herence, or of friction between the wheels and the soil, was 0.3. 5. A speed of seven miles an hour caused no special difficulty in managing either the locomotive or its load. At about this time M. Servel conducted
a series of experiments with a similar machine upon paved and upon macadamized roads, during what he described as the most trying of winter weather. He reports the following distribution of weight per cent.:
The average total weight of three loaded wagons, which was the usual load, was 22,575 kilogrammes, or about twenty-two tons. The experiment was made in 1867-'68 of applying these engines to the towage of boats on the French canals, with very encouraging results.
In 1871 several traction-engines were exhibited before the Royal Agricultural Society of England at Wolverhampton, and the judges made a series of careful tests, reported in its "Journal" for that year. The coal used on special trial amounted to 3.2 pounds per indicated horse-power per hour, and the evaporation of water was 7.62 pounds per pound of coal consumed, the average temperature of feed being 175° Fahr. The load drawn up the maximum grade of 264 feet to the mile on Tottenham Hill, which is 1,900 feet from top to bottom, was twenty-six tons, and including weight of engine thirty-eight tons, giving a coefficient of traction of 0.35. On a country-road sixteen miles long it drew fifteen tons at an average rate of 3½ miles an hour, using 2.85 pounds of coal and 1.94 gallon of water per ton of useful load per mile.
67. In October, 1871, the writer conducted a public trial of road-engines and steam road-rollers, on a well-macadamized road at South Orange, New Jersey. Two road-steamers (Fig. 39) or traction-en-