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Irish logarithms were a system of number manipulation invented by Percy Ludgate for machine multiplication. The system used a combination of mechanical cams as look-up tables and mechanical addition to sum pseudo-logarithmic indices to produce partial products, which were then added to produce results. The technique is similar to Zech logarithms , but uses a system of indices original to Ludgate.
Ludgate's algorithm compresses the multiplication of two single decimal numbers into two table lookups , the addition of the two indices to create a new index which is input to a second lookup table that generates the output product. Because both lookup tables are one-dimensional, and the addition of linear movements is simple to implement mechanically, this allows a less complex mechanism than would be needed to implement a two-dimensional 10x10 multiplication lookup table.