The number of superficial yards rolled per day must vary extremely with circumstances: the class of material, the amount of binding and water used, the gradient and pressure of steam maintained, and the amount of rolling considered necessary,[45] being amongst the various influences. From the above returns I find that the number of square yards rolled varies from 500 to 3000 per diem, the average for 42 towns being 1105 square yards per diem. The cost per square yard rolled, including all charges, may be assumed to be between ¹⁄₂_d._ and 1_d._, and the cost of binding material about 3_d._ per square yard. With reference to the necessity of binding material, the following, facts are interesting. Mr. Wm. H. Grant, Superintending Engineer of the New York Central Park, in his report upon the park roads, says:[46] “At the commencement of the macadam roads, the experiment was tried of rolling and compacting the stone by a strict adherence to Macadam’s theory, that of carefully excluding all dirt and foreign material from the stones, and trusting to the action of the roller and the travel of teams to accomplish the work of consolidation. The bottom layer of stone was sufficiently compacted in this way to form and retain, under the action of the rollers (after the compression had reached about its practical limit) an even and regular surface; but the top layer, with the use of the heavy roller loaded to its greatest capacity, it was found impracticable to solidify and reduce to such a surface as would prevent the stones from loosening and being displaced by the action of waggon-wheels and horses’ feet. No amount of rolling was sufficient to produce a thorough binding effect upon the stones or to cause such a mechanical union and adjustment of their sides and angles together, as to enable them mutually to assist each other in resisting displacement. The rolling was persisted in with the roller adjusted to different weights up to the maximum load (12 tons) until it was apparent that the opposite effect from that intended was being produced. The stones became rounded by the excessive attrition they were subjected to, their more angular parts wearing away, and the weaker and smaller ones being crushed.” “The experiment was not pushed beyond this point. It was conclusively shown, that broken stones of the ordinary sizes, and of the very best quality for wear and durability, with the greatest care and attention to all the necessary conditions of rolling and compression, would not consolidate in the effectual manner required for the surface of a road while entirely isolated from and independent of other substances. The utmost efforts to compress and solidify them while in this condition after a certain limit had been reached, were unavailing.”