76 POPULAR SCIENCE MONTHLY. 



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 hard- 

 ened 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 

 underpressure; rounded stones do not.* Amplifying this prin- 

 ciple, he built up a complete system of road building which is 

 in use to-day, as best shown in Switzerland and France, in Eng- 

 land, and in other foreign countries, and is being revived so gen- 

 erally 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, some- 

 what 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 con- 

 sidered as an artificial floor, forming a strong, smooth, solid sur- 

 face, 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, 



* It is not improbable that Macadam was acquainted with the Napoleonic military roads 

 constructed in France about 1775, which involved the principle of a thin layer of broken 

 stone placed on a rock foundation. These roads were the invention of Tresaguet, a French- 

 man, at about this time, and to him seems to be due the credit of first constructing what is 

 now known essentially as the Telford system. 



