Page:Popular Science Monthly Volume 2.djvu/632

612 pieces, and melted in caldrons with bitumen at a temperature between 150° and 170° Cent. Then from 60 to 70 per cent, of sand is added, and the mixture is ready for application to the surface to be covered.

But still a good pavement for roadways was a desideratum. First the experiment was tried of laying down a bed of broken freestone, and pouring over this melted asphalt mastic. This method, however, proved unsuccessful, for, when cold, the mastic was too brittle, and if one of the corners of the stones was struck and broken by carriage-wheels, a hole was made, which gradually widened, and was difficult to mend. In Lyons, a layer of asphalt mastic, two inches in thickness, was spread over four inches of concrete. This was found to be an excellent pavement, but a fatal objection to it was its costliness.

It was at last observed that the roads and paths leading to the quarries of the Val de Travers were always in good condition, firm, solid, and elastic. Here was an asphalt pavement, formed of the small pieces of the mineral which fell from the carts, and which were pressed down and flattened by the wheels. The first highway asphalted on this principle was that between Bordeaux and Rouen. The road was first macadamized, and then covered to the depth of an inch and a half with asphalt, broken up in small pieces. As the plan appeared to be a complete success at first, several other roads were asphalted in the same way. Soon, however, the crushed granite of the macadam began to cut its way through the asphalt, and broke its continuity, thus allowing it to be permeated with dirt and water. Finally the problem was solved by a Swiss engineer, A. Merian, of Basle, who proposed to lay down powdered asphalt in a warmed state on the street, and, by applying strong pressure, to form at once an impermeable, elastic surface. The French engineers readily adopted the suggestion, and the first trial of the new method was made in the Rue Bergère. The engineers cried "Eureka!" and well they might, for experience in Paris shows that—1. The asphalt costs, in the first instance, one-third less than stone pavement; and 2. That the annual cost of maintenance is three-quarters less than that of a macadamized road.

The process of preparing the asphalt pavement is thus described by M. Léon Malo: The asphalt-stone is brought direct from the quarries, and broken up into small pieces, about the size of those used for macadamized roads; it is then heated over a stove in a drum-shaped iron vessel with feet, till it crumbles into powder. In order that the powder may not lose its heat, the whole apparatus is conveyed on to the street where it is to be applied. Then a foundation of béton is laid, about four inches deep, which may, however, be thicker or thinner, according to the nature of the soil. On a macadamized road the concrete may be omitted; but on loose soil it should be laid as thick as six inches. The arch of the roadway should only be just sufficient