The Marteloio Table
1436 · Rhumb Line Reckoning Table · Work 761 of 1,871
A sheet of parchment of 1436 open on a sloping writing desk in the workroom of a galley officer in Venice, the sheet twenty-six by thirty-nine centimetres, seen along the desk at a flat angle, on it a large circle of fifteen centimetres in fine brown and red lines radiating from one centre with a square grid of finer lines inside it, beside the circle a thirty-two pointed star in green, red, blue and gold, the upper edge of the sheet falling away from the lens soft in focus, brass dividers and a small horn pot of red pigment, daylight from a window on the left, a sleeve of dark red cloth.
- What it was used for
- The toleta de marteloio was the pilot's reckoning aid when a contrary wind drove him off his intended course. From the course angle and the distance sailed he read off how far he had been carried off (alargar), how far he had advanced along the intended line (avanzar), and on what course and over what distance he would get back (ritorno). The table is arranged by the 8 quarter-winds of a quadrant, from 11.25 degrees to 90 degrees, and works on a circle of 100 units. The sheet Raxon de Marteloio in the atlas of the Venetian Andrea Bianco, dated M.CCCC.XXX.vj, carries the table, three subsidiary tables for the tenth-part values, and the tondo e quadro figure.
- What was new
- The table replaced trigonometry with looking up: the work was done in multiples and tenths, not in trigonometric functions. Bianco's sheet was not the first. M J Ferrar, in his 2014 study, cites a Genoese inventory of 1390 listing an unum martelogium, the manuscript of Michael of Rhodes of 1434 with a table and worked examples; he also cites a Florentine abacus manuscript with a sailing table that is dated only generally to the first half of the 15th century. Ferrar also judges that the values are rounded equivalents of sine and cosine and therefore do not bring the ship back exactly onto the 100 units, and that the mental calculation probably asked a great deal of a mariner when several tacks were involved; Bianco's own sheet contains scribal errors, such as 5 1/12 instead of 5 1/2 for the tenth part of 55.