When the roof approaches the circular form, but not sufficiently to have the character of a dome, it may be considered as consisting of several trusses resembling those of an ordinary roof, but so contrived as to intersect each other in the centre; the king-post being conimoni to all the trusses. Fig. 22, representing a design for a polygonal roof, from Nii.d.on, may illustrate this, and exemplify also some of the arneations t f Iron strap : o shows the term of the strap by which the t are secured to the king post; the post having as many (Acta, and the strap as many rums, as there are trueses in the roof.
Though the number of cunt ri%,mova for the of roofs is very great, as may be seen by refen nco to various works on carpentry, allusion can here by made to only one or two others. An admirably simple plan for making a very tat roof is described in the 37th volume of the' Tratwactions' of the S.wiety of Arts. ha as communication from the inventor, Mr. Smart. The basun* or rafters are cut, with a circular saw. as shown at e, fly. 23, while b represents their form when in use, Colonel limy, generally speaking, had a span of about CO feet. A sketch of one of them is appended (fiy. 24) and.a description of the a wedge being inserted between the ends of the Islas that are elovated into a sloping position. These may be raised to an anglo of 10° or and will bear a great weight, as they cannot be depressed without thrusting off the ends of the beam, or breaking the lower part of it by tension. This is called, IT the inventor, the bow and string rafter, and was used by him to au port a roof at the Ordnance Wharf, West minster Bridge. Strong laths were nailed upon the rafters, and on these a 'platform of bricks was laid in cement, the whole being covered with tiles also bedded and pointed with cement, and twice coated with hot linseed-oil. The coat of this roof is stated to be not more than half that of lead.
About thirty years since Colonel Emy, of the French Genie JI ilitaire, applied, on a large scale, the system of bending thin planks into arched ribs to support the rafters of roofs of considerable span, erected over riding schools and building slips, &c. ; and the system has also been applied in the construction of timber bridges for railways, with clear openings even of 133 feet. For the latter purpose the
laminated timber arches have failed, on account of the entrance of moisture between the separate lamimc, and the consequent rotting of the beams; but in roofs, or in other positions where the arches are not exposed to occasional efforts of a nature to cause the laminm to open, the beams do not rot, and there seem to be decided economical advantages in the application of Colonel Emy's system. It has been introduced in the Great Northern Railway Station in London, and in the station of the Baden Railway, at Freiburg, with great success; in both cases the spans are not less than 100 feet ; the riding schools built by mode of putting the principals together is to be found in Emy's very valuable 'Traitti do Is Charpenterie.' Colonel Ardant, also of the French Genie, has however published a work nub r the title of ' Ptudol our les Charpentes is grande portee; in I hid] he allows that these bent thither principals are not so advan. tageous, or so strong, as built beams of a polygonal form, of square timber, properly framed, and bolted together. The great objection to the use of the bent timber ram was considered by Colonel Ardant to lle In their tendency to resume the straight line, nr, in other words, tie elasticity of the gave rise to a considerable force which tended to thrust out the side walls; and in fact this danger was actually found to exist at the Great Northern Railway Station, London, to so great an extent as nearly to lead to the overthrow of the whole shed. It would, however, be easy at any time to counteract the thrust of a principal of this description, and the bent timber ribs are no doubt very convenient and economical in many positions. Local considerations, therefore, ought alone to determine the selection of either Emy, Ardaut, or PhiEbert de 1'Orme's systems of executing large timber roofs without tie-beams. Iu the main, they are ali founded on the same principle—that of introducing a rigid Loam for the purpose of supporting the principal rafter ; the differences consist in the man ner in which the physical properties of the wood are brought into action. In Emy's beam the elasticity of the bent planks is the great element of resistance; in A rdant and de fOrme's principals, the wood resists mainly efforts of compression.