_Answer._ This also varies greatly under different circumstances. An average taken from the monthly reports of 52 different roads gives 38 miles run per ton of coal, and an average from the reports of 16 roads gives 47¹⁄₂ miles run per cord of wood. No deductions should, however, be made from this of the relative value of wood and coal for fuel, because the trains which are run with wood for fuel are usually lighter than those hauled with coal-burning engines. The above figures are the average results during the month of May, 1871, of all the trains on the roads from whose locomotive reports it has been compiled. The following report of experiments, which were carefully made by the writer, will give the performance of a locomotive when great care is taken to produce good results. It should be stated, however, that the engine with which these experiments were made had been in service eighteen months, without receiving thorough repairs, and that the boiler at times primed badly, so that the rate of evaporation of water per pound of coal is not a fair indication of the performance of the engine in that respect. The coal used was known as Brazil coal, from Indiana, and in order to compare the performance of two engines only lumps of coal were used, so as to leave no room for question regarding the relative amount of fine coal used by each engine. The maximum grades on the road on which the experiments were made were 30 feet per mile, and the total ascent from the lowest to the highest point on the road was 374 feet. LOCOMOTIVE EXPERIMENTS. 1873. 1873. 1873. Date of experiment July 21. July 28. August 2. Number of miles run 145 145 145 Number of cars hauled 41 31 41 Total weight of cars, lbs. 1,497,240 1,119,650 1,508,860 Total amount of coal burned, lbs. 8,676 5,102 7,221 Total am’nt of water consumed, lbs. 63,531 45,719 52,609 Water evaporated per lb. of coal, lbs. 7.32 8.02 7.04 Miles run per ton (of 2,000 lbs.) of coal 33.4 50.8 38.8 Coal consumed per car per mile, lbs. 1.45 1.13 1.21 Average speed, including stops, miles 11.1 13 13.8 QUESTION 448. _How can we determine the speed at which an engine is running?_