THE KEY-DRIVEN CALCULATOR Theory versus the concrete 50 All but one of the generic elements solved 51 Originality of inventions 51 A conception which led to the final solution 52 Evolution of an invention 55 Trials of an inventor 55 The first “Comptometer” 56 Felt patent 371,496 56 Description of Felt calculator 59 Recapitulation of Art prior to Felt calculator 60 Why Hill failed to produce an operative machine 61 Idiosyncrasies of force and motion increased by use of keys 61 Light construction a feature 62 Operative features necessary 62 Classification of the features contained in the early Art of key-driven machines 63 Carrying mechanism of Felt’s calculator 63 Transfer devices 64 Carrying mechanism versus mere transfer devices 64 Details of Felt carrying mechanism 65 Manufacture of the Felt calculator 69 Trade name of Felt calculator 70 Felt calculator exhibit at National Museum 70 Significant proof of Felt’s claim of priority 75 Rules for operation an important factor of modern calculator 76