. The Civil engineer and architect's journal, scientific and railway gazette. Architecture; Civil engineering; Science. Fig. 1. In this experiment It is obvious, that the pressure must be transmitted through the points of contact; and it atTords a practical proof, that this line is the curve of equal horizontal thrust; for if in any voussoir a, the hori- zontal force at i, was not equal to that at c, motion must ensue, and as this condition is the same in all the voussoirs, it follows, thai the horizontal force is equal throughout. The experiment admits of further application, by loading the a
. The Civil engineer and architect's journal, scientific and railway gazette. Architecture; Civil engineering; Science. Fig. 1. In this experiment It is obvious, that the pressure must be transmitted through the points of contact; and it atTords a practical proof, that this line is the curve of equal horizontal thrust; for if in any voussoir a, the hori- zontal force at i, was not equal to that at c, motion must ensue, and as this condition is the same in all the voussoirs, it follows, thai the horizontal force is equal throughout. The experiment admits of further application, by loading the arch so as to vary the form of the curve in which the pres- sure is transmitted, while it of necessity retains the element of equal hori- zontal thrust; and it will be found, that the limit of stability is when the point of contact of any two voussoirs falls at their outer or inner extremi- ties ; thus establishing practically, that the line of resistance, or curve of equal horizoulal thrust, must be contained within the thickness at every joint. The second condition necessary to stability, namely, that the direction of the pressure, at each joint, should be within the limiting angle of friction, is almost always of necessity fulfilled in the forms of arches and with the materials usually employed in practice; this part of the inquiry will there- fore be confined to the first condition. Now the property of equal horizontal thrust, enables a geometrical con- struction of the curve to be readily obtained in any given form of arch, if two points in the curve be given, and by assuming these two points, it can be ascertained by a tentative process, if any given arch does, or does not, contain the curve. Proceeding in this manner it is found, that in a semicircular arch, the thickness must be one ninth of the radius to contain the curve, a result which is completely borne out in practice; for though apparently unno- ticed, a semicircular arch cannot be made to stand without foreign su
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