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Page:Popular Science Monthly Volume 61.djvu/221

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215
A MODERN STREET.

of great power, which renders the resulting block as solid as the materials can be made. The Mocks are required by the specifications to be 4 x 5 x 12 inches and to vary not more than one quarter of an inch in dimensions. They are laid on their edges on a cushion of two inches of sand, so that the surface presented to wear shall be 4 x 12 inches, either in parallel rows or in a diagonal across the street. The surface, after the wear of a summer, during the heat of which it softens slightly, becomes nearly coherent and as continuous as sheet asphalt. On grades, the slight irregularities due to jointing offer a better foothold to the hoofs of horses than sheet asphalt and in this respect the asphalt blocks are the best. It is found in practice that on streets subjected to very heavy traffic, asphalt blocks are more easily broken than a good quality of sheet asphalt, but for streets that are subjected only to light traffic, asphalt block pavements are durable, clean and sightly, and cost little or nothing for repairs over long periods of time.

The larger number of so-called asphalt streets are laid with sheet asphalt, in imitation of the surface first laid about fifty years ago in France from the natural bituminous rock occurring at Pyrimont and Seyssel, near the border of France and Switzerland and near the headwaters of the river Rhone. This material consists of chalk saturated with bitumen, which, when extracted, is found to be very permanent in the air, impervious to water and very tenacious. When the bituminous rock is heated to a moderate temperature it falls into powder that can be screened to remove the flints that are in the chalk. It is then spread, while hot, with rakes, and rolled into a sheet that lasts until it wears out. This bituminous rock was called 'asphalt' by M. Leon Malo, the distinguished French engineer, through whose efforts and inventive skill the laying of these streets became an established industry in France.

M. Malo did not identify 'asphalte' as thus named by him, with asphaltum, which is the solid variety of bitumen, and had been known for an immemorial period before any one thought of using bituminous limestone in the construction of streets.

M. Malo says in his paper, published in 1861:

The first point is to establish the value of the definitions and of the words.

To this end he proposes a nomenclature of which he gives the following summary:

First, bitumen or pitch, the materials which impregnate asphalte.

Second, asphalt, the calcareous rock, impregnated naturally by bitumen or pitch.

This definition gave this peculiar bituminous mineral a name that passed to the streets made from it, and these streets became known in England and the United States as "asphalt streets.'