DIMZNSIONS or AcruiL ARCHES. Table 90, page 648. shows the dimensions of a number of the longest voussoir arche in the world, which may be taken as representative of good practice. Because of the impossibility 'of obtaining all of the dimensions, a few arches have been omitted whose spans are greater than some of those given. Unfortunately, there is a difference in the recorded dimensions of some of the arches.
No. 1 is the largest masonry span in the world. For further details see f 1284.
No. 3, the third largest masonry arch (whether voussoir or con crete) in the world, was built in 1377—a very interesting fact. However, the dates and the dimensions of this arch are somewhat uncertain. The data in the table is said to be derived from about 20 feet of each abutment that remained standing in 1838. Some claim that the bridge was never completed and that it is not even known whether there was to be one or more spans.
No. 5 is the largest voussoir arch in this country. For additional details, see 11286.
No. 23 is a remarkable bridge. For details see 4 1288.
No. 31 is noted for its boldness. This design was tested by building an experimental arch—at Soupes, France—of the propor tions given in the table, and 12 feet wide. The center of the experi mental arch was struck after four months, when the total settlement was 1.25 inches, due mostly to the mortar joints, which were about one quarter inch; and it was not injured by a distributed load of 500 pounds per square foot, nor by a weight of 5 tons falling 1.5 feet on the key.
It is interesting to notice that most of the arches given in Table 90, page 648, are comparatively recent. It is also interesting to notice that some of the older arches compare very favorably with the most recent. For example, compare No. 23 with the next one above and also with the one next below it; and compare No. 22 with the one above it. However, possibly the later designer was influenced by the work of the older designer.