PAVEMENT 193 .United States, and when made of stones of nearly equal size and 4 or 5 in. in diameter, laid upon a good bed of gravel and sand, makes a very good pavement for a few years. The ease and cheapness with which it may be re- paired cause it to be retained in many localities where cobble stones can be easily procured ; but the jolting and noise of vehicles driven over it make it objectionable for streets having much traffic. The early stone roads in France were formed of beds of flat stones covered with broken stones, large below and smaller above. About 1760 Tresaguet discarded the flat stones except on marshy ground, and sub- stituted stone blocks on an arched bed. This method continued till the early part of the present century, when the large stones of Tresaguet were replaced by a crust of small broken stones laid directly on the ground. In 1816-'19 Macadam made many roads of this kind in England, and his success caused his name to be given to the system. The roads made by him were however inferior to those made in France by the same process. He dis- regarded too much the nature of the soil, and the defects which followed resulted in the ad- vocacy by Telford of a return to the use of large stones at the bottom on soft ground. Flagging is sometimes laid in connection with macadamized pavement in rows far enough apart to support the wheels of carriages, by which the cutting of the road into ruts is avoided, while the coating of pounded stone need not be so great. Such a road may be seen on parts of the old turnpike between Albany and Schenectady, N. Y. Within a few years increased attention has been given to the construction of asphalt pavements, particularly in France, and when they are properly laid, upon a solid concrete foundation (and they ought to be laid upon no other) they probably form the best roadway that can be constructed. Pavements called asphalt had been laid down in different places in Europe and America, but they did not answer the purpose of either a roadway or sidewalk. They were chiefly a sort of asphalt concrete, made by simply mix- ing melted bitumen with coarse gravel or pounded stones. The more volatile portions of the bitumen being retained, they would gradually pass away, and a crumbling of the road bed would be the final result, preceded during the summer months by the formation of a " poultice." But more attention having been paid to the conditions in which the bitumen is used, a decided advance has been made, and to a greater extent in France than elsewhere. A very good pavement can be made by raising the bitumen to 250 or 300 to expel the vol- atile portions, and using the residue to fill voids in coarse gravel or broken stone ; but a better material is now used in Europe, and also introduced into the United States, in the form of an asphaltic limestone found in the Val de Travers in Switzerland, and exported from Neufchatel, and known as Seyssel or Neuf- chatel asphaltic rock. A good substitute is also made in the United States by mixing such minerals as grahamite or albertite with Trini- dad asphalt, and combining this with cal- careous sand or similar material. The Seyssel rock is a native limestone, composed of pure carbonate of lime impregnated more or less with bitumen, generally containing from 92 to 93 per cent, of limestone and 7 to 8 per cent, of bitumen. Less pure varieties are found in the volcanic region of Auvergne, and contain clay, silica, magnesia, iron, &c. The Seyssel rock has a tine-grained irregular fracture, with a sonorous sound like ordinary limestone at common temperatures; but at 120 F. it may be flattened ; at 160 it begins to crumble, and at 212 the disintegration is complete. Its average specific gravity is 2*235. In Paris this rock is used in two forms : in an unmixed or pure condition for road beds of streets, and mingled with bitumen in different proportions for sidewalks and parks, the artificial bitu- minous mixture being called "mastic of as- phalt," while the unmixed rock is called " as- phalt." When it contains less than 6 per cent, of bitumen, it is not regarded as fit to be worked. The road bed is formed by beating the earth compactly, care being taken to insure good drainage. A concrete of gravel or broken stone in a matrix of hydraulic cement is then laid from 4 to 8 in. or even more in thickness, depending on the nature of the ground, and is allowed to set and become dry upon the sur- face. Upon this the prepared asphalt is spread. The rock is crushed by being first broken into pieces, then passed between rollers armed with strong teeth, and afterward between smooth cylinders ; it is then roasted in shallow sheet- iron pans, or in revolving cylinders. It is at first heated to 160 or a little upward until it begins to crumble, and then raised to 250 or 300, the operation requiring about an hour and a half. The concrete bed must be dry before the asphalt is laid upon it, for if not steam will be formed and prevent the con- solidation of the material. The drying may be hastened by covering with hot ashes or hydraulic lime' The asphalt, which the heat has reduced to the form of a powder, is then spread uniformly over the surface of the con- crete bed to the thickness of 2 or 2 in., so that when compacted it will be from 1 to 2 in. thick. The packing is principally done by hand, with hot iron pestles, raised nearly to redness. A coat of dry sifted powder is then spread over to fill up inequalities of surface, and a flat iron heated nearly to redness is passed over the whole. Rollers weighing from 500 Ibs. to a ton and a half are often used, in conjunction with the hand packing, but their utility in increasing the solidity is doubtful. After the packing has been com- pleted the road may be opened in a few hours for vehicles of all kinds. The grahamite as- phalt pavement company of New York make a substitute for the Seyssel rock by combining
