Chap. XXV.
ON STONE BRIDGES.
1495
! Fig. 2728.
Fig. 2731.
Fig. 2732.
Fig. 2733.
Fig. 2729.
Fig. 2730.
Thickness to he given to the Key-stones of the Arch. — Having determined the form ofthe curve, the thickness of the key must next be considered, as upon it depends thedimensions of the abutments. Alberti, Palladio, and Serlio , have allowed the twelfth,the fifteenth, and seventeenth parts of the span of the arch, without regarding the natureor quality of the stone employed. Perronet asserts that the twenty-fourth of the openingof the arch should be the depth of the key-stone, and Gauthey thinks thirteen inchesa proper dimension for all arches which have a less span than 6 feet 6 inches: for thoseabove that, up to 52 feet, he makes the key-stone the twenty-eighth part of their
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