NavList:

Lars Bergman, you wrote:
"It seems that the logarithms used by Gerbault is log(the number of minutes of arc in the "arcs", divided by 1123.124). Why the table-maker use that odd number I cannot explain, it would have been much easier to use 1000"

A likely explanation: it makes the tables more difficult to reverse-engineer. There were similar "magic numbers" in Martelli's time sight tables, which were known sometimes as "Martelli's Mystery Tables". They acquired that nickname apparently because the numbers were not easily deciphered. Today, with convenient rapid calculation tools at our fingertips, it's a simple matter to puzzle these things out. You could also think of it as a form of copyright protection. Anyone can copy a table of logsines. Such numbers are obviously "public domain" in the deepest sense. But a table of numbers generated by a unique algorithm with some inexplicable element thrown in --even one with no purpose? That's a tougher case, and if such tables were copied and republished without a license, at minimum, it would be easy for the original author/publisher to "call out" the copier and threaten a lawsuit.

Modern navigation enthusiasts are accustomed to government authority and government-issued navigation tables. The commercial editions that are available of the G.I. documents today are, usually, rebindings --not new tables but perfect copies of datasets still wrapped in government authority. It's hard to remember that for many decades of navigation history navigation tables were commercial products, and the creators worked hard to defend their property.

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA