1448

THEORY AND PRACTICE OF ENGINEERING.

Book II.

masonry should be employed; the piers of bridges and vaults which have to resistconsiderable pressure should be formed of carefully selected stone, and of a quality freefrom all fissures and cracks, and care should be taken that when placed the under sides ofall the beds are perfectly smooth and level, which is not always done, less attention beingpaid to them than to the upper bed, which is always in sight, and more easily worked:the engineer who is desirous of having his work solid should observe that every stone isbedded perfectly flat, and placed at right angles to the side that is to form its external face' throughout the whole depth, and not merely for a few inches within the joint, as is oftenpermitted ; there should be perfect contact between the several stones in every directionand on every plane, and no vacuity or space allowed to be filled in with cement or mortar:for the vaults of buildings used as warehouses where heavy weights arc deposited, themasonry should be constructed with the greatest care and solidity ; but for the covering ofa nave or side aisles of a cathedral or church, where no extraordinary additional weight isto be supported, this solidity may partly be dispensed with.

Gothic Vaultj with three Ribs in each Compartment , uniting at several Kegs. — The centresof these ribs are always in a horizontal plane, which passes through the springing.The plan of the ribs being given, we first set out the centre G H, passing through themiddle of the sides which form the plan ; after having drawn the chord G H, draw aperpendicular through its centre to the point L. This point is the centre from which thearc GII is to be struck. To obtain the rib IN, prolong their centre to O; then set offF O from the plan on the elevation from P to 6; through the point b , draw f a parallel tothe axis IIP, which will cut the arc GHint^ and give the height bd for the arc I O.

To have the curve ofthis arc, take for base thediagonal E F on the plan ;set off I O from E to q,and after having raised theperpendicular qg, equalto b d, draw the chord E g,on the middle of whichraise another perpendi-cular, which will cut thebase E F prolonged in A;this will be the centre ofthe arc Ep raised perpen-dicularly on E q, andequal to I O : but as itmust stop at N, we shallobtain its true length bysetting off O N from q to n ,and drawing through thepoint n a parallel to qg,which will cut the arcE g at i, and E i will be thearch represented by I N.

For the parts of the ribswhich intersect the others,as F N on the same baseE F, describe the pointedar"h E H, whose heightF H is given ; then set offF N from F top: havingdrawn through p a paral-lel at F H, set off from pto k the height ni of theintersecting ribs, andhaving drawn the chordAH, raise on its middlea perpendicular cuttingthe base EF in t; thispoint will be the centreof the arch forming theintersecting rib, whoselength is. expressed byh H. The arc formingthe other rib N D isobtained by setting off

..-? n

/

Fig. 2560. GOTHIC VAULTS WITH BIBS.