Page:Popular Science Monthly Volume 51.djvu/84

76 time it was the custom in Great Britain, as elsewhere, to build roads very largely of clay or gravel. Macadam observed that gravel never afforded a good, compact wearing surface until a large amount of traffic had passed over it, when it became hardened and cemented together. He sought an explanation of this phenomenon, and learned that when the pebbles were broken under the impact of heavy wheels they soon consolidated into a firm mass. Here was the great principle: angular stones solidify under pressure; rounded stones do not. Amplifying this principle, he built up a complete system of road building which is in use to-day, as best shown in Switzerland and France, in England, and in other foreign countries, and is being revived so generally in this country at the present time, where the farmer is learning its advantages in the appreciation of land values, and where the bicyclist promotes the cause as the advance agent of good roads.

As defined by Macadam, a good road should be a hard, somewhat elastic surface to receive the wear of all kinds of traffic at every season of the year and during the greatest vicissitudes of weather, which shall also serve as a roof to that part of the road lying below. To use his own words, "A road ought to be considered as an artificial floor, forming a strong, smooth, solid surface, at once capable of carrying great weights, and over which carriages may pass without meeting any impediment." In order to realize such a surface it is necessary that the substructure of the road should be kept free from water, since, by the alternating freezing and thawing of the water, the wearing surface of the broken stone is disrupted, the water is offered a passageway through it, and the road becomes rough and difficult to travel.

It was the custom of Macadam, after the engineering work was completed and the subgrade established, to spread on a layer of stone to a depth of ten inches and to roll this surface with a heavy roller drawn by horses. These stones were broken by hand with small hammers, frequently a whole family working together, and were broken small enough to pass through a three-inch ring, or were not to have a maximum weight of over six ounces. A family of five people could break several tons per day. Side ditches were excavated where necessary, so that at no season of the year could water penetrate to the substructure of the road.

In 1816 Macadam began the construction and maintenance of one hundred and eighty miles of turnpike in Bristol District,