ment as to what constitutes defects there may be differences of opinion as to whether or not they exist in a given article, as well as to the equality of goods furnished with the sample deposited.

The characteristic qualities and strength of the material are not clearly defined, or in such manner as will enable the bidder to correctly interpret the meaning. The power to accept or reject, although nominally in the hands of the engineer, is indefinite and unsupportable, because the acceptance or rejection cannot be made in accordance with known provisions and fixed rules. In the absence of recognized standards two courses are open in order to secure the desired qualities, avoid indefiniteness and controversy; namely, (1) to reserve the right to make, before awarding the contract, any test that the engineer may see fit to make, and award the contract in accordance with the results of such tests; or (2) prescribe in the specifications the definite tests to which the material will be subjected, with such reservations as to time and place as the exigencies of each particular place seem to demand.

CHAPTER VII.

BROKEN-STONE PAVEMENTS.

335. As near as can be ascertained, the first broken-stone pavements were constructed in France in 1764 by one M. Tresaguet, who built many miles of such pavements in the latter part of the last century. In the early part of the present century two systems were introduced into England, the first by Telford, the second by Macadam.

336. The name of Telford is associated with a rough stone foundation, which he did not always use, but which closely resembled that which had been previously used in France. Macadam disregarded this foundation, contending that the subsoil, however bad, would carry any weight if made dry by drainage and kept dry by an impervious covering. The names of both have ever since been associated with the class of road which each favored, as well as with roads on which all their precepts have been disregarded.

337. The following specifications show the difference in the methods of the inventors.

338. Tresaguet's Method, 1764 (Fig. 21).-"The bottom of the foundation is to be parallel to the surface of the road. The first bed or foundation is to be placed on edge and not on the flat, in the form of a rough pavement, and consolidated by beating with a large hammer; but it is unnecessary that the stones should be even one with the other. The second bed is to be equally placed by hand, layer by layer, and beaten and broken coarsely with a large hammer, so that the stones may wedge together and no empty spaces remain. The last bed, three inches in thickness, is to be broken to about the size of a nut with a small hammer, on a sort of anvil, and thrown upon the road without a shovel to form the curved surface. Great attention must be given to choose the hardest stone for the last bed, even if one is obliged to go to more

from the centre, four inches; and at fifteen feet from the centre, three inches. They are to be set on their broadest edges lengthwise across the road, and the breadth of the upper edge is not to exceed four inches in any case. All the irregularities of the upper part of the said pavement are to be broken off by the hammer, and all the interstices to be filled with stone chips firmly wedged or packed by hand with a light hammer, so that when the whole pavement is finished there shall be a convexity of four inches in the breadth of fifteen feet from the centre.

"The middle eighteen feet of pavement is to be coated with hard stones to the depth of six inches. Four of these six inches to be first put on and worked in by carriages and horses; care being taken to rake in the ruts until the surface becomes firm and consolidated, after which the remaining two inches are to be put on.

"The whole of this stone is to be broken into pieces, as nearly cubical as possible, so that the largest piece in its largest dimensions may pass through a ring of two and one half inches inside diameter.

"The paved spaces on each side of the middle eighteen feet are to be coated with broken stones or well-cleaned gravel up to the footpath or other boundary of the road, so as to make the whole convexity of the road six inches from the centre to the sides of it, and the whole of the materials are to be covered with a binding of an inch and a half of good gravel free from clay or earth."

340. Macadam's Method (Fig. 23).—Macadam omitted the foundation of large stones, claiming that it was not only useless but injurious; he placed on the natural soil a layer of stone broken. equally into cubes of about one and a half inches in their greatest dimensions, and spread equally over the surface of the road to a depth of ten or twelve inches. Binding material was not used, the stone being left to work in and unite by its own angles under the traffic. Macadam preferred the test of weight to that of measurement, and insisted that no stone should weigh more than six ounces, which is the weight of a cube of one and a half inches of hard compact limestone; his overseers were provided with small scales and a six-ounce weight to test the larger stones.

Although Macadam was the pioneer of good road construction. in England, and from whose name the word macadamized is derived, it may be observed that he had been anticipated in the pro

mulgation of the system of a regularly-broken stone covering by Mr. Edgeworth, an Irish proprietor, whose treatise on roads, of which the second edition was published in 1817, contains the results of his experiments on the construction of roads, with some useful rules. He advocated the breaking of the stones to a small size, and their equal distribution over the surface. He also recommended that the interstices should be filled with small gravel or sharp sand-a practice which, though condemned by Macadam, is now adopted by the best roadmakers.

341. Since Telford and Macadam's time the practice of roadmaking has been greatly improved by the introduction of rollers. and stone-crushing machinery.

342. Modern Telford. On the natural-soil bed, properly graded, a layer of stones eight inches thick is set by hand, arranged and wedged as described by Telford. On the stone foundation so prepared a layer of broken stone of a size not exceeding three inches is evenly spread and rolled; the surface so rolled is covered with a layer of sand one-half inch thick, and the rolling continued; then a layer of stones not larger in any dimension than two inches is spread to a depth of four inches and rolled, followed as before with a layer of sand and also rolled. Finally a coating of clean sharp sand is applied, well watered, and the rolling continued until the surface becomes smooth. The surplus sand is then swept off and removed.

343. Modern Macadam pavements are constructed in the manner above described, only omitting the stone foundation, and the depth of the stone varies from four to twelve inches.

344. Defects of the Telford System.-(1) No matter how carefully the interstices between the foundation-stones are filled with chips, a large percentage of voids is left giving free access to water, thus defeating the object of the covering, which is to preserve the natural soil from contact with water. The pavement acts as a drain; the natural soil becomes saturated with water, and a slow but constant sinking of the bottom stone into the subsoil and a slow but gradual rising of the natural soil takes place, the cohesion of the superstructure is destroyed, and it finally becomes a mass of mud and stones.

(2) If the foundation be of a harder rock than the covering, it becomes an anvil on which the softer stones are pounded to pieces by the passing loads.