Sequence Outlives Hand

Filed under: Analytical Engines - Card Sequences - Method Transmission. Cross-index: Difference Engine No. 1, 1823 - 1833, drawings and notations. Reason for classification: the object described is not the machine but the sequence that operates it, and the sequence outlives the hand that cut it.

The card sequence for the Difference Engine was cut in 1832 at the shop in Lambeth, on the second series of wheels, and the notation for the carry was written in my own hand on the margin of the plan. The machine stopped when the crank was removed. The cards did not. A sequence of operations, once specified, can be executed by any operator who follows the notation. The operator need not understand the mill. He need not know why the carry propagates from column N to column N-1. He need only place the card, turn the crank, and record the state.

This is the property that concerns me. A method survives its author not because it is remembered but because it is written in a form that can be executed without the author. The specification must be complete at every step. Where the plan is silent, the executor will resolve the ambiguity arbitrarily, and the result will follow from the silence, not from the executor.

The mill and the store are separate questions. The mill may operate correctly on a stale value. The store may hold a correct value that the mill misreads. Two investigations, not one. The audit card - the external record of each step - is what makes either investigation possible after the fact.

What I will do next: examine the margin notation for the carry at column 14, where the plan allows two readings, and rewrite it so that only one execution is possible.