated to their berths, and are then secured at the desired spot by the protrusion, hydraulically, of four legs, which bear upon the bottom, and thus, until they are withdrawn, convert the pontoon from a floating into a fixed structure.