A Discourse Mathematical on the Variation of the Magneticall Needle. Together with its admirable Diminution lately discovered.
London: William Jones, 1635.
Extremely rare first edition of Gellibrand’s most important discovery: the secular change in magnetic declination, or variation. This copy, formerly in the collection of Harrison D. Horblit, is among just three copies having been auctioned the past 50 years – the other two being the Kenney copy and the Streeter copy. When the Kenney copy was auctioned in 1968 Sotheby’s could trace only two copies having been auctioned in England since 1902.
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 declination on geographical location, but according to this theory the declination at a given location should not depend on historical time. Gellibrand’s great discovery, that the declination 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” (p.6).
Henry Gellibrand was the eldest son 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’ 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 16 February, 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 starts with an observation of magnetic declination made by William Borough, a merchant seaman who rose to ‘captain general’ on the Russian trade route before becoming comptroller of the Queen’s navy. The observation was made on 16 October 1580 at Limehouse, London, where he found the magnetic declination, D, to be 11.3 degrees E.
Some 40 years later, Edmund Gunter, distinguished mathematician, Gresham professor of astronomy, and inventor of the slide rule, found D to be only 6.3 degress E. The exact date (ca. 1622) and location (probably Deptford) of the observations are not stated but it alerted Gunter to the discrepancy with Borough’s measurements. He then made further measurements in Limehouse and found a declination of 6 degrees E. Gunter died in 1626 before making further measurements to establish whether the difference was due to errors in the measurements or a genuine change in D over time.
When Gellibrand succeeded Gunter as Gresham professor, he repeated Gunter’s observations at Deptford, first in June 1633 and again on the same site on June 12, 1634, the published data giving D as 4 degrees E. On the strength of these observation he announced his discovery of secular variation in the present work.
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).
The work is extremely rare. 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 (sold at 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. OCLC lists 3 copies of Gellibrand in the US (Smithsonian, Harvard, MIT [lacking title page]), and 3 in the UK (BL, Cambridge, National Library of Scotland). The Kenney sales catalogue (1968) states: ‘VERY RARE. Only two copies are recorded as having been sold by auction in England since 1902 one of which is almost certainly identical with the Harmsworth-Folger copy. The book is of considerable historical importance, its main contribution being the confirmation of the secular or annual “magnetic” variation.’.
Adams & Waters 1156; STC 11712; Taylor Mathematical Practitioners 162-3. See S. R. C. Malin & Sir Edward Bullard, ‘The direction of the Earth’s magnetic field at London, 1570-1975’ Phil. Trans. A299, 357-423; Commander J.B. Hewson, A History of the Practice of Navigation, Glasgow, 1963, pp. 127, 135-6.
8vo, pp. , 22. Disbound, preserved in a folder and quarter morocco slipcase (lightly browned, errata corrected in an early hand, a very large copy with absolutely no cropping of headlines or side notes).
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