Book II. ROADS OF VARIOUS ENGINEERS 589 



metals as are most proper for a road formed on a sloping bank, or on a very dry bottom, 

 would be quite improper for a road that is perfectly level, and is much subjected to 

 dampness. In the former case, even six or eight inches deep of such metals will make a 

 good road ; but in the latter case, twelve or fourteen inches will be found inadequate. 

 In the former case, too, the metals should be of such a size as may fill and pass through 

 a ring from two to two inches and a half in diameter ; and in the latter case, they should 

 not be under three inches ; as under that size I have never found them to make a durable 

 road in such situations. Every road that has more than eight inches deep of metals, 

 should have the half of these in the bottom broken considerably larger than those on the 

 top. If the road, however, has a dry hard bottom, there is not so much need for this ; 

 but if the bottom is soft and wet, it is of the greatest service in making a firm road, and 

 preventing the metals from sinking : and the softer the bottom, the larger, of course, 

 they should be." But it is to be remarked, that the same author in his Letters, §c. 

 published three years afterwards, says, " In my former treatise I proposed, where the 

 bottom was soft, to have the under course of stones a little larger than those at top. 

 This I have seen of service, in several cases : but my mode of draining, which should 

 never be neglected, supersedes this entirely. 



3652. The criterion of size adopted by M'Adam is six ounces, or under, for even- part 

 of the stratum. 



5653. The size approved of by Clarke is not defined, but it should, he says, be small. 

 " The common practice is to lay a stratum of stones nearly the size of a man's head, as a 

 foundation, and to cover them with two or three inches of smaller ones ; but, from ex- 

 perience and observation, I am decidedly of opinion, that all the stones should be small, 

 and as nearly as possible of the same size : for, though a road made as above described 

 may be very good at first, the wheels of carriages will grind the small stones to powder, 

 the large ones will then rise to the surface, and the road will become intolerably rough, 

 and though frequently repaired with new materials, the same cause will produce a simi- 

 lar effect ; whereas, if all the stones are small, and nearly of the same size, they will 

 soon be cemented into one solid mass, and will be worn evenly to the last, so that no 

 repairs will ever be necessary, but the addition of a few broken stones occasionally." 

 (Obs. on Roads, p. 11.) 



3654. Infixing upon the size of the top metal, Stevenson observes, " the more hard and 

 tough its nature is, the smaller it may be broken ; it being an object of main importance 

 to have the metal ' well assembled,' as the road-makers express it, or broken of a uni- 

 form size. In almost every county there is a variation in the quality of the rock, and 

 also in the size to which it is broken. Roads have latterly been made under a specifica- 

 tion as to the weight of the pieces, varying from six to eight ounces. Formerly it was 

 not uncommon to have them specified, of the size of a ' hen's egg,' or even of a ' man's 

 fist.' By reference to weight, the road-maker's operations became more precise; but 

 regard should also be had to the specific gravity of the materials, which differs con- 

 siderably. For example, granite may be taken at twelve cubic feet in the ton, and whin- 

 stone (the greenstone, basalt, and clinkstone of mineralogists) is often met with of similar 

 weight. Compact limestone and flint are about fourteen, and quartzy sandstone about 

 fifteen feet to the ton. Perhaps the most convenient and uniform test for the size of 

 road metal is a ring measuring two inches and a half diameter in the void. When the 

 metal is thus broken, and the road carefully treated, its surface soon becomes smooth 

 and compact, without requiring the addition of blinding, or filling up the interstices 

 with gravel, which, if used, should be free of earthy particles. But this addition is hardly 

 necessary, where there is much traffic, as the rough and angular sides of the metal soon 

 lock into each other, and form a smooth surface." {Ed. Encyc. art. Roads.) 



3655. The mode of preparing gravel is nearly the same by all the best road engineers, 

 who agree with Telford, that it ought to be completely cleansed of ever)- particle of clay 

 or earthy substance, and its different sizes ought to be selected and arranged by means 

 of riddling or washing. In the use of the ridder, the particles of earth or clay adhere 

 so much to the stones, that it frequently requires to be exposed to the sun, air, and frost, 

 for several months, and then riddled over again. In this gravel, the stones are of dif- 

 ferent sizes and different shapes ; all those that are round ought to be broken with a small 

 hammer. Some attempt to attain the same end sooner by washing ; but this is both a 

 more expensive and less effectual mode than that of taking advantage of the weather. 



3656. The mode of breaking stones recommended by Edgeworth, is by persons sitting, 

 and using small hammers. A hard stone should be used as an anvil, and the stone to 

 be broken may be advantageously held in a forked stick. Attempts were made some 

 years ago to break limestone for roads, by the force of horses, wind, and water. Stampers, 

 shod with iron, and raised by proper mill-work, were employed; they were let fall upon 

 blocks of whinstone. These mills were found profitable for breaking limestone to 

 powder, as a manure, where fuel was scarce, but they crushed the stone to dust rather 

 than to fragments ; if lighter stampers were employed, they frequently failed to break 



