Principle XII.

Before starting a design, one should obtain complete data for same.

If he fails to do so, he will generally have to make alteration after alteration as the work progresses; and, as one change usually entails several others, it will result that, by the time the work is finished, enough labor will have been expended thereon to complete two such designs, for which proper data were furnished at the outset.

Principle XIII.

A skew-bridge is a structure the building of which should always be avoided when it is practicable.

It is generally possible to square the crossing either by swinging the centre line, or by lengthening the spans and squaring the piers or abutments. Sometimes, however, it is not practicable to do either, in which case the engineer must make the best of a bad business. The objections to a skew-bridge are these: First, it is fully twice as troublesome to design as a square structure; second, the liability to error in both shop and field is greatly increased by the skew; and, third, the resulting bridge is never so rigid, nor is it so satisfactory in a number of particulars, as a bridge without this objectionable feature.

Principle XIV.

The best modern practice in bridge-engineering does not countenance the building of structures having more than a single system of cancellation, except in lateral systems where the resulting ambiguity of stress distribution is of minor importance.

Some engineer may question the correctness of this assertion; but if he will glance through the author's paper on "Some Disputed Points in Railway-Bridge Designing" published in the February and March, 1892,  number  of  the  Transactions  of  the  American Society of Civil Engineers,  he  will  see