The changes which have been outlined produced the now typical American locomotive. It was necessarily given such form that it would work safely and efficiently on rough, ill-ballasted, and often sharply-winding tracks; and thus it soon became evident that the two pairs of coupled driving-wheels, carrying two-thirds the weight of the whole engine, the forward-truck, and the system of "equalizing" suspension-bars, by which the weight is distributed fairly among all the wheels, whatever the position of the engine, or whatever the irregularity of the track, made it the very best of all known types of locomotive for the railroads of a new country. Experience has shown it equally excellent on the smoothest and best of roads. The "cow-catcher," placed in front to remove obstacles from the track, the bell, and the heavy whistle, are characteristics of the American engine also. The severity of winter-storms compelled the adoption of the "cab," or house, and the use of wood for fuel led to the invention of the "spark-arrester" for that class of engines. The heavy grades on many roads led to the use of the "sand-box," from which sand was sprinkled on the track, to prevent the slipping of the wheels. In the year 1836, the now standard chilled wheel was introduced for cars and trucks; the single eccentric, which had been, until then, used on Baldwin engines, was displaced by the double eccentric, with hooks in place of the link; and, a year later, the iron frame took the place of the previously-used wooden frame on all engines. The year 1837 introduced a period of great depression in all branches of industry, which continued until the year 1840, or later, and seriously checked all kinds of manufacturing, including the building of locomotives. On the revival of business, numbers of new locomotive-works were started, and in these establishments originated many new types of engine, each of the more successful of which was adapted to some peculiar set of conditions. This variety of type is still seen on nearly all of the principal roads. The direction of change in the construction of locomotive-engines at the period at which this division of the subject terminates is very well indicated in a letter from Robert Stephenson to Robert L. Stevens, dated 1833, which is now preserved at the Stevens Institute of Technology. He writes: "I am sorry that the feeling in the United States in favor of light railways is so general. In England we are making every succeeding railway stronger and more substantial." He adds: "Small engines are losing ground, and large ones are daily demonstrating that powerful engines are the most economical." He gives a sketch of his latest engine, weighing _nine tons_, and capable, as he states, of "taking 100 tons, gross load, at the rate of 16 or 17 miles an hour on a level." To-day there are engines built weighing 70 tons, and our locomotive-builders have standard sizes guaranteed to draw over 2,000 tons on a good and level track.