The normal position of the pawl E is jammed into the tooth of the ratchet and against the bracket C, that forms the pivot support for the pivot shaft of the arrow. This jammed or locked combination serves to stop the momentum of the ratchet wheel at the end of the ratcheting action, and holds the wheel and its arrow normally locked until the lever G is again depressed. The means for gauging the depression and additive degrees of action of the lever G is produced through the slides or keys marked a, having finger-pieces c, springs f, and pins e, bearing against the top of the lever G, combined with what may be called a compensating lever marked K. The specification of the patent states that the depression of a key will depress the lever G and the free end will engage the bent end t, of the compensating lever K, and rock its envolute curved arm M, upward until it engages the pin e of the key, which will block further motion of the parts. The effectiveness of the construction shown for the lever K is open to question. [Sidenote: _Prime actuation of a carried wheel impossible in the Spalding machine_] The carry of the hundreds is accomplished by means of a one-step ratchet device represented by the parts lever R, pawl T, spring P, and operating pin g. When the hundred-tooth ratchet nears the end of its revolution, the pin g, made fast therein, engages the free end of the ratchet lever R, and depresses it; and as the hand attached to the hundred-tooth ratchet wheel passes from 99 to 0 the pin g passes off the end of the ratchet lever R, and the spring P retracts the lever ratcheting the twenty-tooth wheel and its arrow forward one point so that the arrow registers one point greater on the hundreds dial. Although the Spalding means of control under carrying differed from that of Bouchet in construction, its function was virtually the same in that it locked the carried or higher wheel in such a manner as to prevent the wheel from being operated by an ordinal set of key mechanism. And the control under key action would prevent a carry being delivered to that order through the locked relation of the ratchet and pawl E. [Illustration] The Key-Driven Calculator While these single-digit adding machines have been used to illustrate how the control, which was lacking in the Hill invention, had been recognized by other inventors as a necessary requisite to the key-drive, it should not be construed that such carrying control as had been applied to their inventions was of a type that could be used in the Hill machine or in any multiple-order key-driven machine. It was thirty years after the first attempt to control a key-driven machine was made before an operative multiple-order key-driven machine, with a control that would prevent over-rotation, was finally invented. [Sidenote: _Theory versus the concrete_]