Tile the Netherlands and the coast-region of Hanover and Oldenburg, in default of suitable material for stone pavements, hard burnt tiles are made to serve as a substitute (fii. 2o, Jigs. 7, 9). Before set ting-, these tiles, the foundation, which is made of the most porous materials obtainable, is carefully rammed or rolled, and is then scraped off into the proper arched section with a scraper. The curbstones in this construction are formed either of natural stones or of several rows of tiles set on edge. The finished roadway commonly receives a thin covering of sand.
Pavements have been tried experimentally, but the result has not been sufficiently satisfactory to encourage their general adoption. Figures 12 and 13 (pi. 2o) exhibit the appearance of such a pavement laid down in New York City. It was formed of cast-iron blocks, each having the form of two concentric cylinders strengthened with radial connecting ribs. The vacant spaces were filled with hard-rammed earth or gravel. Figures 16 and 17 represent a cast-iron cellular pavement with connected ground-plate.
Wooden PaVellIelliS. —Pavements of wooden blocks, nsually placed on a foundation of wooden flooring, have been extensively introduced. They were at one time extremely popular in the United States, and were in use in nearly every important city (notably in Washington and Chicago), but of late they have fallen into disuse. On the other hand, they appear to be growing in favor in Europe, and have recently been adopted to a consid erable extent in Berlin and other cities of Germany.
Asfihaltum and roadways of asphaltum have been laid down in Paris, London, Berlin, and other European cities, and in many of the cities of the United States, Washington being a notable exam ple. The London asphalt pavement is made of a layer of freshly-ground asphalt, 2 inches thick, laid upon a foundation of 8 inches of concrete and compressed by the use of heated iron rollers. The powdered asphalt is laid on very hot, but not melting, and after compression and cooling at once assumes the solidity of the natural rock-asphalt. The high price of this material has led to many attempts to prepare artificial substitutes, for which purpose mixtures of tar, pitch, and granulated limestone have com monly been resorted to. In the United States enormous sums have been squandered in experiments npon various form of concrete and mastic and pseudo-asphalt pavements. These have connnonly been called asphalt pavements, although usually composed of coal-tar, pitch, etc. These ex
pensive failures served for a time to bring the merits of the true asphalt pavement into disrepute. The disposition of stich inferior substitutes to soften and to become sticky in bot weather explains the appropriateness of the name " poultice " pavements, which was coined to describe them.
The true rock-asphalt is a limestone saturated with about 12 per cent. of bitumen so thoroughly as to be hard, tenacious, and waterproof. Asphaltum itself is an oxidized hydrocarbon found in Trinidad and other localities. It is brownish in color, tough and hard, and withstands a tem perature of r7o° Fahr. unaltered. In many of the chief European cities Toadways and footways made of the natural rock-asphalt, disintegrated by heat and afterward mixed with suitable proportions of the free bitumen to form a mastic, are found highly satisfactory. In the United States, asphaltmn pavements of unexceptionable quality, both for streets and for sidewalks, have lately come into vogue. The asphalt employed is that from Cuba or Trinidad. The city of Washington especially is well paved with asphaltum roadways.
Asphallum, and II'oodell Pazvmenls following conclusions respecting the relative merits of the principal varieties of road ways in cities—namely, stone-block, wood, and asphaltum—appear to be warranted by experience. Denoting the asphalt, the stone-block, and the wood-block by their respective initials, A, S, and W, they' may be arranged as follows, in the order of their excellence: Sia'ewalks and sidewalks it is unnecessary to say more than that they are commonly laid out so as to be slightly elevated above the level of the street, the surface-covering consisting- of flagging-, bricks, etc., with a good fall toward the curb for drainage, the object being to render them passable with the least inconvenience and personal discom fort at all times, in wet and dry weather. In certain cities, where the street traffic is so great as to make the crossings dangerous for foot-passen gers, it has been found necessary to throw an elevated footway across the street. A crossing of this kind is shown in Figure 31 (pi. 20), which exhibits a structure that was at one time thrown across a much crowded portion of Broadway, in New York City. Upon country roads the material excavated in forming the road-bed and the drainag-e-ditches is commonly disposed at one or both sides of the road, thus constituting one or two banks which serve as footways.