CON

198

CON

the form of a truss, is vastly superior to placing them withtheir joints parallel to the horizon, and would be a veryproper substitute for arching or trussing the sides in allroofs of moderate dimensions. It must, however, be observed,that the meeting of every two boards ougiit to be as nearlyas possible upon the middle of a rafter, and not over thehollow. To which we may add, that as all the joists areabutting in such disposition, the boards forming trussedwork may be made thicker, and let into the rafters, whichwill give greater security to the abutments ; but for thispurpose they ought to be firmly fixed at their meeting, toprevent them from starting.

The principle of arching the inclined sides of a roof, andmaking the wall-plates act as ties, is exhibited in the archi-tectural plates of Rees’ Cyclopedia, published in ISOo.

Circular roofs may be executed without ties, or withoutany precaution of trussing, as in rectangular buildings, butthe wall-plate ought to be one continued mass. There aretwo methods of covering circular roofs with boards : one is,to bend the boards with their joints in horizontal planes ;and the other is, to bend them in planes passing through theaxis. As that species of circular roofs called domes laysconsiderable claim to our attention, it will here be proper tosay something on their construction.

If the dome be spherical, and have no lantern to support,, the ribs may be constructed of boards in two or three thick-nesses, with the longitudinal joints of the boards tending tothe axis of the dome, and intersecting the spherical edges,and the butting joints intersecting the sides of the ribs, whichtend to the said axis.

Let us now suppose the thickness of a rib to consist ofthree boards, and suppose the circular pieces which areto compose the ribs, to be all prepared of equal lengths andbreadths. Take one of the lengths, suppose for the left-handpiece at the bottom, and lap the next higher length, which isthe middle piece, two-thirds upon the lower piece ; takeanother length for the right-hand piece, next higher, andlap this two-thirds on the middle piece ; so that the right-hand piece will lap one-third upon the left-hand piece ;between the ends of this third, bolt or pin the three piecestogether; the middle board will want a third, the right-handboard two-thirds, to make it complete at the bottom; theseparts being supplied and fixed, lay another board at thehigher end of the right-hand board, the end of another toabut upon the higher end of the middle board, and the end| of a third board to abut upon the upper end of the left-handj j board, then there will be three plies of boards, which mustbe fastened together between each pair of heading joints,which are three in number. Proceed in like manner withevery succeeding three boards, as with the last three, untilyou arrive at the top, and the deficiency must be suppliedas at the bottom. In this manner, every rib in successionmust be constructed, until they are all finished. Each ribought to be fitted to the curvature of the axal section of thedome, drawn on a floor, and the three thicknesses fixedtogether throughout the whole length, before it is removed.If, in addition to the fixing, the joints be strapped, it will addconsiderably to the strength, and will not be much inferiorto that of a solid piece. In large domes of this construction,

! it becomes necessary to discontinue the ribs, otherwise ani unnecessary quantity of timber would be employed ; andit must be observed, that the greatest intervals must be soregulated in their dimensions, as not to be greater than whatwould make the horizontal ribs for the boarding, when fixed,sufficiently strong.

As all domes are best boarded with their joints in verticalplanes tending to the axis, horizontal pieces must, in this

id tneir outer t gj in

formed with the spherical surface. A dome constru rI1)

this manner, might also be made to support a heavy 1 j n

provided the strutting-pieces were strapped togetn • ^

the above manner was the timber dome of the Halle oat Paris, constructed by Moulineau, supposed to beof the kind.

If the boarding of the dome is required to be ^ ’ te rii,the joints in horizontal planes, and the dome have noa very good method is, to construct it with several t iiCribs, their planes being disposed at equal angles 10 Waxis as their common vertex, and constructed acco* plat 6the above method ; between every pair of suchother ribs, the curvature of which will be poition 1( 1

, • . i. interval) "...

intersections er #

I u

angles : by one method, ribs are formed to , 1 acc<

sections of the vault, and disposed in vertical pl aI1 . i ltj »»aingly; but by the other, the ribs are prepared stra%fixed parallel to the axis of each vault. jj 3 by *1

Tlie lathing for plaster is sustained upon "’ a ’ ci iH e “number of parallel posts of very small scantling ^

battening, and ranged according to the figureintended to form. .

CONTABULATE, to floor with boards. .CONTACT (from the Latin , contactus, touch) 1 1touching, or meeting, of two things. 0 ee ts

Contact, in geometry, is when a line or P} a ” e 0 r

figure or solid, without cutting it, though the l> ne c t

v.._j_j 'tvj_ „ 1:_j _ •.in COM™

0 f e

in

be produced. Thus, a line and a circle are in cothe line is a tangent to the circle ; and two cii°^mutual contact, when they touch each other wit.bo jj 0 d/>the like is to be understood of a plane and a coi> va cylinder, cone, conoid, &c. jn

CONTENT (from the Latin , contentus ) tb ; de d "'‘‘ gfcontained, the thing held, included, or coniprehen^ td y ^a limit or line. In geometry, the area ormatter or space included in certain lines. ,p ilC e i s ° v

length simply ; superficial content, area or ^, c ., c °content, the number of cubic inches, feet, }' aI ’tained in a given space.

contexo,

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bo^'

CONl'EXTURE (from the Latin ,disposition or union of the constituent parts of -respect of each other. a rItl

CON IIGNATION (from the Latin contignafj 0 ’e* rignum, a beam), a frame of beams, in ancientpe " t !7’. tl ' e ^i lme as we now understand by nakedfi a t»CONIIGUITY (from the Latin , contigvus, t° ■„ et> crelation of surfaces or solids whereby their sih eS Jother. J f ,s.

mw? GUOlIS ANGLES, in geometry. S<*CONTINUED, a term applied to whatever isrupted, but proceeds in the same course. j

Continued Attic, an attic not broken into P ,Ia fcontinued Pedestal, a pedestal with its 111011 ^

dado, or die, continued both through the column acolumn, without being broken. of >"‘j

Continued Proportion, is when there is a ser \\, e ^

nd a 3 1

circles of the sphere, unless one stand in each intei > t j,eits plane bisect the inclination of the vertical P* aneS u j(]is'two adjacent principal ribs: dispose of these ribs W 3 jJeS

tant parallel planes, and fit their upper ends upon .jj be

of the principal ribs. This disposition of the rioa considerable saving of timber, besides what it vv0 . ubeen, had the planes of all the ribs tended to the aX1 . S ^ d9In the construction of plaster groins, two met jjy bemployed in the disposition and fixing of the ribs. t tl> 3methods, ribs are made to answer the

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or quantities, s

uoh that the first is to the seed