Page:Popular Science Monthly Volume 7.djvu/97

Rh the persistent efforts of some of the best engineers of the age, by which all obstacles have been gradually overcome. The first trials were made with mastic, consisting of the powdered rock melted with mineral tar as a flux, and mixed with sea-grit which was laid upon an ordinary concrete foundation; they were followed by experiments with mastic, cast into blocks at the workshops, and laid with wide joints, which were filled in again with heated mastic. Next we hear of tests with broken asphaltic rock, rammed in a cold state upon a macadam foundation. And finally these intelligent labors were crowned by the splendid improvement of the compressed rock pavement, for which the rock, reduced by heat to powder and rammed and rolled while yet hot, into a homogeneous, tight covering, is laid upon a perfectly dry ordinary concrete foundation composed of crushed stone and cement. This simple improvement virtually adapts the old principle of a barn-floor of rammed clay, for thrashing, to the requirements of the open air, by making it water-proof. In place of the mastic, which attains consistency, by the congelation of the melted mass, without application of pressure, this "merely compressed body, in which the molecules of bitumen and limestone are soldered together by heat and ramming," obviates all tendency toward brittleness, without in the least interfering with the advantages of perfect homogeneity or water-proof qualities. It stands to reason that the mastic, which, notwithstanding its mixture with grit, is more or less pitchy, would be surpassed in elasticity and pliability by a merely kneaded mass. These pavements are reported to have withstood the extreme heat of Bombay, Hindostan, as well as the greatest known cold. Not affording any escape to the terrestrial heat through joints, they are kept warm and open from below in most cases when block-pavements present an icy surface. Their smooth, seamless face, being almost entirely free from abrasion by attrition or atmospheric action, meets the mechanical and hygienic objections to block-pavements, both of stone and wood, as well as of macadamized roads.

The asphaltum pavement is clean and fit for traffic a few hours after being laid, while new or repaved stone roadways must be covered for months with heavy layers of sand, to be drifted by the breeze in dry weather and added to the mud in rainy spells. Repairs can be made to the asphaltum pavement in dry days of a cold winter, while with stone pavements any defects must be endured until spring. Besides the sanitary advantages, the saving in temper, clothes, shoes, and furniture, is not to be overlooked. The popularity of this pavement in the two largest cities of Europe, where, with immense traffic and most extensive experience on the relative value of pavements, the demands on the municipal authorities are inexorable, serves as a proof that smoothness of surface does not cause any danger with this material. Being elastic but not soft in summer, and hard but not brittle in winter, it possesses with a slight yield the power of readjustment in