[Illustration: Fig. 13.--Locomotive for Street Railway. By the Baldwin Locomotive Works.] Nearly all the early locomotives had only four wheels. In some cases one pair alone was used to drive the engine, and in others the two pairs were coupled together, so that the adhesion of all four could be utilized to draw loads. The four-wheeled type is still used a great deal for moving cars at stations, and other purposes where the speed is comparatively slow. But to run around sharp curves the wheels of such engines must be placed near together, just as they are under an ordinary street-car. This makes the wheel-base very short, and such engines are therefore very unsteady at high speeds, so that they are unsuited for any excepting slow service. They have the advantage, though, that the whole weight of the machine may be carried on the driving-wheels, and can thus be useful for increasing their friction, or adhesion to the rails. This gives such engines an advantage for starting and moving heavy trains, at stations or elsewhere, which is the kind of service in which they are usually employed. [Illustration: Fig. 14.--Four-wheeled Switching Locomotive. By the Baldwin Locomotive Works, Philadelphia.] If the front end of the engine is carried on a truck, as in Campbell's plan (Fig. 10)--which is the one that has been very generally adopted in this country--the wheel-base can be extended and at the same time the front wheels can adjust themselves to the curvature of the track. This gives the running-gear lateral flexibility. But as the tractive power of a locomotive is dependent upon the friction, or adhesion of the wheels to the rails, it is of the utmost importance that the pressure of the wheels on the rails should be uniform. For this reason the wheels must be able to adjust themselves to the vertical as well as the horizontal inequalities of the track. [Illustration: Fig. 15.--Driving Wheels, Frames, Spurs, etc., of American Locomotive.]