Gilbert's view was in truth founded on an incomplete set of facts. At that time, as he tells us, the variation of the compass at London was 11⅓ degrees eastward. But he did not know of the secular change which would in about fifty-seven years reduce that variation to zero. Still less did he imagine that there would then begin a westward variation which in the year 1816 should reach 24° 30', and which should then steadily diminish so that in the year 1900 it should stand at 16° 16' westward. For an early discussion of the changes of the variation see vol. i. of the Philosophical Transactions (Abridged), p. 188. Still earlier is the classical volume of Henry Gellibrand, A Discovrse Mathematical on the Variation of the Magneticall Needle (Lond., 1635). Gilbert heads chapter iii. of book iiii. (p. 159) with the assertion Variatio uniuscuiusque loci constans est, declaring that to change it would require the upheaval of a continent. Gellibrand combats this on p. 7 of the work mentioned. He says:
"Thus hitherto (according to the Tenents of all our Magneticall Philosophers) we have supposed the variations of all particular places to continue one and the same. So that when a Seaman shall happly returne to a place where formerly he found the same variation, he may hence conclude he is in the same former Longitude. For it is the Assertion of Mr. Dr. Gilberts. Variatio vnicuiusq; Loci constans est, that is to say, the same place doth alwayes retaine the same variation. Neither hath this Assertion (for ought I ever heard) been questioned by any man. But most diligent magneticall observations have plainely offred violence to the same, and proved the contrary, namely that the variation is accompanied with a variation."
In 1637 Henry Bond wrote in the Sea-Mans Kalendar that in the year 1657 the variation would be zero at London. Compare Bond's Longitude Found (Lond., 1676, p. 3).
As to inconstancy of the variation in one place see further Fournier's Hydrographie (Paris, 1667, liv. xi., ch. 12, p. 413), and Kircher, Magnes (Colon. Agripp., 1643, p. 418).
[220] Page 157, line 4. Page 157, line 5. perfecto.—Though this word is thus in all editions, it ought to stand perfectâ, as in line 10 below.
[221] Page 157, line 11. Page 157, line 13. varietas, for variatio.
[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.