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ways. They are sometimes nailed down on a close boarding:or if large, on battens, or pieces of wood from two and a halfto three inches wide,' and three-quarters of an inch to an inchthick, which are nailed to the rafters at intervals regulatedby the length of the slates. Lozenge-shaped slating is occa-sionally used, and has an ornamental appearance, but iseasily injured, as there is but one nail through each slate. Itis always laid on boarding. For what is called patent slating,the best large slates are selected, and fixed without eitherboarding or battening, the common rafters being placed atsuch a width as to come under the joints. The slates arescrewed down, the courses over-lapping about two inches.The meeting-joints are covered by fillets of slate, about threeinches wide, set in putty, and screwed down ; and the hipsand ridges are sometimes covered in the same manner, thoughit is best, in all such cases, to use lead. Patent slating,when well executed, is water-tight, with as low a slope asone in six. In some districts, laminae of stone are used inlieu of slates or tiles. Shingles, which are like slates, butmade of wood, were formerly much used in covering pyra-midal steeples, and in roofs of steep pitch. They are stillused in the United States, and are usually laid on boarding,in a similar manner to common slates.

Sheets of metal are very convenient for covering domes,and curved or angular surfaces generally, and also for flatroofs, or such as have too little slope for slating. Lead isthe most common material for such purposes, though copper,iron, tinned iron, and recently zinc, are also used. Leadterraces, or flats, are commonly laid on boarding or plaster.The joints are sometimes soldered, but the most approvedmethod is to roll or wrap the edges into each other, makingallowance for expansion and contraction. A fall of a quarterof an inch in a foot, is sufficient for surfaces covered withsheet metal.

The weight of lead is somewhat against its use for roof-covering; it is used in sheets weighing from 41b. to 8 lbs.per foot super. Copper is much lighter, and is used insheets weighing about 1 lb. per foot super, but its expensive-ness precludes its adoption for general purposes. Zinc ismuch lighter than lead, weighing from 12 to 20 ounces perfoot super ; and of late has been very widely employed onthis account, as well as because of its cheapness ; it forms avery useful covering. Iron galvanized, or coated with zinc,has also been very extensively used for this purpose, forwhich it is also well adapted. Sometimes it is corrugated,by which it is considerably strengthened, and thus some ofthe strength of the roof itself may be dispensed with, espe-cially in small spans, where curved roofs of corrugated ironmay be safely employed. Tiles of this material also, aresometimes, used ; they are treated much in the same way ascommon tiles, being simply nailed to battens or boarding.The following table, giving the weight per square of 100 feetfor different kinds of roof-covering, may prove serviceable:—

Cwts.

Plain tiles . 18

Pantiles .. 9^

Slating . 7

Lead at 7 lbs. to foot super. 6^

Corrugated iron... 2

Copper or zinc at 16 ounces to ditto 1

Having now given a description of the several varieties ofroofs covering a square or rectangular plan, it may not beimproper to give a few examples of domes, and show howthey may be constructed under various circumstances. Ifthe dome to be constructed be on a circular plan, with nolantern above, the ribs may be built in the following manner,

with planks of convenient lengths, in three or . m ° r ® n( j tb enesses. Having ascertained the length of the ribs* ^ a pednumber of pieces in that length, and having proper*/all the pieces to the curve, the middle piece at tii, , {Vf omay be one of these lengths ; to each side may be J°* a ndother pieces, one reaching to a third of the middle p 1 .’ u ;agthe other to two-thirds from the bottom, so that by tbe

with planks of the whole length to the other extrem 1 y^jj-dsrib, the middle thickness will always be covered tw ,from the bottom on one side, and one-tliird on the o ^ 0 f adeficiency at the top must be filled up with P’ eceS ’ jjj ; tb ethird, and the other of two-thirds, as at the b nld a cr ° s9whole being well bolted together, and strapp ethe joints, will be nearly as strong as a solid rib. t ; n <rtbi 9Plate VIII. Figure 1, shows the manner of construe ele'' a 'kind of dome ; No. 1 being the semi-plan ; No. 2, , QTa e3 0tion ; No. 3, the manner of building the rib. t; n ue *h ethis kind, it may sometimes be necessary to d* s p 0I \ p or tl> eribs, that the spaces may be more equally divid e ^ &l) -horizontal ribs. It is evident that a dome built m ^ j )aV e 9ner, may be carried to almost any extent, provide t ( 0 n issufficient number of horizontal ribs. Of this cons r , th ethe Halle du Ble, at Paris, of 200 feet in d‘ a ^ ainvention of a judicious carpenter, the Sieur Mob 11 ® c j ]an ica*of little scientific education, but of considerable ^ tit®'experience. Being convinced that a very thin s 10 u jiibri°’her might not only be so shaped as to be nearly j 1 a fe

but that, if well connected with horizontal ribs, > t0 th eall the requisite stiffness, he presented his sc gg( ] th eD3 ’magistracy of Paris. The grandeur of the idea P ^ r f b ebut they referred it to the Academy of S cien ^ th emembers, who were competent judges, were stru * that 9justness of M. Molineau’s principles, and astonis ho use ”thing so plain, had not -become familiar with e ^ cot>v er ”carpenter. It quickly became a universal t°P'°p ar j 3 . B usation, dispute, and cabal, in the polite circles o . f opin' 011 ’the academy having given a favourable report °* an d so°the project was immediately carried into execu 1 e ’ x i,jbiti ofl _completed, and now stands as one of the greates ^ o6i co®in Paris. The circular ribs, which compose t 1 , )e g tin 0 ’sist of planks 9 feet long, 13 inches broad, and jj, e d. ■ymade in three thicknesses, as in that already j" r j ZO ntally^pvarious distances, these ribs are connected j l00 ps

to

th e

purlins and iron straps, which make so rna . n Jj tr ] lt , that **whole. When the work had reached such a 1 .® a j (lists 11 ^distance of the ribs was two-tbirds of the nea r

every second (now consisting of two ribs, zg( j > A * - r ,other,) was discontinued, and the void g ® j n to 9 t ^ eabove this, the heads of the ribs are oeIi ing 1 . n , a ncular ring of timber, which forms a wide ^brell*’middle, over which is a glazed canopy, ° p u •„» the b e jo

uvci which io u giaziou r ifl&

between it and the dome, for a* oget

air to get out. All who have seen this ^°jg cer it °^ edescribing it as the most beautiful and ma athey ever beheld. _ . f wo0 den

The only difficulty in the construction o j, ea vy * tt ' inwhen they are loaded in the upper part by ^oiild . 0 dor cupola. Such a dome as lias been descripf tiC tuaf inl L, ofdanger of being crushed inwardly ; the m°f j rl the *° . ofof preventing which, is by making the n m the 0 t ^otrusses, with an inclined timber extendmg t ing , b»sethe dome to the bottom of the lantern, co ^ t* 1 t jj0

extremities of the exterior side, forming tl contrary ‘f theof a truss in a common roof, and act o pr essu r0 g 0e3nature of a tie-beam ; they resist the ver oU t *’' grl }etlantern, without having any tendency to j. j a { 0 , -*■by acting entirely longitudinally on the vva