A four-speed selection transmission uses four, instead of three, gears on the driving shaft, without in any way changing the principles above outlined. CONTROL LEVER FOR PROGRESSIVE TRANSMISSION.--A careful study of the following mechanism, taken in connection with the accompanying sketch of the change speed gear, and the relations of the several elements, will explain the method now generally employed in the use of the _progressive_ type. The diagram, Fig. 51, shows the engine 1, with its shaft 2, connected directly with the shaft A of the transmission gearing. Intermediate the gear box and the engine 1, is a clutch 4, with which the foot pedal 5 is connected. [Illustration: Fig. 51. Progressive Control Mechanism.] The gear box has thereon a fore and aft sliding bar 6, the forward end of which projects through the case and is pivotally connected with the change speed lever 7. The lever has a quadrant 8, alongside, with four notches therein, for the low, intermediate, and high and also for the neutral positions of the lever. The sliding bar 6, has an arm, the fork of which spans the hub I of the gears G H, so they may be carried in either direction when the speed lever swings to and fro. The reversing lever 10 may be connected up with a bar, similar to 6, but for convenience herein, we employ a vertical lever R, pivoted to a cross rock-shaft S. The lower end of this lever has a fork T to engage the collar of the shaft N of the idler pinion. The upper end of the lever is connected with the reversing lever 10 by a link U. The quadrant, alongside the reversing lever, has two notches, as shown, one being designed to hold the lever in a cut-out position, whereas the other notch is to hold the lever 10 when the running gear is in action. OPERATION OF THE PROGRESSIVE GEAR.--The relative arrangement of the parts gives a comprehensive idea of the mechanical ideas involved, and by referring to the description and illustrations of the gears, it will be seen how the change lever 7, in moving back one notch, from its neutral position, will throw the gear G into mesh with L, and another movement of the lever to the next notch, will cause the crown teeth on G, to engage with the teeth on gear E, and thus effect a high gear connection. THE SELECTOR MECHANISM.--This is more or less confusing to the novice, and the accompanying illustration, Fig. 52, shows a perspective view, in which some of the parts are drawn out of proportion, merely for the sake of clearness. The aim is to show principles and not details of exact mechanical construction. [Illustration: Fig. 52. Selective Control Mechanism.] SELECTOR BARS.--The two selector bars A B, are mounted in guide ways so they move longitudinally alongside each other a limited distance. Each bar has an arm, as at C D, the end of each having a curved finger E to engage the annular grooves on the hubs of the shifting gears.