528 PRACTICE OF AGRICULTURE. Part III. 



hardest and most durable that the place or neighborhood can afford. But this dura- 

 bility will be found in a great measure to depend on the dryness of the road. Freestone, 

 of a moderate hardness, such as chemists would term No. 6., * that would with difficulty 

 yield to the knife,' will make a very good road on a dry sloping bank, exposed to the 

 sun and air, or even on a level surface that has a dry gravelly bottom. Nay, even seven 

 or eight inches deep of such metals on such situations, will make a better road than 

 twelve inches of the best metals, where the bottom is constantly damp, and will actually 

 surpass it in point of durability. This, however, is not meant to give a preference to those 

 metals, but merely to show the great difference there is betwixt a wet and a dry bottom ; 

 and that such metals will answer very well in the situations above described. Still it 

 must be held as a general rule, to take the best and hardest metals the neighborhood can 

 afford, as formerly mentioned." 



3388. But the hardest metals will not ahvays be found the most durable ; and here it may 

 be remarked, as another general rule, with some exceptions, that the worse they are to 

 break, the greater their durability. Some stones, for instance, as hai'd as No. 9. of mine- 

 ralogists, " such as would give a few feeble sparks with strit," are so free that they will 

 fly under the stroke of a hammer like so many pieces of glass. These, although very hard, 

 being of a quality so free and brittle, will grind down by the wheels rather easily, and in 

 time of rains will be formed into mud ; while, on the other hand, there are stones not 

 harder than No. 7. that are so tough, that there is great difficulty in breaking them. 

 Yet these latter, although two degrees softer, will absolutely last longer than the former, 

 on any road whatever. 



3389. Flints reduced to a small size, and mixed with chalk, make an excellent 

 road in dry weather ; but chalk being very absorbent of water they become slippery 

 and soft in moist weather, and are much affected by frost. 



3390. Whinstone, M'Adam, and all road engineers, agree in considering the most 

 durable of all materials ; and wherever it is well and judiciously applied, the roads 

 are comparatively good and cheap. Fry, however, has uniformly observed, in various 

 parts of England, that where limestone is used, the roads are the best, and this superiority 

 is not in his opinion owing merely to the hardness of this substance, but also to its 

 adhesive or cementing property : how, otherwise, he says, are we to account for the firmness 

 and solidity of the roads around Bristol, that are made of white limestone. 



3391. Gravel is of two kinds, that obtained from pits, and that from the beds of rivers. 

 Gravel is generally silicious and hard ; otherwise indeed it would have been worn down 

 to sand, in undergoing the operation which has rendered it gravel. This material is chiefly 

 used on the roads round London: it is often found, Paterson observes, "to answer very 

 well in point of durability. But such kind of gravel, being composed chiefly of hard sand, 

 and smooth, little, round stones, does not so easily bind together, and seldom make a 

 very firm road. On the other hand, stones that are broken have so many sides that 

 they readily lock into one another ; whereas the small round gravel keeps rolling and 

 shifting about by every motion of the wheels. All road metals, therefore, should be 

 of stones as large as to require breaking before they are used. The roads on which 

 gravel will be found to answer best, are those which are neither too wet nor too dry. I 

 have seen a road made with such kind of materials, not only easily rutted in time of 

 the winter rains ; but the same road, in the drought of summer, became as loose as ashes, 

 and was then also very easily rutted ; while in a medium betwixt these two extremes, it 

 answered exceedingly well. Upon the whole, it would be improper to use gravel for any 

 turnpike or public road, where stones can be got that require to come under the hammer." 

 {Treatise, ^c. p. 31.) 



3392. The gravel of which roads are usually formed, is mixed with a large portion 

 of clay, and because the component parts of gravel are round, and want the angular 

 points of contact, by which broken stone unites, and forms a solid body ; the loose state 

 of the roads near London, is a consequence of this quality in the material, and of tlie 

 entire neglect, or ignorance of the method of amending it. 



3393. Gravel is the worst material for making roads subject to great traffick. Telford, 

 on being asked his opinion of it by the road committee, replied, " I am of opinion that 

 the materials in the whole valley or plain round London being entirely silicious, or flints, 

 and easily ground to dust, are very improper. This must be evident to every person who 

 travels near London in any direction." In this opinion M'Adam concurs. 



3394. Artificial materials for roads are sometimes had recourse to, when stone or 

 gravel is not to be procured, and sometimes used because unfit for any thing else. 

 They are chiefly the scoria of founderies, dross, cinders, &c., to which may be added 

 burnt *clay; the last a very perishable material. It is burned in clamps like bricks, 

 and differs from them in being in irregular masses, and in not having been previously 

 worked. 



3395. The preparation of materials relates chiefly to their proper size or weight, and 

 cleaning from earthy matters. 



