tive.

If, on the other hand, the operation be carried on at too high a temperature or for too long a time, the resinous portions of the bundles of fibrilla will be softened and the wood lose its elasticity in just the proportion that the coherence of the fibrilla is lessened. The temperature should never be less than 212° nor more than 266° F.

The following specifications for creosoting are from the publication above referred to:

"Oil.-All oil shall be the heavy or dead oil of coal-tar, containing not more than 1 per cent of water, not more than 5 per cent of tar, and not more than 5 per cent of carbolic acid.

"It must not flash below 185° F. nor burn below 200° F., and it must be fluid at 118° F. It must begin to distill at 320° F., and must yield, between that temperature and 410° F., of all substances less than 20 per cent by volume.

"Between 410° and 470° F. the yield of naphthalene must be not less than 40 nor more than 60 per cent by volume. At two degrees above its liquefying-point it must have a specific gravity of maximum 1.05 and minimum 1.015.

"Processes of Treatment.-Seasoning: This is to be accomplished by subjecting the timber to the action of live steam for a period of from five to seven hours at a pressure of 35 to 55 pounds per square inch, the temperature not at any time exceeding 275° F. unless the timber be water-soaked, in which case it may reach 285° F. for the first half of the period. At the expiration of the steaming the chamber shall be entirely emptied of sap and water by drawing off at the bottom. As soon as the chamber is cleared of all sap and water a vacuum of not less than 20 inches shall be set up and maintained in the chamber for a period of from five to eight hours, or until the discharge from the vacuum-pump has no odor or taste, the temperature in the chamber being maintained at between 100° and 130° F. The chamber being again emptied of all sap and water, the oil is to be admitted, the vacuum-pump being worked at its full speed until the chamber is filled with oil. As soon thereafter as is practicable such a pressure shall be set up as shall cause the entire charge of timber to absorb pounds of oil within per cent, more or less (at a minimum penetration of 1 inches in round timber.

for a treatment of 12 pounds of oil per cubic feet, constituting a basis for determining the penetration due to a treatment of any specific quantity of oil), inches from all exposed surfaces. The depth of the penetration being ascertained by boring the treated piece with an auger making a hole not more than inch in diameter, such pieces as are found not to have the required penetration being returned to the chamber with a subsequent charge for further treatment."

Where any chemical treatment is adopted, both in this country. and in Europe, it seems to be generally accepted that creosote is the best treatment; but Australian woods have given excellent results, and about all that could be expected of any material, without any treatment whatever. It is doubtful, however, if any of the available woods of this country will give satisfactory results without chemical treatment of some kind.

The experience of the different cities that have tried wood as a paving material has been such that a pronounced success must be obtained before it will be again taken up as a paving material. Besides its lack of durability, it has been objected to seriously on account of its slipperiness and its unsanitary qualities. The varieties of wood that have been used in this country have absorbed water freely, which has been as freely evaporated in warm weather, this being one cause of its unsanitariness. This objection would probably be overcome to a great extent, if not wholly, by chemical treatment, as it is claimed by many bacteriologists that no unhealthy germ exists in the wood itself; but it is extremely doubtful, when the success of asphalt and brick is considered, if wood will ever come in favor in this country as a paving material.

CHAPTER XI

BROKEN-STONE PAVEMENTS.

As has been seen in the study of stone-block pavements, the developments led to a general reduction in the size of blocks. So with the irregular stone pavements, they, too, decreased in size as their use increased. While probably small broken stone were used in roads for many years previous, it was not until 1764 that what is known at the present time as macadam roads were first built systematically by M. Tresaguet, a French engineer, who was the first to adopt this plan, and it came into general use about ten years later. His method of construction as described by himself is as follows:

"The bottom of the foundation is to be made parallel to the surface of the road. The first bed of the foundation is to be placed on edge, and not on the flat, in the form of the rough pavement and consolidated by beating with a large hammer, but it is unnecessary that the stones should be even with one another.

"The second bed is to be likewise arranged by hand, layer by layer, and beaten and broken coarsely with a large hammer, so that the stones may wedge together and no empty space may remain.

"The last bed of 3 inches in thickness to be broken about to the size of a small walnut with a hammer on one side of a sort of anvil, and thrown upon the road with a shovel to form a curved surface. Great care must be taken to choose the hardest stone for the last bed, even if one is obliged to go to more distant quarries than those which furnish the stone for the body of the road. The solidity of the road depending on this latter bed, one cannot be too scrupulous as to the quality of the materials which are used for it."

The object of this lower course of large stone was to separate the wearing surface from the subgrade, rather than to form a foundation for the road.

This method as just described was practically that adopted by Telford some forty years later in England, the difference being principally in making the subgrade level and forming the crown with the stone itself, rather than making the base parallel to the finished surface of the road as Tresaguet did. The following is taken from Parnell's treatise on Roads, which gives Telford's specifications in detail:

"Upon the level bed prepared for the road materials the bottom course, or layer of stone, is to be set by hand in the form of a close, firm pavement. They are to be set on the broadest edges, lengthwise across the road, and the breadth of the upper beds is not to exceed 4 inches in any case. All the irregularities of the upper part of the said pavement are to be broken off by a hammer, and all the interstices to be filled with stone chips, firmly wedged together by hand with a light hammer. The middle 18 feet of pavement is to be coated with hard stone as nearly cubical as possible, broken to go through a 24-inch ring, to a depth of 6 nches; 4 of these 6 inches to be first put on and worked by traffic, after which the remaining 2 inches can be put on. The work of setting the paving-stones must be executed with the greatest care and strictly according to the foregoing directions, or otherwise the stone will become loose and in time may work up to the surface of the road. When the work is properly executed, no stone can move; the whole of the material to be covered with 14 inches of good gravel, free from clay or earth."

Parnell, in commenting on this last clause of covering the road with gravel, says: "The binding which is required to be laid on a new-made road is by no means of use to the road, but, on thể contrary, is injurious to it. This binding by sinking between the stone diminishes absolute solidity to the surface of the road, lets in water and frost, and contributes to preventing complete consolidation of the mass of the broken stone."

A contemporary of Telford and a man whose name has been given to this class of roads, in the English-speaking world at least, was Macadam. He worked on very different principles, in that

he not only did not require the foundation-course, but stated that he considered it positively injurious. He enunciated the following principles as fundamental: "That it is the native soil which really supports the weight of traffic; that while it is preserved in a dry state, it will carry any weight without sinking, and that it does in fact carry the road and carriages also; that this native soil must be previously made quite dry and a covering impenetrable to rain must then be placed over it in that dry state; that the thickness of the road should only be regulated by the quantity of material necessary to form such impervious covering and never by any reference to its own power of carrying weight."

In some evidence given before a Parliamentray commission upon the subject of roads, soon after Macadam had taken up their reconstruction, he stated in answer to a question by one of the committee that he considered that 10 inches of well-consolidated material was sufficient to carry any load, and that without any reference whatever to the foundation. He also added that he would prefer a soft foundation to a hard one, going so far as to say that he would prefer a bog if it were sufficiently hard to allow a man to walk over it. It must be remembered that all of these roads were very different when first built from those of the socalled macadam roads of to-day, as they received no rolling whatever, but were consolidated wholly by traffic.

The question as to which is the better system, Telford's or Macadam's, is one that has been discussed for a good many years. It is hardly necessary to say that at the present time Macadam's idea of having a soft, yielding foundation for his road is not considered good practice. On the other hand, the foundation as described by Telford is expensive, and in roads of light traffic, with a good natural foundation, it would seem to be unnecessary. Where a particularly solid roadbed is required it is the custom of many engineers to build what is called the telford-macadam road, that is, it has a telford base with a macadam wearing surface. Macadam's own particular work, when he took it up, consisted of repairing old roads rather than constructing new, and it is said that he was so successful that in many instances the cost of reconstruction per mile was but little, if any, more than had been the previous cost per annum for maintenance, and it is also true