The recording telegraph instruments hitherto noticed impress on the paper only hieroglyphical symbols, which require long practice to decipher readily. It has, from the first practical application of the invention, been considered highly desirable that the letters of the alphabet should be indicated and printed in their proper forms, so that the momentary transmission of an electric current should leave behind a durable impression that could be read without difficulty. Professor Wheatstone and Mr. Bain separately attempted to accomplish this desired object by the invention of Printing Telegraphs, which print messages from types. It is a question in dispute which of them was the first to design a telegraph of this kind. In 1845, Mr. Bain had a printing telegraph in operation experimentally on the South-Western Railway, for a distance of seven miles, and we are not aware that Professor Wheatstone ever succeeded in working his printing instruments when separated at a distance from each other. In principle, both inventions were similar. A wheel, into the periphery of which were inserted types of the twenty-six letters, was made to rotate in close proximity to a piece of paper, over which was placed a blackened surface that would leave a mark on the paper when pressed upon. When the required letter came opposite the paper, the type-wheel was stopped and forced against it, so that the letter was impressed, and the black from the interposed surface marked the form of the type. The paper was then moved forward to leave space for the next letter, and thus a continuous message could be printed. The objection to these instruments was the uncertainty of stopping the type-wheel at the proper point, so as to avoid printing wrong letters; and when the instruments became thus irregular, they continued so till they were again adjusted. This difficulty has since been overcome; and by the combined efforts of Mr. House in America, and of Messrs. Brett in this country, the printing telegraph has attained a high degree of perfection. The mechanical arrangements of the instrument, though very complex, consist essentially, like those of Mr. Bain and Professor Wheatstone, in having a type-wheel, which, by the action of the operator at the transmitting instrument in making and breaking contact, moves or stops at the required point, and the letters are printed by forcing the paper against the type by an electro-magnet. The movements of the type-wheel are regulated by an electro-magnet, and one great improvement introduced by Mr. Brett prevents the continuance of error, should any be made during transmission, by bringing the type-wheel to its first position after printing each letter, so that if a wrong letter be printed, the subsequent letters will not continue erroneous. This printing telegraph works with a single wire, but its operation is rather slow.