Principle XXII.

The principle of symmetry in designing must be carried even into the riveting; and groups of rivets must be made to balance about central lines and central planes to as great an extent as is practicable.

The violation of this principle was exceedingly common not very long ago; and even to-day, when checking the shop drawings of some of the leading bridge-manufacturing companies, the author's assistants have to correct occasional departures therefrom.

Principle XXIII.

In proportioning members of bridges to meet stresses and combinations of stresses it is important to consider duly the quality, frequency, and probability of the action of said stresses or combinations of stresses.

As a rule, standard specifications take care fairly well of this subject; nevertheless there will often occur in one's practice cases which they do not cover. The quality of stress should be considered in determining the sectional area of the member, because the greater the impact, other things remainlug the same, the smaller should be the intensity of working stress. The frequency of application of stress should be considered, because, if a certain stress or combination of stresses be of frequent occurrence, a small intensity of working stress should be adopted, while for very infrequent occurrences the intensity can be taken considerably higher.

Finally, the probability of the application of a certain load or loads should be considered; because for inevitable loads or combinations of loads the metal should be strained fairly low, while for highly improbable loads or combinations of loads it is legitimate to strain much higher. Just

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