. The Civil engineer and architect's journal, scientific and railway gazette. Architecture; Civil engineering; Science. Having pi ojected the entire semicylinder, of which only a porlion can be used with propriety, let AB, CD, be the sides of the projection, and EF, parallel to the parapet, the plan of one of the lines of pres- sure. Bisect EF at right angles by GUI, and form two logarithmic curves of which AB, CD, may be the asymptotes, EG the common subtangent, their ordinates being parallel to EF. Then draw lines KL parallel to AB, and intercepted between the logarithmics, the middles M of
. The Civil engineer and architect's journal, scientific and railway gazette. Architecture; Civil engineering; Science. Having pi ojected the entire semicylinder, of which only a porlion can be used with propriety, let AB, CD, be the sides of the projection, and EF, parallel to the parapet, the plan of one of the lines of pres- sure. Bisect EF at right angles by GUI, and form two logarithmic curves of which AB, CD, may be the asymptotes, EG the common subtangent, their ordinates being parallel to EF. Then draw lines KL parallel to AB, and intercepted between the logarithmics, the middles M of these lines trace out the horizontal projection of one of the joints. The lines AB, CD, are thus asymptotes to the horizontal projection, and this geometrical property illustrates the mechanical impossibility of constructing a semirylindric arcli, without trusting to the cohesion of the mortar. The introduction of the logarithmic curve into investigations concerning bridges, has been of great utility, and the analogy between this curve and the connnon catenary is striking. The catenary is also formed by bisecting the interval between two logarithnacs; but these have a common asymptote with rectangular co-ordinates, while the bisected line is parallel to the ordinate. The computations needed for the delineation of such projections, are by no means tedious ; they may be performed rapidly by help of Napierian logarithms; but a better method, capable of giving all the projections, will be explained shortly. It may be expected, from what has been said of such elevations in general, that the end elevation of a circular oblique arch shall present some interesting peculiarity. The end elevation of a joint ought, in fact, to cross at right angles the circumferences of circles described with equal radius from points lying in a straight line ; now, this is the distinguishing characteristic of the tractory, and that curve must there- fore be exhibited on the end projections of all circular
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