upon which the rack bears are to be planed smooth. The width of the base of the rack shall be at leastt two thirds (2/3) of its height; and ribs bracing the vertical portion to the base shall be provided at distances not exceeding eighteen (18) inches.

Drainage-holes not less than three fourths (3/4) of an inch in diameter, spaced not more than two (2) feet between centres, shall be bored in the lower-track segments, starting just back of the rack and leading to the outside of the track.

The girders over the drum shall be so arranged as to distribute the load over it properly. The number of bearing points required will depend upon the length of span, the distance from centre to centre of trusses, the total load to be carried, and the economical size of pivot-pier. The arrangement of the supporting girders in turn depends upon the number of bearing points to be used. For ordinary single-track bridges up to three hundred (300) feet in length a very good arrangement of girders over drum is secured by making the diameter of the drum and the length of centre panel equal to the distance from centre to centre of trusses; then the middle points of both the longitudinal and the transverse girders will be directly over the web of the drum, thus furnishing four points of bearing. Four more points of bearing are secured by putting in short diagonal girders, which connect to both transverse and longitudinal girders and bear on the drum at their centres. This arrangement gives in all eight (8) points of support.

The longitudinal, transverse, and diagonal girders over the drum shall be so designed that their rigidities will be such that when deflected under the load the extreme fibre-stress will be about the same in all the said girders.

The bottom-chord stresses in the centre panel can either be carried by the longitudinal girders, or the bottom-chord sections can be continued through the centre panel, the longitudinal girders being placed above them, and steel chairs being inserted beneath their centres to furnish bearings on the drum. In case that the bottom-chord stresses are carried by  the  longitudinal  girders,  ample provision must be made for