=235. Covered Filters.=—It has become the custom where the best results are expected in cold climates, if not in all cases, to cover filters with masonry roofs of domes and cylindrical or groined arches supported on masonry columns. Such roofs are usually covered with earth to a depth of 1 to 2 or 3 feet. They prevent any injurious action on the sides of the filters produced by thick ice or the effects of such ice upon the upper portions of the sand. In summer they also protect against the baking and cracking of the upper surface of the sand when exposed to the sun and prevent, to a considerable extent, the growth of algæ in different portions of the beds. They are expensive, filters with masonry covers costing once and a half to twice as much as open filters, but they enhance the sanitary value of the water. The height of the masonry roof must be about 2 to 3 feet above the upper surface of the water and high enough to offer convenient access to the sand when it is to be cleaned and renewed. The length of span for the arches or domes is seldom more than 12 or 15 feet. =236. Clear-water Drain-pipes of Filters.=—After the water has passed through the sand it must be withdrawn from the bottom of the filter with as little resistance as practicable. This necessitates, in the first place, the bottom of the filter to be so shaped as to induce the flow of the filtered water toward the lines of drain-pipes which are laid to receive it. These pipes consist of the main members and the branches, the main members being laid along the centres of the beds and the branches running from them. The bottoms of the filters, therefore, should be formed with depressions in which the main pipes are laid, and with such grades as to expedite the movement of the water flowing through the branches. If the bottoms are of concrete, they can advantageously be made of inverted arches or domes, the drain-pipes being laid along the lines of greatest depression. In such cases the loads produced by the weight of the roof are more nearly uniformly distributed over the bottom. The sizes of the drains will be dependent upon the areas from which they withdraw water. It is advisable to make them rather large, in order that the water may flow through them more freely. They seldom need exceed 6 or 8 inches. They are preferably made of salt-glazed vitrified pipes laid with open joints, around and in the vicinity of which are placed gravel or broken stone, the largest pieces with a maximum diameter of 1 to 2 inches. The largest broken stone or coarsest gravel is near the pipe and should decrease in size as the drain-pipe is receded from, so that the final portions of the gravel farthest removed from the drains will not permit the filter-sand to pass into it. When properly designed and arranged, the loss of head in passing from the farthest points of a filter-bed to the point of exit from the filter will not exceed about .01 to .02 of a foot.