they afford a poor riding surface and are short-lived. Transverse planks are objectionable on account of rough riding, and diagonal ones are better for high-speed traffic, although they splinter and are primarily somewhat more expensive—due to a waste in cutting them to proper lengths. Longitudinal planks afford the easiest riding, but they do not wear at all uniformly because of the tendency of the travel to run in certain lines.

Planks can be either untreated or creosoted. Creosoting delays the process of decay but lowers the resistance to abrasion; hence, for the wearing floor, untreated planks are the more economical. A hardwood timber that does not warp or twist excessively is the best for the said wearing floor. A double-plank floor with the lower layer creosoted and the upper layer placed diagonally is ultimately the most economical of all plank floors.

With the creosoted planks it is practicable to use a pavement of creosoted pine blocks, but the combination is very inflammable, and hence, is not truly economic. A similar pavement can be used to advantage for bascule spans by adopting instead of the blocks maple planks on edge bolted into groups and attached firmly to the steelwork.

With a concrete base any desired type of paving can be employed— wood blocks, brick, asphalt, bitulithic concrete or any other kind of bituminous paving, plain concrete, or granitoid. Wood block is the most expensive as far as first cost is concerned, but it makes a much better showing in the comparison when maintenance and renewal are considered. Brick per se is less expensive, but it is heavy and, in consequence, requires more metal to carry it. This is not a serious handicap on short-span bridges, but on long-span ones it is almost prohibitory.

Asphalt and bituminous pavements in general are good; and usually they are no heavier than the wooden-block ones. Unfortunately, they require an extensive plant to lay them; and, as the total area of paved surface on most bridges is comparatively small, the charge per square yard for use of plant will be excessive, unless there be a nearby plant available. To adopt an asphalt or bitulithic paving on a bridge in a small town is, for that reason, rarely economic practice. This difficulty, however, can be overcome by adopting an asphalt block pavement, which requires no plant for its construction.

A concrete wearing-surface in many cases is both satisfactory and comparatively inexpensive, for it requires no special plant to lay it; nevertheless an extra hard and durable aggregate is obligatory, and the concrete must be very carefully mixed, placed, and finished, and must be kept properly wet while curing, especially in hot, dry weather. Unless these precautions be observed, the concrete pavement will not prove economic because of short life and the expense of repairs and replacement. It will be found advisable to design with an allowance in dead load for an extra two inches of concrete, so that a thicker wearing surface may be put on, if ever desired, without overloading the floor-system or the trusses.