Book II. ROADS OF VARIOUS ENGINEERS. 585 



should not be used, unless they are so low as always to be completely moist. When 

 they are dry and excluded from the air they decay in a very few years, and produce a 

 sinking in place of preserving the road ; a thickness of hard chalk has been recommended 

 for the same purpose ; the chalk, mixing with the gravel or stones, becomes concreted, 

 and presents a larger surface to the pressure. It is alleged on the other hand, that 

 chalk is one of the worst materials for roads, as it absorbs water, which, when frozen, 

 never fails to break up the road. 



3627. The base of the road is constructed by Telford and Stevenson of an elliptical form ; 

 if it is upon clay or other elastic substance which would retain water, Telford would 

 recommend to cover the whole bottom of the road with surface soil; in cases where 

 the natural shape of the ground admits, he would not remove the original surface; 

 and, where there are inequalities, he would fill them up with surface soil, so as to cut off 

 all connection with clay. 



3628. Informing the basis of a road on a flow-bog, Clarke directs to strip the heathy 

 sods (tussocks) off the whole surface of the side-drains, and place them with tlie heath 

 uppermost on the space intended for the road ; or if a sufficiency of brushwood or furze 

 can be procured, it will answer still better. Proceed to let off the water at the lowest ends 

 of the drains, leaving an open channel in the middle of each. After the water has run off 

 for some time, throw off another spit ; and repeat this operation month after month, till 

 the space for the road becomes compact and dry ; and be sure to keep it in that state by 

 cleaning the drains frequently. There should be eight or ten inches of tough clay laid 

 over the tussocks or brushwood, which will be greatly the better of being consolidated by 

 rollers. This part of the road may be left rather higher in the centre than the other 

 parts, to allow for settling. There is no situation where it is more difficult to make a 

 good road than through a flow-bog ; but, if once made well, it is the most permanent of 

 all roads, and, from its elasticity, the most easy to horses. 



3629. In forming the basis of a road on thin moor, the whole of the peat should be 

 removed from the space on which the road is to be made ; for, if allowed to remain 

 between the hard subsoil and the small stones, the weight of carriages would press down 

 the latter, force up the black peat through them, and totally spoil the road : this happens 

 only where there is a thin, soft, peaty stratum between two hard bodies ; for in deep bog, 

 the elasticity of the foundation yields to the superficial pressure, and contributes to the 

 durability of the materials : after this has been so removed, the surface, when formed 

 and drained, will be ready for the road materials. 



3630. In forming the base or metal-bed, Paterson observes, " it is common to cut it to 

 the exact breadth and depth of the metals, and to make it quite flat in the bottom, or level 

 from the one side of the metals to the other. Supposing this metal-bed to be formed 

 fourteen feet broad, and nine inches deep, on a breadth of fourteen feet, the metals 

 would require to be about three inches higher in the middle than on the sides. In this 

 case, then, they would be nine inches deep on the sides, and twelve on the middle ; and 

 as it is evident that the middle of the road, where the metals are deepest, is not sub- 

 jected to so much waste from the tread of the horses' feet, as that nearer the sides is 

 from the grinding of the wheels, this is, therefore, a waste of metals on the middle of the 

 road. But this is not the greatest evil of which I complain : the metal-bed being cut 

 into the solid ground, andi flat in the middle, and having the earth on each side about 

 nine inches higher than it, this, upon any other ground than that of dry sand or gravel, 

 forms a bed for retaining the water, as well as for holding the metals, which often deluges 

 the middle of the road with mud or gutters, when it might be prevented. I would 

 therefore propose, that a metal-bed of fourteen feet broad should, instead of being level, 

 have a rise in the middle of at least four inches, which will make a declivity from the middle 

 to each side of nearly two inches in the yard. Then, supposing the surface of the metals 

 to have the same shape as mentioned above, viz. three inches higher on the middle than 

 on the edges, the metals on the sides will be the same depth as formerly mentioned, 

 namely, nine inches ; but instead of twelve inches on the middle, they will then only be 

 seven inches deep, which makes a saving of five inches. This saving of five inches on the 

 middle, or two inches and a half on the whole breadth of the metals, is very considerable ; 

 but this is not the only benefit arising from this mode of procedure. The metal-bed, having 

 a slope from the middle to each side of the road, so far from retaining the water, runs it 

 off from the middle ; and this will be of more service in keeping the road in good order 

 ever afterwards, than if you were to put three or four inches more of additional depth to 

 the metals on the common plan. This appears to me to carry so much of common sense 

 on the face of it, that I am surprised it has not long ere this time been generally adopted." 

 Here Paterson seems to infer that water may, or rather does, penetrate the stratum of 

 metal to the base, which, in properly made roads, will at least not often be the case. 

 The argument of a saving in materials is quite sufficient to justify him and Telford in 

 adopting the elliptical form for a basis. 



3631. A soft base is always preferred by M'Adam, who drains effectually, and puts no 



