The timing of the entry of the tooth of the detent is supposed to be gauged to enter the right tooth, but as the action of these parts is fast, slow or medium at the will of the operator, considerable time must be allowed for variation in the entry of the detent tooth, which requires space, as certain parts will fly ahead under the sudden impact they may receive from a quick stroke, where they would not under a slow stroke, but no allowance was provided for such contingency. The means provided for the carry of the tens consist of the gears G⁹, meshing with the numeral wheel gears and the single gear tooth g⁹, attached to it, which, at each revolution of the lower wheel, as it passes from 9 to 0, engages the gear of the numeral wheel of higher denomination and was supposed to turn the higher gear one-tenth of a revolution, thus registering one greater. On account of the Gears G⁹, of one order and the gear tooth g⁹, of another order operating on the same numeral wheel gear, the transfer gears are arranged alternately on separate shafts, one at the side and one below the numeral wheels. [Sidenote: _Ludlum machine inoperative_] The mechanical scheme disclosed in the Ludlum patent, to the unsophisticated may seem to be operative. But to those familiar with the Art of key-driven adding mechanism it will at once be obvious that even if the typewriter feature was constructed properly the possibility of correctly adding the items as they were printed was absolutely impossible. Laying aside several other features of inoperativeness, obvious to those who know such mechanism, the reader, although not versed in the Art of key-driven adding mechanism, will observe from the preceding chapter, that the means provided for transferring the tens without any control for the numeral wheels against over-rotation, would make correct addition impossible. The drawings and specification of the Ludlum patent disclose a mere dream and show that they were not copied from the make-up of an operative machine. It was a daring scheme and one that none but a dreamer would undertake to construct in the method shown. There have in later years been some successful ten-key recording machines made and sold, but they were of a very different design and principle. There have also been several adding attachments made and sold that could be adjusted to a regular commercial typewriter that are claimed to be dependable, but none of these machines were early enough to be claimed as the first operative recording-adding machine, or the first adding machine in which the principle used for the legible recording of the numerical items used in the machines of today may be found. [Illustration] FIRST PRACTICAL RECORDERS