The two principal classes of connecting links, the crank and ordinary form of lever and clutch, need no explanation or discussion beyond their fundamental characteristics, but there are several combinations of lever and crank which are of interest and properly come under the head of modifications of the crank. These modifications are numerous in the market, and there exists cardinal distinctions between them. We annex diagrams of five distinct types which fall into two groups, the first group being a combination of lever and crank, in which the foot has an oval motion, as shown by Figs. 1, 2, and 3, the arrows showing the direction of progression. [Illustration: GROUP I. Fig. 1. Fig. 2. Fig. 3.] [Illustration: GROUP II. Fig. 4. Fig. 5.] The second distinctive arrangement of lever and crank is where the lever is pivoted so as to return over the same track in which it descends, as shown in Figs. 4 and 5. The first group, with its oval motion, has a decided advantage in regard to dead centre or continuous power; since by an ankle-motion the rider can transmit some power in a circular direction to the crank; that is to say, he can actually push to some extent in a forward horizontal direction. But it will be seen that the pivotal connection shown in Figs. 4 and 5 does not allow of any such possibility; the rider must have momentum enough to throw the cranks over the dead centre or he is lost. In Fig. 4, which represents a form of pivoted treadle used on a reputable make of front-driving machine, it will be noticed that the rider has less than one-half of the revolution of the crank in which any power can be transmitted at all, which becomes apparent in observing a pedal in such devices while in motion, from the fact that it descends more rapidly than it ascends, thus giving the rider less than half the time in which he can transmit any power. We are now speaking of one side only of the machine; taking both sides together, there are two short arcs of a circle in which there can be no propulsive power transmitted to the wheel on either side. Fig. 6 illustrates this as follows: [Illustration: Fig. 6.]