[18] The method adopted in Chicago, U.S.A., for forming their roadways is as follows:--The road bed is prepared of the proper contour and well-rolled with a 15-ton steam roller until it is even, firm, and compact; on this bed rubble stone is carefully placed by hand with its broadest surface downwards, then 12 inches of metal are added 6 inches at a time, thoroughly rolled to bond it well, it is then topped with 4 inches of crushed trap rock or some other equally hard stone, which will not disintegrate through the action of the weather, nor pulverise under the pressure and wear of vehicles upon it; this is again, thoroughly well rolled so as to compact and bind it together. [19] “If roads be kept dry they will be maintained in a good state with proportionally less expense. It has been well observed that the statuary cannot saw his marble, nor the lapidary cut his jewels without the assistance of the powder of the specific materials on which he is acting; this, when combined with water, produces sufficient attrition to accomplish his purpose. A similar effect is produced on roads, since the reduced particles of the materials, when wet, assist the wheels in rapidly grinding down the surface.” Parnell’s ‘Treatise on Roads,’ 1883. More modern writers have likened macadamised roadways to “stone mills on which the stones are ground into dust when dry, or mud when wet.” [20] _Vide_ ‘Minutes of Proceedings of the Institution of Civil Engineers,’ vol. lx. [21] _Vide_ ‘Annales industrielles de Paris,’ Oct. 21st and Nov. 4th, 1877. [22] _Vide_ ‘Annales industrielles de Paris.’ [23] A report of the Paddington Vestry on “wood and other pavements,” (1878) states macadam as a mud producing material is twelve times worse than wood, and six times worse than granite cubes. [24] This is also sometimes called “stocking” or “chequering,” and consists of making furrows across a roadway with a sharp pickaxe, about a couple of inches in depth, thus removing any irregularities, and also allowing the new metal to bed properly. [25] In Birmingham, good cleansing is said to have reduced the amount of metal necessary for the maintenance of the roadways from 20,000 tons per annum to 13,000 tons. CHAPTER VI. ROAD METAL AND BREAKING. The only true test of the fitness of any stone for use as a road metal is by an experimental trial upon a certain length of roadway; but in making the first selection for such trials it is well to make the following investigations:-- (1.) Ascertain from local persons, such as masons, quarrymen, and others, their opinion of the qualities of the stones in the neighbourhood. (2.) Make a trial of the stone for toughness. This can be done by setting a good stone-breaker to work upon a heap of the stone as quarried and carefully watching how much he can break in an hour.[26]