. The Civil engineer and architect's journal, scientific and railway gazette. Architecture; Civil engineering; Science. 1847.] THE CIVIL ENGINEEERAND ARCHITECT'S JOURNAL. 67 There are three conditions to be considered as affecting the equili- brium of tlie arch—1st. The form and dimensions of the inlrados or internal lower curve of the arcli: 2nd. The form and dimensions of the extrados or external curve of the arch: 3rd. The weight and dis- position of the loading. The first condition of courses includes the rise and span; the second, combined with the first, is equivalent to a determination
. The Civil engineer and architect's journal, scientific and railway gazette. Architecture; Civil engineering; Science. 1847.] THE CIVIL ENGINEEERAND ARCHITECT'S JOURNAL. 67 There are three conditions to be considered as affecting the equili- brium of tlie arch—1st. The form and dimensions of the inlrados or internal lower curve of the arcli: 2nd. The form and dimensions of the extrados or external curve of the arch: 3rd. The weight and dis- position of the loading. The first condition of courses includes the rise and span; the second, combined with the first, is equivalent to a determination of the depth of the voussoirs; the third exhibits the fxternal forces to which the system is subjected : for we suppose that not only is the weight of the loading known, but also the manner of its distribution—that is, whether and in what degree its specific weight or density varies in different parts of it. But, it may be asked, is not this enumeration of the conditions of equilibrium imperfect from the omission of the form, number, and position of the joints? The answer to this question is important, be- cause it exhibits in the strongest light possible the distinction between the ancient and the modern theory of the arch. It is manifest that the extrados and intrados, which have been enumerated as two of the "conditions," simply define the depth of each voussoir and the form of its upper and lower curved surfaces: the lateral dimensions of the voussoir, and the form of those surfaces of it which are in contact with the adjacent voussoirs, are as yet left indeterminate. Consequently it is not known how many joints there be, or even whether there be any joints. Neither is the direction of the joints ascertained ; they may be plane converging joints, or they may be "joggled," or all vertical, or all horizontal—(as an instance of the latter case maybe cited the Treasury of Atreus at Mycene, where the stones are not wedged together, but are merely dispos
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