Hill made no provision for overcoming the lightning-speed momentum that could be given the numeral wheels in his machine through manipulation of the keys, either from direct key-action or indirectly through the carry of the tens. Imagine the sudden whirl his numeral wheel would receive on a quick depression of a key and then consider that he provided no means for stopping these wheels; it is obvious that a correct result could not be obtained by the use of such mechanism. Some idea of what would take place in the Hill machine under manipulation by an operator may be conceived from the speed attained in the operation of the keys of the up-to-date modern key-driven machine. [Sidenote: _High speed of key drive_] Operators on key-driven machines oftentimes attain a speed of 550 key strokes a minute in multiplication. Let us presume that any one of these strokes may be a depression of a nine key. The depression and return, of course, represents a full stroke, but only half of the stroke would represent the time in which the wheel acts. Thus the numeral wheel would be turned nine of its ten points of rotation in an eleven hundredth (¹/₁₁₀₀) of a minute. That means only one-ninth of the time given to half of the key-stroke, or a ninety-nine hundredth (¹/₉₉₀₀) of a minute; a one hundred and sixty-fifth (¹/₁₆₅) part of a second for a carry to be effected. [Sidenote: _Camera slow compared with carry of the tens_] If you have ever watched a camera-shutter work on a twenty-fifth of a second exposure, which is the average time for a snap-shot with an ordinary camera, it will be interesting to know that these controlling devices of a key-driven machine must act in one-fifth the time in which the shutter allows the daylight to pass through the lens of the camera. Think of it; a machine built with the idea of offering the possibility of such key manipulation and supplying nothing to overcome the tremendous momentum set up in the numeral wheels and their driving mechanism, unless perchance Hill thought the operator of his machine could, mentally, control the wheels against over-rotation. [Illustration] [Illustration: Chapin Patent Drawings] Lack of a proper descriptive term used to refer to an object, machine, etc., oftentimes leads to the use of an erroneous term. To call the Hill invention an adding machine is erroneous since it would not add correctly. It is as great an error as it would be to refer to the Langley aeroplane as a flying machine. [Sidenote: _Hill machine merely adding mechanism, incomplete as operative machine_]