The thrust, as to show as one long, pointed vault below. which is immense, is resisted by walls 6 feet thick, with huge buttresses. The upper part of the construction is very good; but how the part between AB can sustain the weight of the upper part without bending has puzzled everybody. It can, in effect, be only due to the excellence of the workmanship and strength of the materials.

On looking back to fig. 40, it will be seen not only that the curved brace was intended to strengthen the rafters, but also to relieve the thrust by conveying it lower down the wall, and distributing it over a much larger portion of its surface, as well as bringing it more in a line with the

tween the end of the brace A, and the top of the ashlerpiece B. To obviate this, various contrivances were used;' one of the most elegant is the addition of curved braces, which not only strengthened each rafter, but tended to brace the whole together. Fig. 39 is a common example in England, particularly in Somersetshire.

As those roofs with simple couples could not be erected of any great span, they were commonly framed with trusses, purlins, and common rafters; and then the defect of these open roofs became more felt, as each truss had not only to bear its own load, but that

Fig. 42.

buttress AA (fig. 42). Fig. 43 shows the beautiful roof over the church at Wymondham in Norfolk. Here the Fig. 43.

of the adjacent common rafters. The usual means of strengthening the roofs last described, that of introducing curved braces to the couples, was adopted for the truss collar-beam roofs, till at length they assumed the form of an arch, filling in the space between the principal rafter and collar like a rib of solid timber. Fig. 40 is from St Mary's, Leicester, and is a very good example. But the most extraordinary medieval roof, on the principle of a collar-beam and curved braces, is that over the Salle des Pas Perdus at Rouen, which was built in 1493, and is of inconceivable lightness and boldness (see fig. 41). It is 54 feet 5 inches English in span, and covers a hall 155 feet long, the trusses being not quite 4 feet apart. It is close-boarded inside, so

brace at the foot of the rafter is composed of two curved pieces, which meet together in a horizontal piece carved as the figure of an angel. This construction, at once so elegant and useful, soon became developed as the hammerbeam roof. Figure 42 is from a church in Suffolk. In this A, A are the wall buttresses; B the hammer-beams; C the wall-pieces, or, as some call them, the pendant-posts; D the hammer-braces; E the collar; F the collar-braces; G the side-posts; H the ashler-pieces; I, I the purlins; K, K the principal rafters. There is an infinite variety of those roofs in England, chiefly of the perpendicular period. As they were required of a larger span, the roof became a double hammer-beam roof. Figure 44 shows the general section of these: the nomenclature is the same, with the addition that A is the upper hammer-beam, B the upper hammer-brace, and C the upper side-post. Of these,

Fig. 44.

lege halls. It will be seen by the section, the use of the hammer-beam framing was to deepen, as it were, the principal rafter, and thereby prevent its bending between the collar and the plate; besides which it got better hold of the wall; but there was nothing to tie the walls together, nor to keep the truss from spreading; the consequence is, in spite of the buttresses, many of these roofs have opened, and the walls have gone over. A very curious roof, and one which is much stronger in point of construction, is that over the Parliament House at Edinburgh, which was begun in 1632. In this roof the pieces which act as hammerbeams are not horizontal but incline, or rather radiate towards a centre; they are filled in with arched pieces bearing pendants, and have a very original and pleasing

effect.

The use of level tie-beams in roofs is of two kinds: one where the rafters are in couples, and they seem merely introduced to tie the plates together, and probably were inserted afterwards; the other where they form parts of the trusses themselves. In large roofs, we have already an example in fig. 35. In smaller roofs of the fourteenth century, fig. 45 is a very common and pleasing example. From

Fig. 45.

the tie-beam springs a king-post, from which branch four curved struts, one pair serving to support the principal rafters, the other pair doing the same for the ridge. This is called a tree-post roof. Figure 46 shows a very curious combination of a level tie-beam and curved struts. It is from the church of St Mary the Virgin at Pulham in Nor

Fig. 46.

are of infinite variety in design, and are generally filled in with tracery, and ornamented with carving. Figure 47 shows a very beautiful example, that of St Martin's at Leicester. In many instances these roofs are richly adorned with painting and gilding of the most brilliant description. Another sort of medieval roof is yet to be noticed, and that is where, instead of timber principals, arches of stone are thrown across from wall to wall, and carry the purlins and common rafters: among these may be named the great hall at Mayfield; but they are of very rare occurrence.

Account of Roofs of Large Span (à grand portées) and some modern Roofs.

For many years the roofs of the basilicas at Rome were art; but about three-quarters of a century ago there was a the largest spans that had been covered by the carpenter's great desire to roof over much larger spaces without internal supports, for the purpose of military and other riding-schools. The vast roof over the Salle d'Exercise at Darmstadt, erected by M. Schubknecht in 1771, is 228 feet long and 154 feet in the clear of the walls, or 2 feet more in span than that roof lately erected over the railway station at Lime Street, Liverpool, and was the largest roof in the world till that we are about shortly to describe was erected over the New Street station at Birmingham. It appears to have stood very well, although its construction is certainly not on the best principles. The thickening out of the tie-beam by packing beam after beam, one on the other, must have caused considerable shrinkage; and the struts would be much better if placed the reverse way. This roof attracted considerable attention; and about ten years after its erection the Emperor of Russia, Paul I., happened to travel through Darmstadt, and visited the building. He expressed great astonishment at its vast proportions, and determined on his return that one should be erected at Moscow which should entirely eclipse it in magnitude. Accordingly the design was prepared. This gigantic roof was intended to have covered a hall 852 feet in length, by 308 feet in width from out to out. The walls were double, and formed a system of arcaded galleries round the building about 25 feet in depth; so that the span of the roof in the clear was reduced to about 230 feet. The main support of this roof was the curved rib of three thicknesses of timber, notched on to each other en cremaillière. Krafft says it was executed, and was used in 1790 for the exercise of the Cossack cavalry and infantry. But M. de Bétancourt, of whom we shall speak presently, affirms that it never was finished. The dotted lines show a method proposed by Rondelet and Krafft to strengthen this roof and make it effective. The chief defect, however, seems to be, that the

principal rafters are too weak, and receive no direct support from the cross struts. The troubles in Russia seem to have caused the matter to drop, till the year 1817, when the Emperor Alexander, being at Moscow, resolved on carrying out a roof that should rival the one at Darmstadt. A great number of designs were prepared, none exceeding spans of from 110 to 115 feet. They were referred to General de Betancourt, before named, who was then chief director of public roads. After some study, this offiecr prepared the design of it. It is for a

hall 501 feet long and 150 in width, which was executed in the short

space of five months. On striking the scaffold, which had supported the roof during its erection, the tie-beams went down about two inches. This depression increased in three months to eight inches, when the tie-beam of the twenty-fourth truss gave way, in consequence of the existence of a large knot. The roof was shored up and strengthened, and afterwards stood perfectly well. In justice to M. Betancourt, it should be stated that such was the hurry in which the work was done that the greater part of the timbers were cut down and floated on the river only a few days before they were framed, which was done by 400 carpenters, or rather woodmen, who hewed the wood with axes-they being ignorant of the use of any other tool. The writer of this article has designed a roof for the first-class swimming bath in the Westminster Road. This is on entirely a new principle, being, in fact, the adaptation of the trellis or lattice principle to a roof. The trusses are about 18 feet apart, or nearly double the usual distance. The purlins, however, are prevented from bending by a series of light longitudinal trusses, likewise on the lattice principle. The roof is, in truth, trussed fore and aft, so that no part can move. It is of extraordinary lightness and cheapness, the trusses being so few and the timbers so slender. Its deflection, after receiving the load of slates, skylights, &c., was scarcely perceptible.

(a.) Roofs Trussed with Curved Timbers.-These are (a) with timbers side by side, flatwise, the ends breaking joint, or, as the system is called by French writers, " en bois plat." It is the invention of Philibert de Lorme, a French architect, who published it in 1561, in a work called Traité sur la Manière de Bien Bàtir, et à Petits Frais. In this system the rafters are in effect curved ribs, of several thicknesses of timber, nailed side by side, care being taken that the ends do not come in the same place; in other words, that they break joint. The rib then resembles a beam cut out of a crooked limb of a tree, and owes its strength simply to cohesion of the particles. These beams are often used in pairs oined together by cross pieces, which are keyed on the

to domes intended to be covered with lead or other metal. One of the most ingenious adaptations of this principle is at the Hotel Legion d'Honneur at Paris, where it forms a dome, with an inner half-dome or cove which carries a gallery. The roof over the great hall of the Pantheon in Oxford Street is a long vault on this principle, the middle thickness or flitch of each rib being of teak wood. That lately erected over the Surrey Music Hall is also a long vaulted ceiling; but here the main ribs are formed of four thicknesses of 11-inch deal, 1 foot 3 inches wide at bottom and 1 foot at top, in the middle of which is a rib of rolled ths boiler-plate, 6 inches wide, the whole of which are bolted together. The main ribs are 17 feet apart, between which are two intermediate ribs of slighter construction.

(b.) Roofs constructed of Timber bent on the flat "en bois plat."-This is the reverse of the former system; the boards being bent over a centre, and thickness added upon thickness till the beam is thought strong enough, when the whole is bolted together, care being taken, as in the former system, that the ends of the boards break joint. The difficulty is, to prevent the natural tendency of the wood to spring back to the straight, as well as to counteract the weight of the roof covering, which would cripple the curve either at the haunch or crown, as the pressure might be exercised. In this respect something analogous to the laws of arches will apply to explain the result.

The roof over the great riding-house at Libourne was designed by the celebrated Colonel Emy, and executed ir. 1826. Every rib is composed of five thicknesses of deal, each nearly 2 inches thick, about 6 inches wide and 40 feet long. The rib is semicircular, the springing about 24 feet from the ground, and the span about 70 feet. The ribs are not only bolted together, but clipped with a sort of stirrup-iron and bolts. From these ribs a number of struts radiate to and support the principal rafters, purlins, &c., and at the same time prevent the rib from being crippled at either the haunch or crown. The worst of this roof is, there is nothing to prevent their spreading at the foot. The consequence is, the wall was not only of unusual thickness (nearly 5 feet), but large buttresses were added to counteract the thrust. The roofs over the Great Northern Railway station at London are of about the same span, and are of similar construction, except that the ribs are of 1 deal in 16 thicknesses, and the spandrils are filled in with ornamental cast-iron work in form of circles, guilloches, &c., which have a very pretty effect. Like the Libourne

roof, the thrust was compensated by massive brickwork.

To prevent this last defect, as well as to stiffen the rib at the springing, M. Émy designed the roof for the cavalry school at Saumur. This is intended for a span of upwards of 130 feet. Each truss is composed of two sets of ribs, similar to those before described, kept apart at the foot by a series of trellis, and joining together as one rib about halfway up the curve. Instead of one thick wall, it was intended to carry the ends of the trusses partly on an outer wall, which has strong double pilasters, and partly on one large square pilaster, intended to stand under the inner part of the end of the truss, as shown in the figure given in his work; so that, though the space is available for spectators, and little room is lost, the truss itself has a level bearing 18 feet in width from whence to spring.

But of all roofs ever projected, either in wood or iron, the most gigantic, the most original, and the boldest, is one for another riding-house, also by Colonel Émy. This has a span of 328 feet, or at least half as much again as the largest existing roof in the world, that at New Street, Birmingham. It is intended to be composed of two ribs similar to those before described, with another intermediate rib carried up about two-thirds of the span, and braced, as shown in the figure. The building was to have been surrounded by an ordinary wall, at right angles to which, under the foot of every truss, was a return wall 50 feet long and 4 feet thick, perforated below with arches to form passageways, and to serve for spectators, as in the former instance.

Iron Roofs.

Like most important discoveries, the use of iron in roofs has grown up from the smallest beginnings to the largest and most extraordinary results. From the simple substitution of an iron rod for a king or queen post in a wooden truss, we have attained the art of covering spaces so vast as to throw all other modes of construction into insignificance, so light as to appear the work of fairies rather than of human beings, and yet so strong as to bear their weight with ease, and to resist unshaken and unhurt the action of the roughest wind. The theory of these roofs, however, is just the same as those of timber, allowing only for the difference of weight and power of resistance of the materials. The tie keeps the walls together, the king and queen rods prevent the tie from sagging: all these are in a state of tension; while the struts prevent the rafters from bending, and are in a state of compression. Fig. 48

Fig. 48.

shows the earliest use of iron as the king-post of roofs suited to those of from about 20 to 30 feet span. In Plate LIV, CARPENTRY, figs 38, 40, is shown the method of employing iron as king or queen posts of still larger dimensions. Fig. 49 shows a very convenient method of using iron rods as king-post and tie-beam in a collar-beam roof where height or head-room is of consequence; but this was shortly after superseded by the form fig. 50, the adjustment of the screws, &c., at A being more convenient than at

fig. 49. With large spans, however, the wooden tie-beam continued in use for a long time. One of the best in

Fig. 49.

stances for the time it was executed may be found in the roof over the passenger-station at London Bridge of the

A

Fig. 50.

Croydon Railway. This roof, which is about 54 feet in span, is constructed with an iron king-post and ten sets of iron queens, with wooden struts. (See fig. 51.) It is, how

Fig. 51. Croydon Railway Roof.

ever, somewhat twisted on the face by the winding of the timber. The great fire at the Houses of Parliament, and subsequently at the Royal Exchange, drew attention strongly to the importance of fire-proof roofs; and the first of these structures of any size or importance was designed by Sir Charles Barry for the new palace at Westminster. Fig. 52 shows the section of that over the committeerooms in the front next to the river. This is entirely of wrought iron, excepting the shoes, by which the ends of the various pieces are connected. The iron is flat bar, simply cut to lengths, and punched to receive the bolts that pass through them and the shoes. They vary in width from 2 to 3 inches, and in thickness from ths to ths. These light principals are not quite 3 feet apart, and are covered with a species of galvanized iron tile reaching from one to another, hung on a sort of connecting-rod without purlins. It is stated that the sulphurous acid in the smoke of London is already causing serious oxidation, and several methods have already been tried to prevent premature decay. This is to be regretted, as the material is very picturesque in character, as well as light, and not expensive. Some fault at the time was found with the construction of the shoes or connecting-plates, but they stand perfectly well, and there has been no failure. The suspension-rods at each side of the king are a peculiar feature; they not

These carry two common rafters 2 feet 6 inches apart, this being the width of the iron plate or tile with which the roofs are covered.

Much_animadversion has been expressed as to these roofs. The nearness of the principals has been criticised, as well as the small subdivision of detail; in fact, we shall shortly show roofs of three times the space with a less number of junctions, the principals of which are very much farther apart so in this respect they are very much more

economical. But it must be remembered these Gothic roofs are of high pitch, while the others are very flat; and therefore more struts and queens are necessary; and that the distance of the principals was regulated in great measure by the nature of the covering. If we also consider they were the first roofs of the kind, we think they may be regarded with great commendation. The progress of railways caused a great demand for light roofs of every span and length, and the facilities for obtaining rolled T and L