The table in the appendix will no doubt be valuable as indicating the properties and relative value of several different kinds of fuel used in this country. The table is copied from a report made to the Navy Department of the United States by Professor Walter B. Johnson in 1844, and the conclusions are deduced from a series of very elaborate experiments made for the Navy Department. This report furnishes the most full and reliable data regarding the value of American fuel thus far (1874) published; but it contains little or no information concerning the fuels which are now used on railroads in our Western States. The first eight specimens of coal given in the table are anthracite; all the rest are bituminous coals. PART XX. THE RESISTANCE OF TRAINS. QUESTION 407. _What is meant by the resistance of trains or cars?_ _Answer._ It is the power required to move them on the track. Thus if a rope, fig. 219, was attached to a car at one end, and the other passed over a pulley, _a_, and a sufficiently heavy weight, _W_, was hung on the end of the rope, it would move the car. The weight _W_ would then be equal to the resistance of the car. [Illustration: Fig. 219.] QUESTION 408. _How can the resistance of cars under different circumstances be determined?_ _Answer._ It has been found that it takes a force of about 6 lbs. per ton (of 2,000 lbs.) to move a car slowly on a level and straight track after it is started. That is, if a car weighs 20 tons and a rope, fig. 219, is attached to it at one end and the other passed over a pulley, _a_, with a weight, _W_, suspended to it, it will require a weight equal to 20 × 6 = 120 lbs. to keep the car moving slowly. If two cars of the above weight were coupled together, it would require twice 120 or 240 lbs., and if three were attached to each other, three times 120, or 360 lbs., and so on. In other words, MULTIPLYING THE TOTAL WEIGHT OF THE CARS IN TONS (OF 2,000 LBS.) BY 6 WILL GIVE US THEIR RESISTANCE, OR THE FORCE REQUIRED TO KEEP THEM MOVING ON A LEVEL AND STRAIGHT TRACK AT A SLOW SPEED AFTER THEY ARE STARTED. The resistance is represented by the weight above, and the locomotive must exert a force equal to that weight to keep the train moving. As the speed increases the resistance increases, as is shown by the following table. It should be stated here, however, that our knowledge regarding this whole subject of the resistance of American cars and trains is exceedingly inaccurate and imperfect, and the data given in the books are nearly all based on experiments made in Europe, with cars of a different construction from those used here. There is reason for believing, however, that the resistance of American cars is less than that of European cars, and we have assumed it to be 6 lbs. per ton on a level at very slow speed, which is less than the resistance which is usually given, but the following figures should be regarded merely as an approximation to the