339. It is desirable that the weight of the roller should be pro portional to the hardness of the stone, as too great a weight crushes the material instead of compacting it. An excessively heavy roller will sometimes sink into light or loose soil, and force it ahead in a wave which the roller can not surmount. This may sometimes be prevented by spreading a thin layer of sand or gravel on the surface being rolled. A similar difficulty sometimes occurs with a heavy roller on a layer of loose stones. If the front wheels or rollers of the machine were larger, this difficulty would be de creased. In localities where the soil is of a loose sandy nature, a roller weighing 10 or 12 tons is usually preferred; and in districts where the soil is gravelly or stiff clay, a weight of 12 or 15 tons is used. In localities where the road material is hard, a 15-ton roller is necessary; but with the softer stones a weight of 10 or 12 tons is sufficient.
in first cost. is lighter, and is generally cheaper to operate; but on account of its lighter weight it is less effective on soft material than the steam roller, and can not thoroughly compact the harder road materials, and, besides, the horses' feet often loosen the material nearly as fast as it is packed down by the roller. The time required to consolidate a stone road varies with the weight of the roller, and for this reason roads can be built more quickly with the steam than with the horse roller.
For compacting the subgrade of roads and pavements, the horse roller is reasonably effective. However, there is one important ad vantage in using the steam roller to consolidate the subgrade of a street pavement: One of the chief objects in rolling the foundation is to discover partially filled trenches, which usually run both lengthwise and crosswise of the street; and therefore the roller should be run over the street both longitudinally and transversely. The latter can be done only with a steam roller.