Ifaeadamiza tion.—This process, which has received its name from Macadam, to whose labors the improvement of the roads of England within the last half cen tury owes so much, consists in forming the road crust of ;tones, broken with a hammer into angular pieces of a small and uniform size. This method, how ever, is one which was long practised in various parts of Europe. When the stones of which the road crust is to be formed are broken to the proper magni tude and form, they are spread over the surface of the road in a layer of three or four inches thick. After this has been consolidated by carriages working upon it or by rollers, another laver of broken stones of equal is laid upon it ; it is consolidated in like manner; and thus one layer is laid over another until an artificial crust is formed of broken stones of sufficient thickness to give the requi site strength to the road.
A coating or road crust thus formed might be constructed on any substratum whatsoever, and a smooth and apparent ly good road would be obtained. It was the practice of Mr. Macadam to disregard the nature of the substratum ; and ho maintained that if it was not each a bog as would not allow a man to walk over it, he would even prefer it to a hard bottom.
Telford's Systern.—The improvement in road-making, which consists in a due attention to the substratum or foundation of the road, so as to give increased facili ty to the tractive power by rendering its surface hard and unyielding, is due to the late Mr. Telford. The following is a description of the method of constructing such a road practised by that eminent engineer.
Upon the level bed, prepared for the road materials, a bottom course or layer of stones is to be set by hand, in form of a close firm pavement. The stones set in the middle of the road should be 7 in ches in depth ; at 9 feet from the centre the depth should be 5 inches; at 12 feet, 4 inches • and at 15 feet, 3 inches ; the entire width of the road being 30 feet. These stones are to be set on their broad est edges lengthwise across the road, and the breadth of the upper edge should not exceed 4 inches. All the irregulari ties of the upper part of this pavement are to be broken off by hammers, and all the interstices to be filled with stone chips firmly wedged or packed by hand with a light hammer • so that, when the pavement is finished, its cross section shall have a convexity of surface of about 4 inches in the centre above the extreme edges ; 18 feet in the centre of this pave ment are to be coated with a layer of hard broken stones, 6 inches deep; of these 6 inches, 4 must be first put on and worked down by carriages and horses in the or dinary traffic of the road, care being taken constantly to rake in the ruts until the surface has become firm and the crust consolidated. After this, the remaining
2 inches of stone may be put on : the whole of this stone, forming 6 inches of crust, is to consist of pieces broken as nearly as is practicable into a cubical form, and of such a magnitude that they can pass through a ring of 24 inches in ternal diameter. The spaces on each side of the middle 18 feet are to be coated with broken stone or well-cleansed stone gravel up to the level of the footpath, or other boundary of the road, so as to make the whole convexity of the road 6 inches in the middle above the level of the edges ; and the whole of the materials thus formed and consolidated, should be cov ered with a coating 14 inches deep of good gravel, free from clay or earth.
Under the article PAVEMENT for roads, the Russ and Perrine pavements put down in New-York are described. These are in part based on Telford's plan, and are enduring roads. In 1850, a part of Broadway was paved thus : there was an excavation made to the depth of 2 feet, and coarse flagstone 2 feet by 8 laid down. The seams were filled in by hot pitch covered by gravel, and above this a layer of broken road metal was laid smoothly, and the whole overtopped with large granite blocks, about the size used for Russ pavement. There can be no question about the enduring qualities of this pavement, but the large blocks on top are objectionable, for when worn smooth they become dangerously slippe ry. Neither is it convenient, when a ne cessity arises to open the street to get at the gas or water pipes. Top blocks of half the size would answer a betterpurpose ; where a firm foundation is used the top tier need not be so large. On these roads the superficial wear is but slight, and much of the dust of summer and mud of ; winter is obviated. Such is the structure which is requisite for the streets which are the main thoroughfares of a great city ; a pavement with less strength of foundation, and formed of smaller blocks of stone, being used for the streets of less intercourse.