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THEORY AND PRACTICE OF ENGINEERING.

Book II

the construction is sosimple that few work-men were required toraise it. There arefourteen main archesor principals, and thewalls, though exceed-ingly thick, arefurther strengthenedwith buttresses: eachprincipal is composedof five thicknesses oftimber throughout;these were bent totheir form on thefloor, and then raisedin a body to theirsituation.

MaraCy near Ba-yonne. — This roof,constructed also byM. Emy, in 1826, is65 feet 6 inches span,and 187 feetinlengthjeach principal

Jl

v VAFig. 2155.

semicircle, composed of deals 2 inches in thickness, 5 inches in width,and about 40 feet in length ; they are about 10 feet apart: 2.^-inchplanks placed end to end form one arch: none of the bent timbershave more than three joints, ordinarily two, and there are never morethan ten or twelve to a principal. The principals are held at every 10feet distance by pendant binding pieces which have a horizontal con-nection under the soffite;these passing over the prin-cipal rafter form a strongand irresistible piece offraming.

Where eight or moretimbers or thin planks arebent one over the otherand bolted through at shortdistances, the shape of thearch is not likely to be muchvaried; but with all the pre-cautions that have beenhitherto taken this cannotbe insured.

The difference betweenthis system of constructionand that of Philibert De-lorme chiefly consists inthe planks being employedin the reverse manner toeach other : the methodpractised by the latterarchitect would at first ap-pear the strongest, as thedepth of the deals or planksis greater than their width,from their being placededgeways: in the othersystem, thin boards are bentupon a cylindrical mould;then united firmly together,being rendered strong bytheir inability to spring;should this, however, occur Fig. 2156, marac, near bayo.nne.