Book II. M'ADAM'S ROADS. 519 



lated by the quantity of material necessary to form such impervious covering, and never 

 by any reference to its oum power of carrying weight. 



3335. The erroneous ojnnion so long acted upon, and so tenacumsly adhered to, that by 

 placing a large quantity of stone under the roads, a remedy will be found for the sinking 

 into wet clay, or other soft soils, or in other words, that a road may be made suflfi- 

 ciently strong, artificially, to carry heavy carriages, though the sub-soil be in a wet state, 

 and by such means to avert the inconveniences of the natural soil receiving water from 

 rain, or other causes, has produced most of the defects of the roads of Great Britain. 

 At one time M' Adam had formed the opinion that this practice was only a useless expense ; 

 but experience has convinced him that it is likewise positively injurious. 



3336. If strata of stone of various sizes be placed as a road, it is well-known to every 

 skilful and observant road-maker, that the largest stones will constantly work up by the 

 shaking and pressure of the traffick ; and that the only mode of keeping the stones of 

 a road from motion, is to use materials of a uniform size from the bottom. In roads 

 made upon large stones as a foundation, the perpetual motion, or change of the position 

 of the materials, keeps open many apertures, through which the water passes. 



3337. Roads placed upon a hard bottom, it has also been found, wear away more 

 quickly than those which are placed upon a Soft soil. This has been apparent upon 

 roads where motives of economy, or other causes, have prevented the road being lifted 

 to the bottom at once ; the wear has always been found to diminish, as soon as it was 

 possible to remove the hard foundation. It is a known fact, that a road lasts much 

 longer over a morass than when made over rock. The evidence produced before the 

 committee of the house of commons, showed the comparison on the road between 

 Bristol and Bridgewater, to be as five to seven in favor of the wearing on the morass, 

 where the road is laid on the naked surface of the soil, against a part of the same road 

 made over rocky ground. 



3338. The common practice, on the formation of a new road is, to dig a trench below 

 the surface of the ground adjoining, and in this trench to deposit a quantity of large 

 stones ; after this, a second quantity of stone, broken smaller, generally to about seven 

 or eight pounds weight ; these previous beds of stone are called the bottoming of the 

 road, and are of various thickness, according to the caprice of the maker, and generally 

 in proportion to the sum of money placed at his disposal. On some new roads, made 

 in Scotland in the summer of 1819, the thickness exceeded three feet. That which is 

 properly called the road is then placed on the bottoming, by putting large quantities of 

 broken stone or gravel, generally a foot or eighteen inches thick, at once upon it. Were 

 the materials of which the road itself is composed properly selected, prepared, and laid, 

 some of the inconveniences of this system might be avoided ; but in the careless way in 

 which this service is generally performed, the road is as open as a sieve to receive water; 

 which penetrates through the whole mass, is received and retained in the trench, whence 

 the road is liable to give way in all changes of weather. A road formed on such prin- 

 ciples has never effectually answered the purpose which the road-maker should con- 

 stantly have in view ; namely, to make a secure, level flooring, over which carriages may 

 pass with safety, and equal expedition, at all seasons of the year. 



3339. ^n artificial road in Bi'itain is only required to obviate the inconvenience of a 

 very unsettled climate. Water, with alternate frost and thaw, are the evils to be guarded 

 against ; consequently nothing can be more erroneous than providing a reservoir for 

 water under the road, and giving facility to the water to pass through the road into this 

 trench, where it is acted upon by frost to the destruction of the road. As no artificial 

 road can ever be made so good and so useful as the natural soil in a dry state, it is only 

 necessary to procure and preserve this dry state of so much ground as is intended to be 

 occupied by a road. 



3340. The first operation in making a road should be the reverse of digging a trench. 

 The road should not be sunk below, but rather raised above, the ordinary level of the 

 adjacent ground; care should at any rate be taken, that there be a sufficient fall to take 

 off the water, so that it should always be some inches below the level of the ground 

 upon which the road is intended to be placed : this must be done, either by making 

 drains to lower ground, or if that be not practicable, from the nature of the country, 

 then the soil upon which the road is proposed to be laid, must be raised by addition, so 

 as to be some inches above the level of tlie water. 



3341. Having secured the soil from under-ivater, the road-maker is next to secure it 

 from rain-water, by a solid road made of clean dry stone or flint, so selected, prepared, 

 and laid, as to be perfectly impervious to water ; and this cannot be effected unless the 

 greatest care be taken that no earth, clay, chalk, or other matter, that will hold or 

 conduct water, be mixed with the broken stone ; which must be so prepared and laid, as 

 to imite with its own angles into a firm, compact, impenetrable body. 



3342. The thickness of such road is immaterial, as to its strength for carrying weight ; 

 this object is already obtained by providing a dry surface, over which the road is to \)e 



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