On the Magnet/IV-4

The arc of variation is not changed equally
in proportion to the distance of places.

Gilbert De Magnete IlloI.jpg
n the open sea, when a vessel is borne by a favourable wind along the same parallel, if the variation be changed by one degree in the course of one hundred miles, the next hundred miles do not therefore lessen it by another degree; for the magnetick [needle] varies erratically as respects position, form, and vigour of the land, and also because of the distance. As, for example, when a course from the Scilly Isles to Newfoundland has proceeded so far that the compass is directed to the true pole, then, as the vessel proceeds, in the first part of the course the variation increases toward the north-west[222], but rather indistinctly and with small difference: thence, after an equal distance, the arc is increased in a greater proportion until the vessel is not far from the continent: for then it varies most of all. But before it touches actual land or enters port, then at a certain distance the arc is again slightly diminished. But if the vessel in its course should decline greatly from that parallel either toward the south or the north, the magnetick [needle] will vary more or less, according to the position of the land and the latitude * of the region. For (cæteris paribus) the greater the latitude the greater the variation.
The page and line references given in these notes are in all cases first to the Latin edition of 1600, and secondly to the English edition of 1900.

222 ^  Page 160, line 20. Page 160, line 23. in Borrholybicum.—This name for the North-west, or North-North-West, is rarely used. It is found on the chart or windrose of the names of the winds on pp. 151 and 152 of the Mécometrie de l'Eyman of G. Nautonier (1602). Here the name Borrolybicus is given as a synonym for Nortouest Galerne, or Ὀλυμπιάς, while the two winds on the points next on the western and northern sides respectively are called Upocorus and Upocircius.

In Swan's Specvlvm Mundi (Camb., 1643, p. 174) is this explanation: "Borrholybicus is the North-west wind."

In Kircher's Magnes (Colon. Agripp., 1643, p. 434) is a table of the names of the thirty-two winds in six languages, where Borrolybicus is given as the equivalent of Maestro or North-West.