1318
THEORY AND PRACTICE OF ENGINEERING.
Book II.
roof. The ribs that form this arch are framed into a beam, which connects the rafters inthe middle of their length : within this large arch is introduced another, which has itsspringing on a level with the base line of the roof, and this is supported by two brackets, orhalf arches, issuing from the springers of the main arch ; each of the trusses acts like an arch,and by placing their springing below the tops of the walls, a better abutment is obtained.
Eltham Hall is internally 101feet 4 inches in length, and 36 feetin width, and is divided into 6compartments or bays; the lengthof the rafters is afiout four-fifths ofthe span, so that it has not so greata pitch as that of WestminsterIlall. In this example the archesare obtusely pointed, and struckfrom four centres. The trussesat Eltham have a greater quantityof timber in proportion to theirbreadth than those of Westminster,but the walls and buttresses arcmuch lighter.
Crosby Ilall , finished about1470, is internally 69 feet by 27,and is another fine example of theseadmirable works in oak timber :three ranges of pendants form thechief ornament and variety.
Hampton Court Great Hall wascompleted about 1537; its di-mensions internally are 106 feetby 40 feet; the flattened pitchat the top is a novelty in thiskind of construction.
The Hall of Christ Church ,
Oxford , is 115 feet by 40 feet; thatofTrinity College, Cambridge, 100feet by 40 feet; the New Hall atBorcham, 90 feet by 40 feet; thatof the Middle Temple, London ,
100 feet by 44 feet; LambethPalace, 93 feet by 38 feet; Guild-hall, London , 153 feet by 48 feet.
In examining the principles ofthe beautiful roofs, of which we have so many remaining in England, we find thatthe pressure against the walls is sometimes too oblique, and that the buttresses haveyielded after the strong plates, laid upon the walls, had given way, as is the case atEltham. These timber roofs originated in the reign of Edward III. , and were commonto all large halls: they were executed with the best quality of well-seasoned oak timber,admirably put together with oak pins ; the mouldings all cut out of the solid, and setout upon simple principles ; the halving, mortising, tenoning, and framing, performedwith the greatest nicety, and no iron work of any kind, not even a nail, used ; their pitchvaries in mt>st examples, but the loftiest were the earliest built. The greater portion wereoriginally covered with lead, of a weight equivalent to 10 pounds to the foot superficial.
The roofs of some of the early buildings in Brittany, where timber is not abundantlyproduced, are exceedingly simple in their construction ; the principals have the form of apointed arch,or inverted boat, and are formed on masonry or rubble work: the walls are car-ried up upon them to the necessary height, and then cut to the rake of the roof. The wailsof the houses are brought up to a level, and then quarter circles described determinethe commencement of the roof. In many parts of Germany , barns are erected uponthis principle, using timbers instead of masonry: fir planks are united in the form of apointed arch, into which a piece of timber is framed resting upon the top of a plate,mortised on to the head of an upright post: upon this is laid a rafter, which carriesa number of purlins to receive the boarded covering. The same system is adopted inHolland to houses of two or more stories ; the principals are formed of solid timber, andplaced about 12 feet apart; the outer walls are framed together, and the spaces betweenthe timbers filled in with brick ; the timbers are painted yellow, the rooms admirablyarranged within, forming convenient houses at a moderate cost for people of the workingclasses.
Fig.*2lOG.