avoid as much as possible ambiguity in the stress distribution. Riveted connections are preferable to pin connections for the diagonals on account of rigidity, but are more expensive for erection.

Hard and fast rules for the minimum spacing of outer arches of bridges for various spans and rises cannot well be given. The narrower the structure within reasonable limits the less the cost, but the less also the rigidity and the lateral resistance to overturning from wind-pressure. In the 260-ft. span herein referred to, the author made the distance between central planes of arches twenty-two feet, which was as small a distance as he dared to adopt, notwithstanding the fact that economy of first cost was an important factor in the design. An approximate rule to work by might be to make the perpendicular distance between outer arches not less than one third of the height from springing point to grade.

In concluding this chapter, the author desires to call attention to the fact that there is still a great deal to be learned about the designing of arches; and to suggest that some professor of civil engineering, who is well posted on bridge designing and who has time to spare, could spend several mouths to the great advantage of the engineering profession in determining the proper relations of span length, rise, arch depth, width between exterior arches, etc., for the various styles of arch, and in ascertaining the relative economies of the latter.