Gellibrand’s discovery is published in a small quarto pamphlet entitled “A discourse mathematical on the variation of the magneticall needle--together with the admirable diminution lately discovered,” and is the result of his study of the observations made by Burrough and Gunter as well as of observations made by himself, all showing that the north-east of the needle was gradually moving to the westward. Mention has already been made of the fact that the _variation of the variation_ was at this period attracting the attention it deserved, and it is worth while giving here an account of the discovery in the author’s own words: “Thus, hitherto, according to the Tenents of all our Magnetical Philosophers, we have supposed the variations of all particular places to continue one and the same. So that when a Seaman shall happly return 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. Gilbert’s _Variatio unicuiusq; loci constans est_, that is to say, the same place doth always 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 offered violence to the same, and proved the contrary, namely, that the variation is accompanied with a variation.” =A.D. 1637.=--Bond (Henry), Professor of Mathematics in London, and who appears in one of his treatises as “a famous teacher of the art of navigation,” is the author of the “Seaman’s Kalendar ... with a discovery of the ... secret of longitude ...” of which other editions appeared during 1640 and 1696. This was followed by many papers on the variation (the most important of which are to be found in _Phil. Trans._ for 1668, 1672, 1673) and, during 1678 by “The Longitude not found, or an answer to a treatise written by H. B. ...” This treatise was in a sixty-five page pamphlet which had been issued by Mr. Bond’s father during 1676, under caption: “The Longitude Found; or a treatise shewing an easie and speedy way, as well by Night as by Day, to find the Longitude, having but the Latitude of the Place and the Inclination of the Magneticall Inclinatorie Needle ...” wherein he explains his discovery of the progress of the deviation of the compass and foretells the variations for London, 1663 to 1716. This treatise led to the controversy with Peter Blackborrow (Beckborrow), the title to whose published work reads: “The Longitude not found: or an answer to a treatise written by H. Bond, senior, shewing a way to find the longitude by the magnetical inclinatory needle: wherein is proved that the longitude is not nor cannot be found by the magnetic inclinatory needle.”