It has been well said that the transmission of force is an “essential condition in machinery.” It is no less true that directness in transference is important; that the fewer links in the chain binding driver and follower together, the less likely is the machine to be put out of gear. There is no question here as to the comparative values of the different modes of conveying motion from a prime mover,—rolling contact, sliding contact, wrapping connectors, or linkwork. Each has its own excellencies; each its special advantages; and one is to be preferred to another only according to the nature of the work to be done.