The difference between the extremes of engines in different classes is not so great as it is with horses; it is, nevertheless, considerable. The express engine, with its driving-wheel of diameter varying from six feet and a half to eight feet, can carry a _light load_ of seventy to ninety tons on a good—that is, a comparatively level—roadway, at the rate of fully fifty miles an hour, and can maintain this pace with perfect safety for a length of 90 to 100 miles, provided she can be supplied with water, as can now be done by means of the trough. Were she with a load behind her, not exceeding three or four carriages, she could, as a matter of performance, and without reference to the question of safety, maintain, without difficulty, a rate of sixty miles an hour. A four-wheeled coupled engine, each of the four wheels being of six feet diameter, and with appropriate dimensions of heating-surface, draws easily 120 to 150 tons, at a running speed of forty miles an hour. An engine, similar in other respects, but with wheels of five and a half feet diameter, could run thirty-five miles an hour with an increase of load of from thirty to fifty tons. “Goods engines” of the most powerful class—that is, six wheels coupled—diameter of wheel four feet, can draw from 300 to 350 tons. This is also the diameter—in some instances it is even smaller—for the heavy engines used to ascend severe gradients. In England we have no heavier goods engines on the narrow gauge than six-wheeled coupled. Their weight is about thirty-five tons. On the London and North-Western they are only thirty-one tons. Abroad, however, there are some few engines of much greater power. Thus, on the Orleans Railway, there is a ten-wheeled coupled tank locomotive with 19¾-inch cylinders, and 24 inches stroke. It is used principally to overcome a long and heavy gradient near Auvergne. The diameter of its wheels is three feet six inches. The “Steyerdorf,” an Austrian tank engine, which has run about 20,000 miles in the last four years, has ten wheels, six of which are coupled in one coupling, four others, being “bogie” wheels, are coupled together separately. There is also an extremely ingenious intermediate shaft by which the two separate couplings are connected together. All the wheels are of the same diameter—3 feet 3½ inches. The other principal dimensions are, cylinder 18¼ inches, length of stroke 25 inches. This locomotive is principally employed upon a line with very stiff gradients and sharp curves. Its maximum running speed does not exceed thirteen miles an hour.