50

OF GROINED RIBBED VAULTS.

Gautier (Traite des Ponts, page 119,) says, thatpointed arches were used in bridges, churches, andother buildings, “ par la faire moins de pousee.” SirChristopher Wren makes a similar observation, add-ing, that these arches required less centering andthinner stones. #

In every arch the summering of each voussoir de-creases from the springing to the vertex, and in anarch whose tangent at the vertex is horizontal, thesummering necessarily vanishes at that point. Thethickness which is given to an arch at the vertex, hasrelation to what may be considered a sufficient sum-mering., Hence the more oblique the tangent of auarch at the vertex may be, or, in other words, themore acutely pointed the arch, the less thickness ofmaterial will be required. By a parity of reasoningit may be shewn, that the more acute the arch maybe.at the vertex, the less lateral thrust will be on theabutment. These geometrical and mechanical factsmust necessarily have led to the adoption of the semi-elliptical vault, the short axis horizontal, or to thepointed arch. A gentleman, eminent as a scholarand a man of taste, observes, that, “ the pointed archis the primitive arch, of which the earliest instanceknown in Europe is the emissarius of the lake Albano,built during the siege of Veil , (394 years before

* “ At Bassora , where they have no timber but the wood ofthe date-tree, which is like a cabbage stalk, they make archeswithout any frame. The mason, with a nail and a bit of string,■describes a semicircle on the ground, lays his bricks, fastened to-gether with a gypsum cement, on the lines thus traced, and havingthus formed his arch, except the crown brick, it is carefully raised,and in two parts placed upon the walls. They proceed thus tillthe whole arch is finished. This part is only half a brick thick,but it serves them to turn a stronger arch over it.” — Survey of theTurkish Empire, by William Eton, Esq.