ments so with wood blocks, the gauge of a horse's hoof is the measure of the maximum width. After numerous experiments with widths varying from 3 inches to 4 inches, European engineers have decided upon the following dimensions: for rectangular blocks, width 3 inches, depth 6 inches, length 9 inches.

The advantage of the narrower width is that, besides affording a more ready foothold when the pavement is slippery, narrow blocks have more stability than wide ones of the same depth.

The length of a block should be suitably proportioned to the width; a length of 12 inches has been tried and found to be too much the blocks were subject to splitting across. Nine inches appears to be the most suitable length.

For round blocks the diameter should not exceed 6 inches; the depth may be the same as for the rectangular blocks, 6 inches. There is no advantage gained by a greater depth, for few wood pavements under the most favorable conditions retain a sufficiently good surface after about six years' wear without extensive repairs, and it is therefore not advantageous to lay blocks of a greater depth than will provide for a duration of seven years. Six inches is sufficient for this.

190. Expansion of Blocks.-Wood blocks expand on exposure to moisture, and when laid end to end across the street the curbstones are liable to be displaced, or the courses of blocks will be bent into reverse curves. To avoid this the joints of the courses near the curb may be left open, or the courses next the curb may be left out until expansion has ceased, the space being temporarily filled with sand. The rate of expansion is about 1 inch in 8 feet, but varies for different woods. The time required for the wood to become fully expanded varies from 12 to 18 months. By employing blocks impregnated with the oil of creosote this trouble will be avoided. Blocks so treated do not contract or expand to any appreciable extent.

191. Width of Joints.-Experience has demonstrated that the wide joints once thought necessary for foothold are not required. The best European practice of to-day is to make the joints as near one quarter of an inch as possible. Wide joints hasten the destruction of the wood by permitting fibres to spread under the traffic.

192. Filling for Joints.-The best materials for filling the joints are bitumen for the lower two or three inches, and hy

draulic cement-grout for the remainder of the depth. The cementgrout protects the pitch from the action of the sun and does not. wear down very much below the surface of the wood.

193. Durability. That wood pavements formed of well-seasoned wood properly laid on an unyielding foundation, with water-proof joints between the blocks, may last for many years without suffering decomposition, has been amply demonstrated by the experience had with wood pavements in the city of London and other European cities.

From the following table and remarks it will be seen that the durability of wood pavements in London varies from 5 to 19 years, while in the United States it varies from 3 to 7 years.

Table XXXI shows the actual duration and cost of certain wood pavements in the city of London.

TABLE XXXI.

DURATION AND COST OF WOOD PAVEMENTS IN THE CITY OF LONDON. * (Foundations are included, but no

excavation.)

"The average

life of the pavements in the three streets with the

argest traffic was about 9 years, that of the three streets with the least traffic about 11 years. Nearly all before they were removed

Average Cost per sq. yd. per annum.

had been relaid over their entire surface, and some new wood introduced from time to time in lieu of that found too defective to relay."

"It will be observed that the wood pavements last removed had a shorter life than the previous pavements. There is more than one reason for this, but it should be stated that nearly all would by relay and the introduction of some new wood have endured a few years longer."

194. The wood pavements of Berlin have not proved as durable as those of London and Paris, and their use is practically abandoned. Those of Frankfort (Ger.) laid under the Kerr system are giving satisfaction, and are said to be in as good condition to-day as when laid five years ago. The traffic on them is said to be con

stant and heavy.

195. W. Weaver, Chief Engineer and Surveyor, Kensington, London, says wood pavement of 5-inch creosote blocks will last ten years.

196. In Chicago, Ill., in some streets wood pavements have lasted upward of ten years; in others they have become very rough and uneven in three or four years, while in the river-tunnels they have worn out in two years.

197. The wood pavements of Washington, D. C., were of green hemlock, very badly constructed, and were destroyed by decay and dry-rot in about four years.

198. In St. Louis, Mo., the average life of the Nicholson pavements was six years. Burnettized cottonwood used on Broadway developed decay in the third year. Mr. George Burnet, Street Commissioner of St. Louis, in his annual report for 1890 recommends the use of cedar-block pavement for medium-traffic streets.

"Since 1884 the practice has been to lay gum and cottonwood blocks impregnated with chloride of zinc on a foundation of cement concrete 6 inches thick, and the joints filled with hot bituminous composition. The channels are formed by iron studs driven in the side of the blocks to the head, which is half an inch." (Thomas H. Macklind, District Engineer.)

199. In Detroit, Mich., the Board of Public Works consider that cedar-block pavements will last eight years before extensive repairs are necessary, but that it is better to make repairs earlier.

200. Mr. R. W. Roberts, City Surveyor of East Saginaw, Mich.,

in his report for 1889 says: "Eight to ten years is the estimated life of cedar-block pavement laid on sand and board foundation. Allowing that our pavements will last ten years, then during the next ten years this city will have to do all its paving over again at a cost equal to about 85 per cent of the first cost. Would it not be better to change from cedar-block pavements to something more durable, though the first cost is greater? On our business streets especially, the interruption of business by the tearing up and relaying of pavements is a thing to be considered in choosing our paving material."

201. According to the annual report for 1890 of the Board of Public Works of Duluth, Minn., 89,300 square yards of cedar-block pavement were laid in that year. The experience of the year shows that on grades between 4 and 13 per cent the best street surface is formed of blocks not more than 6 inches in diameter, laid in the usual manner, and with joints filled with grouting of Portland cement and sand. It has been found necessary to keep the surfacewater on top of the pavements on grades exceeding 7 per cent. and on this account a concrete foundation is regarded as indispensable.

No tar composition was employed on the wood pavements in 1890, as its use was found to hasten rather than retard decay in the climate of Duluth, making the extra cost of about 17 cents per square yard an unnecessary expenditure. On levels and light grades the dust from the gravel strewed over the surface at the time of completion of the pavement soon worked into the joints, making an impervious roadway.

Owing to the fact that the subsoil of the streets in that city is a clay which causes the customary 3 inches of sand to be an insufficient foundation, most of the pavement laid in 1891 was built on a Telford foundation. This consists essentially of two layers of stone. The first is 6 inches thick, composed of large stones thoroughly wedged together, all chinks being filled with smaller stones, and the whole surface covered with a layer of wet gravel compacted by a 20-ton steam-roller. The second layer is 2 inches thick, composed of broken stone not more than 2 inches in greatest diameter, and covered with wet rolled gravel, like the first. On top of this foundation is sprinkled a thin layer of sand, which is covered by a course of 1-inch plank, affording a perfectly smooth and uniform surface on which to lay the blocks.

Such a foundation has been found in Duluth to have the advantage over concrete, which has heretofore been used on the best business streets, of having every portion of the sub-grade thoroughly compacted by the roller, the broken stone being forced down into the numerous soft spots. In preparing the subgrade for concrete foundations it was found that many spots had to be left unrolled, as they were too soft for the roller to pass over them. On the other hand, openings in Telford foundations for repairs to underground work cannot be as completely restored to their original condition as can concrete foundations, it being impracticable to thoroughly consolidate Telford macadam without the use of a heavy roller.

202. Wear. The wear of wood pavements is generally considered to be as much due to the action of the horses' feet as to that of the wheels, and the action of the former is more destructive on steep grades; the wear is also increased by wide joints.

The wear of wood pavements by the abrading action of traffic is stated by various authorities as follows:

The wear in the latter years of the life of the wood was found to be greater than in the first years. The wear between street-car rails is about one third more than the remainder of the roadway.

203. St. Paul, Minn.-The cedar-block pavement laid in 1882, on a plank and sand foundation, shows after seven years' use a wear of 2 to 2 inches under ordinary traffic; on recent investiga