One of the earliest issues in the typewriter field concerned the relative merits of the type-bar principle versus the type wheel. Mention of the type wheel brings us back to John Pratt's Pterotype and the article concerning it in the Scientific American of July 6, 1867, which is said to have suggested the idea of a typewriter to Sholes and his colleagues. Pratt is said to have actually built and sold some of these machines in England, but they were not a success, and he for a time despaired of being able to construct a machine on which the printing wheel would move quickly and yet stop instantly. He worked over the problem for years, and when, at last he approached the United States Patent Office he found himself in interference with two other inventors, James B. Hammond and Lucien S. Crandall, both of whom appeared with writing machines built on the type-wheel principle. A deadlock ensued which was finally settled by Pratt yielding precedence to Hammond upon a type-wheel machine and receiving a royalty, while Crandall proceeded with his application for a patent on a type-sleeve instrument. The first Hammond patents were taken out in 1880, and the machine was placed on the market shortly thereafter. The early Hammond had what was called the "ideal" keyboard, semi-circular in shape, but later Hammonds have conformed to the "universal" keyboard arrangement. The Hammond was the first practical type-wheel machine and is today the leading machine of this class. The type-wheel construction has always had strong advocates, but these machines have never been very serious competitors of the type-bar machines in the general commercial field.