traveller running on top of the erected portion of the work. With tower spans of thirty feet and intermediate spans of sixty feet, the traveller will have to reach out ninety feet to erect a tower, which is about the extreme practicable limit. However, should it be necessary to use more or less falsework, longer spans than sixty feet would probably be economic.

The most economic layout for a highway viaduct with wooden joists is alternate towers and spans that consist entirely of joists, the limiting lengths of span being about twenty feet for the towers and twenty-four feet for intermediates, which latter length is the greatest spanned in general practice for 4" X 16" wooden joists. It is not legitimate in such a design to rely on the wooden joists of the tower spans to act as a part of the longitudinal tower bracing.

In railroad trestles the longitudinal girders should abut against and rivet into the webs of the columns, the latter being bent just below the longitudinal girders when the legs are battered. The author has lately adopted this detail in some trestle designs for a British Columbia railroad, and has found it to be very satisfactory. For double-track structures, the columns at tops are to be spaced a distance equal to the sum of the perpendicular distance between the longitudinal girders of one track and that between centres of tracks, and the legs may be made vertical up to a limit of about twice the perpendicular distance between axes of opposite columns.

For single-track structures, it is generally best to space the longitudinal girders and tops of columns ten feet centres, although an eight-foot spacing is legitimate. The former spacing gives greater rigidity to the structure, but necessitates the use of deeper timber ties. By using very deep ties a greater girder spacing may be adopted; but this is not necessary, unless very long intermediate spans erected on falsework be employed.

It is not worth while to use a batter for columns less than one and a half inches to the foot, and it is never economical to use one greater than three inches to the foot. The smaller the batter the less the total weight of transverse bracing, but the greater  the  tension  stresses on the columns.  As  a rule, it