Book II. ROADS OF ENGINEERS. 527 



substrata, whether clay or sand, can never be felt in eifect by carriages going over the 

 road ; because a road well made unites itself in a body like a piece of timber or a 

 board. 



3382. An instructive proof of the preference given by M^Adam to a soft base is derived 

 from a case which occurred near Montrose. This case was sent to him by Paterson in 

 the following report. " This road," says the reporter, '* for about a mile, goes over a bank 

 of sea-beach, many feet in depth, and all round stones from two to five or six inches in 

 diameter. Always as the stcrties above three inches work up, and make their appearance 

 on the surface, they are taken off to the side of the road, and broken to the ordinary size. 

 This has been done several times every year for many years back, but the road always 

 continues loose and open as ever." The answer of M'Adam was, " I am of opinion 

 that you will not have a good and solid road over this beach, until you have a depth of 

 eight or ten inches of properly broken metals on the surface; and probably it might be 

 an improvement to put under them a few inches of soil, as the bottom is so very loose. 

 But although the great mass of stones, over which this road is made, is of the best quality, 

 yet as tlie greater part of them are under three inches diameter, I am afraid that were they 

 even broken to my size, they will not bind together as if broken from larger stones." 



3383. M^Adarns Answer. " The road you, have sent me a report of, is novel in its 

 situation, but very far from hopeless. The sea-beach, of which it is wholly composed, 

 should be picked ; that is to say, the large sized pebbles should be carefully removed 

 from the surface, and carried to the side of the road, and there broken, not to what your 

 surveyor calls my size, which is six ounces, but smaller, say to three or four ounces. And 

 / must also warn you, that any round stone, when broken in half so as to form a hemis- 

 phere, is nearly as unmanageable and as little likely to consolidate in a road, as one left 

 quite round ; therefore, with regard to weight, your stones must be taken so as to form 

 as many angles as possible. No large pebble must be left in sight upon the bottom of 

 the road, otherwise they will work up through the broken stones, of which your road will 

 be composed ; but having prepared a surface upon which to place your road, by removing 

 the large sized pebbles (I mean all above six ounces), and even covering the surface with 

 sand soil or other soft matter, lay on your properly broken stones." Paterson entirely 

 concurs with M' Adam in regard to the advantage of a soft base, adding in his last pub- 

 lication [Letters, &c. 1822.), " although tlie ground under the materials can never be 

 too dry, the materials never unite so firm when placed upon a hard rock, or upon gravel, 

 as they do upon earth, moss, or sand. There should always, therefore, be a few inches of 

 the one or the other of these, put under the road as a bed for the materials, where it is on 

 a rocky or gravelly bottom." 



3384. IVhen the basis consists partly of Jirm, and partly of loose materials, or moved 

 earth, some nicety is required to determine the allowance for the sinking of the latter, and 

 indeed roads, under such circumstances, cannot often be finished out of hand. Some 

 judicious directions on this subject are given by Paterson. '* When a road," he observes, 



" is formed along the side of a hill, or sloping bank, the earth that is produced from the 

 side-cutting makes up a part of the breadth of the road ; so that the road is formed, part- 

 ly on the solid ground, and partly on the embankment. All new-made-up earths, or 

 embankments, subside a little, whatever be the nature or quality of the stuff of which 

 they are composed. For which reason, that part of the breadth of the road, that is formed 

 upon the embankment, should be raised a little higher than the solid ground. No pre- 

 cise rule can be given to ascertain exactly how much the different kinds of earths, clays, 

 gravel, &c. will subside ; but the following has been found so near to the truth, in most 

 cases, that it may with safety be admitted as a general rule. 



3385. At all places where there are embankments, whether over hollow ground, or 

 along the side of a sloping bank ; for every foot that these embankments or mounds are 

 raised in height, one inch may be allowed for subsiding. So that if an embankment, or 

 the outer edge of a road formed from the side-cutting, requires, for instance, six feet deep 

 of forced earth to bring it to the level required, in that case it should be made six inches 

 higher, namely, six feet six inches upon the newly-made-up ground. And it will be 

 found, in general, to be about six months, from the time that the embankment has been 

 made, until it has become properly consolidated together. 



3386. IVie materials of the road may be considered in regard to their nature or kind, 

 the proper size and weight ; the outline of their upper surface, and the mode of laying 

 them on, and consolidating them. 



3387. Stone is universally allowed to be the best kind of material for roads, and granite, 

 trap, or flint, the best species; next in order are some sorts of limestone, and hard sandstone. 

 Soft claystone is the worst. Limestone is the principal material in Wiltshire, Somerset- 

 shire, Gloucestershire, and Ireland : Granite and trap in the north of England and 

 Scotland ; slatestone in North Wales ; sandstone pebbles in Shropshire and Stafford- 

 shire ; flint in Essex, Susse?^, and part of Kent ; and gravel in Middlesex and Surrey. 

 *' The stones used for the metals of any road," Paterson observes, " should always be the 



