tural steel, omitting paint from the head if desired. The rails, in many installations where the traffic is comparatively light, rust out in the web and base, rather than wear out.

Wood blocks may be laid directly on the planks, but the usual practice is to place a single layer of tar paper between, so as to prevent loss of the hot joint-filler. A very convenient depth for the wood blocks is 3 1/2 inches, but, whatever depth is selected, there should be at least one-fourth inch difference between the depth and the width, so that they will not be laid accidentally with the side of the grain up. On long bridges, especially where there is a grade, angles should be fastened to the floor with lag screws at intervals varying from 10 feet to 30 feet, according to conditions, in order to prevent the blocks from creeping. On grades greater than two per cent, "hillside" blocks, made like hillside brick, can be used to good advantage. We have had a short section of such floor in place on an eight per cent grade for several years with good success. Where there are no street-car rails to prevent side drainage, it is well to give the roadway a crown of two or three inches in order to assist in draining.

While it is true that the floors described are heavier than the plank floors which they replace, this is offset by the comparative smoothness of the surface, which reduces the vibration caused by passing vehicles.

The sidewalks on our lighter bridges have been constructed with either treated or untreated lumber laid in the usual manner, but this is not very satisfactory. Such floors wear out rapidly, if the traffic is heavy, and are rough and irregular. Treated lumber exposed to wear on the side of the grain does not last well and does not make a good sidewalk. We have recently laid a sidewalk on a river bridge with tongued-and-grooved lumber surfaced to 14 inches, and with wood blocks 2 inches deep, 3 inches wide, and averaging 6 inches long, laid thereon. Each block in every fifth row was nailed with a 10d wire finishing nail to prevent movement or displacement, and the joints were filled with dry sand. It was found, while the work was in progress, that blocks of this depth, on a surface where there was no wheel traffic, remained loose so that mischief-loving boys lifted them out of the walk and threw them into the river. The joints were then filled with hot bituminous filler, as was done on the roadway, but care was taken to cover the surface immediately with dry sand before the filler had time to cool. The resulting surface is very satisfactory.

When the old plank floors on the smaller bridges are replaced with the wood-block floor as described, the grade is usually raised a few inches. This necessitates an increased height in the back-wall. At first this was accomplished by taking up the sand-stone back-walls and resetting them at sufficient height to dress to the new floor-level. This is expensive and not wholly satisfactory; and for three years the same result has been reached by cutting the old back-wall down, where necessary, and setting two rows of paving brick at right angles to the face of the said back-wall in a bed of Portland cement mortar, and grouted in place. Where the approaches