ber of rivets, fail before the full strength of the compression member is developed.

Principle V.

There are no bridge specifications yet written, and there probably never will be any, which will enable an engineer to make a complete design for an important bridge without using his judgment to settle many points which the specifications do not properly cover; or as Mr. Theodore Cooper puts it. "The most perfect system of rules to insure success must be interpreted upon the broad grounds of professional intelligence and common sense."

At first thought one might conclude that this speaks badly for modern standard bridge specifications, and to a certain limited extent he would be right; for while it is quite true that no railway-bridge specifications yet published begin to cover the entire ground of ordinary bridge-designing at all adequately, or nearly as thoroughly as they might readily be made to do, nevertheless it is also true that the science of bridge-designing is such a profound and intricate one that it is absolutely impossible in any specification to cover the entire field and make rules to govern the scientific proportioning of all parts of all structures.

The author has done his best in Chapters XIV.-XIX. of this little treatise to render the last statement incorrect, but with what success time alone can prove.

Principle VI.

In every detail of bridge-designing the principles of true economy must be applied by every one who desires to be a successful bridge engineer.

This subject is such an important one that to its consideration the whole of the next chapter will be devoted.

Principle VII.

In bridge-designing rigidity is quite as important an element as is mere strength.