[Sidenote: _Felt calculator Exhibit at National Museum_] In September, 1887, Felt took one of the first eight machines to Washington and exhibited it to Gen. W. S. Rosecrans, then Registrar of the Treasury, and left the machine in the office of Dr. E. B. Elliott, Actuary of the Treasury, where it was put into constant use. Proof of the date of this use of Felt’s invention in the Treasury is set forth in the reproduction of two letters (see opposite page), one was written by Mr. Elliott and another by Gen. W. S. Rosecrans, in answer to an inquiry of the Hall Typewriter Co. of Salem, Mass. Another of the first eight machines was placed with Dr. Daniel Draper, of the N. Y. State Weather Bureau, New York City. Felt finally closed a deal with Mr. Robert Tarrant of Chicago, whereby a partnership contract was signed November 28, 1887. The partnership was incorporated January 25, 1889, under the name of the Felt & Tarrant Mfg. Co., who are still manufacturing and selling “Comptometers” under that name. [Sidenote: _Significant proof of Felt’s claim of priority_] Laying aside all the evidence set forth in the foregoing history of key-driven machines and their idiosyncrasies, significant proof of Felt’s claim as the first inventor of the modern calculating machine is justified by the fact that no other multiple-order key-driven calculating machine was placed on the market prior to 1902. Lest we lose sight of a most important feature in dealing with the Art of the Modern Calculator, we should call to mind the fact that as Felt was the originator of this type of machine, he was also the originator of the scheme of operation in its performance of the many and varied short cuts in arithmetical calculation. The performance of calculation on machines of the older Art differed so entirely from the new that any scheme of operation that may have been devised for their use would lend nothing to the derivation of the new process for operating the key-driven machine of the new Art. [Sidenote: _Rules for operation an important factor of modern calculator_] A superficial examination of one of the instruction books of the “Comptometer” will convince most any one that it is not only the mechanism of the machine that made the modern calculator so valuable to the business world, but also the schemes laid down for its use. The instructions for figuring Multiplication, Subtraction, Division, Square Root, Cube Root, Interest, Exchange, Discount, English Currency, etc., involved hard study to devise such simple methods and rules. The instruction books written by Felt for the “Comptometer, the Modern Calculator,” reflect the genius disclosed in the invention of the machine itself. [Illustration] [Illustration] [Illustration: From Drawings of Barbour Patent No. 133,188] Early Efforts in the Recording Machine Art