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"THE MOST IMPORTANT WORK ON THE SCIENCE OF NAVIGATION TO BE PUBLISHED IN THE SEVENTEENTH CENTURY". First edition, extremely rare complete as here, of Gunter s book on the sector and other mathematical instruments, "one of the most influential scientific works on navigation" (Waters, p. 359), the work which introduced logarithms into the science of navigation and led to the development of the slide-rule. Gunter s book is here accompanied by a brass sector made to his design by Elias Allen, c. 1630. The engraved frontispiece carries an advertisement (in Allen's hand) for the maker as provider of the sector: "These instruments are wrought in brasse by Elias Allen dwelling without Tempel barre over against St Clements Church: and in wood by John Thompson dwelling in Hosiar Lane". "This book must be reckoned, by every standard, to be the most important work on the science of navigation to be published in the seventeenth century. It opened the whole subject of mathematical application to navigation and nautical astronomy to every mariner who was sufficiently interested in devoting time to the perfecting of his art. The sector described by Gunter consisted basically of two hinged arms (like a carpenter s ruler) on which were engraved several scales … Gunter s book was given in two main parts. In the first he concentrated his attention on the sector; and in the second on the cross-staff. In the first part he gave solutions, not only to nautical astronomical problems but also to plane and Mercator sailing. He also provided a novel traverse table, this being the first of its kind and of the type that is now commonly used by navigators. In the second part of his book Gunter described a novel form of cross-staff, the most useful feature of which was the several scales engraved on the staff. These were logarithmic scales by means of which, using a pair of dividers, problems of multiplication and division could be solved easily and quickly" (Cotter, pp. 363-4). Gunter s sector "allowed calculations involving square and cubic proportions, and carried various trigonometrical scales. Moreover, it had a scale for use with Mercator s new projection of the sphere, making this projection more manageable for navigators who were only partially mathematically literate. The sector was sold as a navigational instrument throughout the seventeenth century and survived in cases of drawing instruments for nearly three hundred years. The most striking feature of the cross-staff, distancing it from other forms of this instrument, was the inclusion of logarithmic scales. This was the first version of a logarithmic rule, and it was from Gunter s work that logarithmic slide rules were developed, instruments that remained in use until the late twentieth century" (ODNB). This book is justly renowned as a contribution to navigation, but it seems not to be widely known that it also contains (p. 60 of the second part) the first printed observation of the temporal variation of magnetic declination, the discovery of which is normally ascribed to Henry Gellibrand who published it 12 years later. "In 1622 Gunter s investigations at Limehouse, Deptford, of the magnetic variation of the compass needle produced results differing from William Borough s, obtained more than forty years earlier. He assumed an error in Borough s measurements, but this was in fact the first observation of temporal change in magnetic variation, a contribution acknowledged by his successor, Henry Gellibrand" (ODNB). All of Gunter s instruments are shown in use on the engraved title page. This particular engraving was used for many of the reprints of Gunter s work, the central title being changed and various inscriptions being added to the shield at the base (blank in this first edition). The present copy of Gunter s De sectore is here bound with the second edition of his Canon triangulorum (first, 1620), the first published table of logarithmic sines and tangents. This edition includes the first t.
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