A manual of the principles and practice of road-making: comprising the location, construction, and improvement of roads, (common, macadam, paved, plank, etc.) and rail-roads. By W. M. Gillespie ...

198 IMPROVEMENT OF TIE SURFACE.
be used near collieries. Cubes of iron have been im.
bedded among the stones with some advantages.*
SIZE OF THE STONE.
The stone should be broken into pieces, which are as
nearly cubical as possible, (rejecting splinters and slices)
and the largest of which, in its longest dimensions, can
pass through a ring two and a half inches in diameter.
In reducing them to this size, there will of course Fig. 108
be many smaller stones in the mass. These are
the proper dimensions, according to Telford and
Parnell.t Edgeworth prefers 1 - inches. Penzfoldt names two inches for brittle materials. If
smaller they would crush too easily; but on the
other hand, the less the size of the fragments, the smaller
are the interstices exposed to be filled with water and mud.
The tougher the stone, the smaller may it be broken.
The less its size, the sooner will it make a hard road;
and for roads little travelled, and over which only light
weights pass, the stones may be reduced to the size of
one inch.
McAdam argues that the size of the stone used on a road
must be in due proportion to the space occupied on a smooth
level surface, by a wheel of ordinary dimensions; and, as it
has about an inch of contact longitudinally, therefore every
stone in a road exceeding one inch in diameter, is mischievous;
for the one-sided bearing of the wheel on a larger stone will
tend to turn it over and to loosen the neighboring materials.
But this argument proves too much; for however small the
stone is, there must be a moment, just as the wheel is leaving
it, when the pressure is one-sided, and therefore tends to over.
turn it. Subsequently McAdam preferred the standard of
I Parnell, p. 245. t Ibid. p. 133 t Pages 14, 15