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DISCOVERY OF THE SECULAR CHANGE IN MAGNETIC VARIATION. First edition, extremely rare, of Gellibrand?s most important discovery: the secular change in magnetic declination, or variation. According to William Gilbert in his De Magnete (1600), the existence of magnetic declination, the difference in direction between geographical and magnetic north, is a result of the magnetic effects of large land masses. This explained the observed dependence of variation on geographical location, but according to this theory the variation at a given location should not depend on historical time. Gellibrand?s great discovery, that the variation in fact changes with time, not only disproved Gilbert?s theory but was of great importance for navigation since a knowledge of the declination at various locations was used by sailors as an aid to determining their position at sea: ?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? ?Gellibrand?s most widely appreciated scientific discovery, which he should share with John Marr, was that of the secular change in the magnetic variation (declination). It was announced, without much comment, in A Discourse Mathematical) on the Variation of the Magneticall Needle, Together With Its Admirable Diminution Lately Discovered (1635). His predecessor, Gunter, had noticed that the variation at Limehouse in 1622 differed from the value found by William Borough in 1580, but he ascribed the difference to an error on Borough?s part. In 1633 some rough observations of his own and John Marr?s convinced Gellibrand that the value was now even less, but not until 1634 was he sufficiently confident to make a categorical assertion of its secular change. As his main evidence he referred to an appendix to Edward Wright?s Certaine Errors in Navigation? (1599, 1610). This contains a compendium of recorded values of variation at various places made by a number of physicists and navigators the world over? (DSB). Less well known is that Gellibrand, in this book, gave the first clearly documented use of the arithmetic mean as a summary statistic (see below). We have been able to trace only three copies at auction in the last 50 years: the Kenney copy (sold Sotheby?s 1968); the present copy (Sotheby?s London, 11 November, 1974, The Library of Harrison D. Horblit, Part 2, Lot 450, ?1200 to Traylen); and the Streeter copy, ?closely trimmed with loss of some headlines and shoulder notes? (sold at Sotheby?s in 1968 and then at the Streeter sale at Christie?s New York, 2007, $21,600). All three copies were either disbound or in modern bindings. Provenance: Eighteenth-century bookplate of Lord Dalrymple on verso of title page; Harrison D. Horblit (book label inside slip case); Henry Faul, University of Pennsylvania geophysicist, d. 1981 (book label inside slip case). Henry Gellibrand was the eldest son of a physician, also Henry, and was born on 17 November 1597 in the parish of St. Botolph, Aldersgate, London. In 1615 he became a commoner at Trinity College, Oxford, and obtained a BA in 1619 and an MA in 1621. After taking holy orders he became curate at Chiddingstone, Kent, but the lectures of Sir Henry Savile inspired him to become a full-time mathematician. He settled in Oxford, where he became friends with Henry Briggs, famed for introducing logarithms to base 10. It was on Briggs?s recommendation that, on the death of Edmund Gunter, Gellibrand succeeded him as Gresham professor of Astronomy in 1627 ? a post he held until his death from a fever on February 16, 1636. Gellibrand?s principal publications were concerned with mathematics (notably the completion of Briggs' Trigonometria Britannica after Briggs died in 1630) and navigation. The road to Gellibrand?s discovery start.
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