case, if it is attainable without a great deal of extra expense. It is in the knowledge of the above points, and of the application of them in practice, that what may be called the science of road-making consists, as the observations apply in every case. When a road is to be formed, accurate sections of the rises and falls of the ground should always be taken, in the same way as is done for a canal, before the line is determined, or the levels of the road fixed upon, and when the course and levels of the road are laid down, the derail of the work ought to be particularly explained by a specification and plan, describing the manner in which each particular length is to be formed and completed.
The quantity of materials necessary to form the road depends so much upon the soil and the nature of the materials themselves, that it is impossible to lay down any general rules for them. The thickness ought to be such that the greatest weight will not affect more than the surface of the shell, and it is for this purpose chiefly, that thickness is required, in order to spread the weight which comes upon a small part only of the road over a large portion of the foundation. When the ground is very soft, trees, bavins or bushes, are applied to answer the same purpose, and to carry off the water previous to the materials of the road being so consolidated as to form a solid body, and to be impervious to water. Bushes are, however, not advisable to 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 chalk is useful for the same purpose in cases where bushes are improper, the chalk mixing with the gravel or stones becomes concreted, and presents a larger surface to the pressure. If the material for making the roads is gravel, the common way is to lay it as it comes from
