The invention of Printing Machines underwent many changes before it was brought to a practical form. Such a machine was first projected in 1790, by Mr. Nicholson, who proposed to place the types and paper upon cylinders, and to distribute and apply the ink also by cylinders. Another plan, more closely approaching that of the printing machines afterwards perfected by Mr. Napier and others, was to place the types upon a table and the paper upon an impressing cylinder, and to move the table backwards and forwards under it. In 1813, Messrs. Donkin and Bacon proposed placing the types upon a prism, which was to revolve against an irregularly shaped cylinder, on which the paper was to be placed. Nothing, however, could be effectually done in producing a proper working printing machine until the invention of inking rollers. In 1814, Messrs. Bauer and Kœnig succeeded in constructing a machine, which was erected at the _Times_ office, that produced 1,800 impressions an hour; and it continued in use till 1827. This rapidity of action, compared with that of the most improved printing press, produced a revolution in the art of printing; attention was then directed almost exclusively to the further improvement of the machines, and the platten press was neglected. In the form of printing machines generally used, the types are laid upon an iron table that is moved to and fro by the turning of a wheel connected with a steam engine. The paper is placed upon cylinders covered with flannel, and the impression of the types is produced by the cylinders being fixed so closely to them that, as the table passes backwards and forwards, there is great pressure. The types are inked by a series of rollers, by which the ink is distributed and evenly laid on the face of the types without any manual labour. The mechanical power gained by an arrangement of this kind arises from the pressure being exerted on a small surface at a time; consequently the power required for producing the impression of the types is not nearly so great as when the whole surface of the types makes the impression at the same instant. The force actually pressing on the types, from contact with the cylinders, is very much less than that brought to bear on them by the platten of the Stanhope press; but as it acts on a smaller surface at a time, the amount of pressure on each part, successively, greatly exceeds that received by any similar portion when it is impressed all at once. The difference of the action of the platten and of the cylinder may be compared to the different effects produced by a knife when pressed with its edge and with its flat side against a yielding surface; the pressure on the flat surface may not be sufficient to leave any impression, whilst a much smaller pressure on the edge will produce an indentation. [Illustration] The accompanying woodcut is a representation of one of Messrs. Applegath and Cowper's machines for printing both sides of the paper at the same time.