THE HYDROGRAPHIC CIRCLE OF ROLLET DE L'ISLE, French, late 19th century, finely signed "Lorieux, A. Hurlimann, Succ'r a Paris." The 10-1/2" (27 cm) diameter brass circle is fitted with four mirrors, two of them mounted on the rotating index arm and set back-to-back at a fixed angle, a large-light-gathering-power telescope, and a hinged turned wood handle. An inset circular scale is divided every 0.5 from -10 to +200 half-degrees. The index arm has clamp, fine motion tangent screw, vernier to one-fiftieth of a degree, and magnifier. The instrument is equipped with its full complement of accessories, all original: two solar filters, two right angle sights, seven interchangeable filters, two adjusting tools, three spare mirrors, fitted mahogany case 13" (33 cm) square, and key. Only the screwdriver and loupe are lacking. Condition is absolutely superb throughout. The circle functions basically as a sextant, except there are two "horizon" mirrors, and two "index" mirrors, disposed so that one pair is useful at smaller angles, the other pair at larger ones. Readings are possible from 0 degrees straight ahead all the way to 180 degrees directly behind the observer -- as one mirror view becomes foreshortened, the other takes over automatically.

This is the remarkable invention of M. Rollet de l'Isle, French hydrographic engineer to the Marine (and world-famous proponent of the Esperanto language). It was still advertised in 1911 (by Ponthus & Therrode, successors to Hurlimann, who in turn was successor to the well-known firm of Lorieux established in 1845), as the model adopted by the French hydrographic marine services. A very rare example of this innovative design, in splendid condition. (8194) $7200.

RARE NAVIGATIONAL SLIDING GUNTER, English, c.1800. This uncommon slide rule is made of boxwood, brassbound, with a 25" (63 cm) long central slider. Rule and slider are divided on both sides with logarithmic and trigonometric scales, including Chords, Rhumbs, Numbers, Sine Rhumbs, Sines, Versed Sines, Tangents, Tangent Rhumbs, Longitudes, Meridional Parts, and Equal Parts, the latter two designed specifically for sailing by Mercator's chart projections. Condition is fine throughout. A description of the "sliding Gunter" is found in Norie's A New and Complete Epitome of Practical Navigation (8th ed., 1825); it bears considerable similarity to the sliding navigational rule invented by John Robertson (1712-1776) and published by William Mountaine in 1778. In 1807 Mackay wrote, in The Complete Navigator, "But the most convenient form of this instrument [the "common Gunter's scale"] both for accuracy and dispatch is that known by the name of the SLIDING GUNTER, in which the use of a compass is superseded." We find one quite similar rule in the Science Museum collection (Inventory #1921-676). A rare and significant navigational instrument. (7263) $1950.