392 
Roads and Higlucaijs : their History, 
done beyond cleaning the dirt off for six years after its con- 
struction. 
Mr. Macadam's system differed from Telford's in this material 
point. The latter sought to obtain a hard, unyielding surface ; 
and hence his first formation of paving. The former, however, 
thought a certain amount of elasticity admissible, if not de- 
sirable. After the formation and drainage of the bed of the 
road, he covered it several inches in depth with a coating of 
hard stones, broken into angular fragments. This was watched 
by men ; and, as ruts or inequalities formed, they were raked 
and levelled, and fresh material added, until a hard and level 
surface was obtained. The material he used for coating was 
the hardest that could be obtained, preference being given to 
granite, greenstone, or basalt. 
While Telford's was the most thorough. Macadam's was the 
more economical in first cost. 
The great art is so to construct a road that, while a uniform 
surface is maintained, and the wet not allowed to penetrate from 
the top, the dirt also cannot be forced up through the road- 
material from its base. With this view an experiment was tried 
on the Highgate Archway Road of forming the foundation with 
concrete, composed of one-tenth of Roman cement, one-tenth of 
sand, and eight-tenths of stones or gravel, laid 6 inches thick 
over the central part of the road for a width of 18 feet ; on this 
a covering of broken stones, 3 inches thick, was laid, and a hard 
and durable road was the result. The cost was from 12s. to lbs. 
the running yard. 
On the Brixton roads Mr. Penfold used a concrete composed 
of four parts of gravel to one of lime, laid 6 inches thick, 
extending over half the road, and covered with 6 inches of broken 
stones. 
Few modern roads have the same care and attention bestowed 
on them as Telford gave, the practice now being generally to lay 
a foundation of such rough, dry material as can be most easily 
obtained. In some localities chalk is used, in London often the 
screenings from ash-yards ; burnt clay is also frequently made 
use of. All these materials are liable to objections unless special 
precautions are observed in their use. 
Chalk, on account of its affinity for water, readily absorbs 
moisture, which expands in freezing, and bursts the chalk into 
atoms, which the succeeding thaw changes into mud. Unless, 
therefore, the chalk is covered with a sufficiently thick coating of 
gravel or other material, in winter it forces its way up to the 
surface, and makes a soft, loose, and dirty road. The screenings 
of ash-yards being composed of materials of different degrees of 
hardness, many of which soon decay and rot away, afford a very 
