and figure of the vault by fixed and simpleprinciples, as well geometrically as by calculation,so that the material used may be in a due figure,and of a due thickness in each part in respect tothat figure, in comparison of which all otherthings relating to the construction of a buildingare of little value, and upon which the cost andbeauty of it mainly depend. The same prin-ciples are applicable to demonstrate the propor-tional thicknesses of the parts of the cupolas ofair-vessels and boilers of steam-engines, and theformer may be rendered through them less liableto burst, and the latter to bulge and burst, thanthey are at present.
The author has endeavoured, in pages 35 and36 of the Tract on Pendent Bridges, to showthe nature of the data which should be first de-termined by those giving instructions for theerection of a bridge, and of the questions whichshould be answered by those appointed to givedesigns for them.
He would have been glad to have found thatthe strength of the proposed Menai bridge, asdescribed in the reports of the House of Com mons , had been so much too strong, as fromcalculation '* he finds it to be too weak, and to