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If one of the ribs, instead of being a component part of a vaultedof an insulated arch (unless of such a form that the lines of pressur *' 0rrne< * one s ^ epass through it from the vertex to the base) weights might be supped** would
points abc (F), which in the vault are intersected by the horizontal rib 6 ° n the
to produce an equilibrium, the greatest weight would be required on th<Tl" ’ ° rder
diminishing over each successive point to the vertex, where a greater and
tion of the materials equivalent to it, would be required. Butin a vaulteJroof ofTv"/ 08 "where a horizontal resistance can be obtained, no additional weight is necessarv 7J ,
pl„„. of the horizontal rib and the line of pressure ferns mgk a> ^V.ZT"to"
drawn from the chord of the quadrant to the vertex ; therefore the thrust cannot pos-sibly throw the haunch up ; and as the horizontal resistance at each successive point (risingtowards the vertex) consists of a segment of a larger circle, it would be proportionallyweaker, if it consisted of stones of the same substance as the smaller circle; they are there-fore found to increase in size from the haunch upwards, as a b c (F). The upper one (d)is within the longitudinal line of key stones. The horizontal ribs serve also as a counterpoiseto the ridges and to each other, and by these several operations produce a general equili-brium. The thrust of the circular ribs, in the direction of the transverse ridge towards thewall over the windows, is discharged to the buttresses by the opposing ribs, 1 1, 2 2, 3 3, &c.F*
* These remarks are written merely for the purpose of making the plate3 better understood, and are not intended orexpected to convey more than a very general idea of the principles on which the roof is constructed, being founded onan observation of a few points only, that could not fail to attract notice in the course of a close and attentive surveyThey may, however, as far as the conclusions are correct, afford sufficient data, together with the plates, to enable thosewhose professional experience and talents render them better qualified to analyze the subject, but who have not hadsufficient leisure to investigate it, to form for themselves a more complete and satisfactory analysis.
It was not known till after the plates were engraved, that a learned Professor, highly distinguished for his scientific