canite pavements have only averaged 2.9 cents per square yard per annum."

So expensive were these coal-tar pavements to maintain that Lieutenant Hoxie, in 1887, estimated that their cost would be 20 cents per yard per annum, so that when the first Board of Commissioners appointed under Act of June 11, 1878, came into office, they expressed themselves as follows on this subject:

"In determining the class of pavements to be hereafter laid, the commissioners maintain that each class of pavement must prove its quality under test of actual traffic before being extensively laid upon the streets of this city.

"While some of the later and better class of coal-tar pavements show good service and give a fair promise of reasonable durability, yet the general condition of this class of pavements in the city is such as to lead to their condemnation as faulty in principle and deficient in vitality.

"The use of bituminous bases has also given rise to many perplexing problems in the grades of streets upon which they have been laid, and as, when properly laid, their cost is as great as, if not greater than, hydraulic concrete, they have been definitely abandoned."

In 1886-87 Congress passed a law which provided that no contract should be made for making or repairing concrete or asphalt pavements at a higher price than $2 per square yard, of a quality equal to the best laid in the District prior to July 1, 1886, and with the same depth of base. The lowest bid for asphalt pavements received immediately after the passage of this act was $2.25, which could not be accepted, and the city was obliged to return to coaltar pavements and those of asphalt block.

The specifications for these coal-tar pavements provided that the base and binder should be 4 inches thick and laid as follows: "The base will be composed of clean broken stone that will pass through a 3-inch screen, well rammed and rolled with a steamroller, to a depth of 4 inches, and thoroughly coated with hot paving-cement composed of the best No. 4 coal-tar distillate, in the proportion of about 1 gallon to the square yard of pavement. The second binder course will be composed of clean broken stone thoroughly screened, not exceeding 11 inches in dimension, and No. 4

coal-tar distillate. The stone will be heated by passing through revolving heaters and thoroughly mixed by machinery with the distillate in the proportion of one gallon of distillate to one cubic foot of stone. The binder will be hauled to the work, spread upon the base course at least two inches thick, and immediately rammed and rolled with hand and heavy steam rollers while in a hot and plastic condition. The wearing surface will be 1 inches thick when compacted, made of paving-cement composed of 25 per cent asphalt and 75 per cent coal-tar distillate, mixed with other materials as follows:

"Clean, sharp sand will be mixed with pulverized stone, of such dimensions as to pass through a 4-inch screen, in the proportion of 2 to 1.

"To 21 cubic feet of the above-named mixture will be added 1 peck of dry hydraulic cement, 1 quart of flour of sulphur, and 2 quarts of air-slacked lime. To this mixture will be added 320 lbs. of paving-cement to compose the wearing surface."

This material was laid on the street in practically the same manner as asphalt pavement is at the present time.

The coal-tar pavements laid in 1887 cost 4.65 cents per yard per year for maintenance for ten years, and those laid in 1888 cost 5.96 cents per yard per year for a period of nine years. At the end of ten years it was found necessary to relay some of them and substitute standard asphalt, and future repairs will be made in the

same manner.

From a table published in the report of the Engineering Department of the District of Columbia, in the fiscal years 1886-87, it is shown that the annual expenditure for the maintenance of coal-tar pavements for fifteen years ending July 1, 1886, had been 7.2 cents per square yard.

These pavements being laid on a bituminous base become practically a part of the base, and in repaving them it is necessary to take up the entire pavement; while if they had been laid on a hydraulic-cement concrete base, it would only have been necessary to have renewed the wearing surface.

The fact that these coal-tar pavements did not give complete satisfaction, and were expensive to maintain, led people interested in the subject to make experiments with other material.

Mr. E. J. de Smedt, who had taken out several patents and had made many experiments, laid a bituminous pavement in Newark, N. J., in front of the City Hall in 1870, with Trinidad asphalt as the cementing material. This was without doubt the first asphalt pavement laid in the United States. It was followed by another similar one in New York City near the Battery in 1871, and soon after by another in Philadelphia, and in a few years still more in New York City. These pavements gave such satisfactory results that they attracted the attention of the authorities in Washington, and a special commission was appointed by Congress to investigate and report as to the advisability of adopting them in Washington. As a result of the commissioners' report Pennsylvania Avenue from First Street to Sixth Street was paved with rock asphalt by the Neuchatel Asphalt Co. in 1876-77, and from Sixth Street to Fifteenth Street at the same time with Trinidad asphalt. These pavements gave good satisfaction, except that the rock asphalt was so slippery that when the street was resurfaced in 1890 Trinidad asphalt was laid over the entire area. The success of asphalt in Washington may be considered as settling to a great extent the experimental nature of the pavement, and from that time on its success has been assured and its use has continually increased.

In many respects asphalt makes a perfect pavement. It will sustain travel without being damaged, and in fact is benefited by quite severe traffic. It is smooth, pleasant to drive over, almost noiseless for carriages, and can be kept absolutely clean. It is impervious to water or moisture and, consequently, as a sanitary pavement is without a rival. It is considered by some to be expensive, and it is, as compared with some of the coarser rock pavements, but very few who have once used it are willing to give it up, or doubt that they have received the value of their money.

Many asphalt pavements have failed, and have required considerable resurfacing sooner than they should; but when it is remembered how new the industry is, how rapidly it has been developed, that there was no precedent for the mixtures, and that the principal mode of treatment, as well as the percentages of materials to be used, had to be determined by actual practice and experiment, the wonder is that not so many but that so few pavements have failed.

One of the objections made to asphalt is on account of its slipperiness and the liability of horses falling when they come off from a rough stone surface to the smooth asphalt. There is some reason in this, but as asphalt pavements increase in quantity, horses will become more accustomed to them and learn to adapt themselves to the smooth surface. Asphalt itself, contrary to the general belief, is not slippery. It is smooth, and any soft substance upon a smooth surface makes it slippery. Asphalt pavements should be kept clean and then there will be less trouble on account of horses slipping. Asphalt is much less slippery when dry than when slightly damp or moist. It is well known to truckmen that horses travel on a smooth pavement much more easily during a heavy rain than in a drizzle. A certain amount of street detritus must collect on any smooth pavement, and when rain falls in a quantity sufficient to wet it only rather than wash it clean, it must be slippery to a certain extent.

The question as to what is the steepest grade on which it is safe to lay asphalt has received a great deal of study. When the material was first introduced grades of 4 per cent were considered prohibitory, and very little was laid on those exceeding 3 per cent, but practice soon showed that this was too conservative a view, and as a result pavements have been laid successfully and quite frequently on grades as high as 7 and 8 per cent, and in Scranton, Pa., there is a portion of one street that has a grade of 12 per cent. It was said to have been placed on this particular block for the sake of preventing traffic, but, strange to say, it has not done so, and the City Engineer says that after several years' use no great trouble has been experienced with it.

Fig. 13 represents a profile of a portion of Bates Street, Pittsburgh, Pa. This shows that the elevation of the grade increased from 188.21 at the property line to 209.63 at a point 200 feet distant, making an average rise of 10.7 per cent. Instead, however, of making a uniform grade, these points were connected by a vertical curve, making in one section a grade of 17.1 per cent, and in the first 80 feet the minimum rate is 12.4 per cent. This street is paved with sheet asphalt, and without doubt has the steepest grade of any street in the world paved with that material.

As a rule, however, asphalt should not be laid on a street that

will be subjected to any material amount of traffic on grades exceeding 6 per cent, for there must be certain times of the year when they can be used but little and with considerable difficulty. On residence streets, however, where traffic is light, the people are willing in many cases to put up with the inconvenience of the slippery streets on a comparatively few days of the year for the sake of hav

ing smooth, clean, noiseless pavements for the remainder of the

time.

In New York City, where a street has been paved on a 6 per cent grade with asphalt on the sides and granite in the centre, as a rule the traffic seeks the smooth asphalt with its ease of traction, rather than take the granite.

Asphalt pavements are now in use upon grades in different cities as shown on page 218.