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 11-1/3 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 [Greek: Olumpias], while the two winds on the points next on the western and northern sides respectively are called _Upocorus_ and _Upocircius_. {56} In Swan's _Specvlvm Mundi_ (Camb., 1643, p. 174) is this explanation: "Borrholybicus is the North-west wind."