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.

TOWNSHEND'S DOUBLE REFLECTING CIRCLE -- AN EXCEPTIONAL AMERICAN NAVIGATIONAL INSTRUMENT, c.1890, signed "Townshend, Patd. Jan 31, 1888," and "Stackpole & Brother, New York, 2204." The instrument is made of brass, with 8" (20 cm) diameter six-spoked main circle inset with silver scale divided every ten seconds of arc and labeled with double degrees 0° - 180° (on two sides), and 0° - 720° full circle. Two mirrors are mounted independently at the center of rotation, each with vernier (divided every ten arcseconds thus reading to 20 seconds in double degrees) arm and magnifier. Each mirror arm can be adjusted manually by long control rod, or driven in fine motion by clamp and tangent screw. One views through the adjustable telescope to the fixed mirror (with one clear and two silvered areas itself mounted tied to the telescope on long arm with clamp, tangent screw, vernier, and magnifier), then to the adjustable mirrors and to the targets. Thus one can sight three objects and measure two angular separations simultaneously (as two bright stars from the moon, for the lunar longitude method, or three coastal landmarks, for triangulation in coastal navigation). It is equipped with mahogany handle, two interchangeable achromatic telescopes, one sighting tube, screw-on solar filter, three insertable rectangular filters, and the original fitted mahogany case. Condition is very fine noting a little spotting to the original lacquer finish. Captain Townshend (1833 - 1904) was a resident of New Haven, Ct, and recommended his device for sighting sun, moon, and fixed stars, etc. Very few examples of his double reflecting circle have survived -- we can locate only those at Mystic Seaport and in the New Haven Colony Historical Society, one in a private collection, and the one offered in Tesseract Catalogue 50, Item 26. An important find. (8152) $14,500.