Three lead discs, from apparatus to demonstrate the mechanical advantage of a screw, by Robert Brettell Bate, Poultry, City of London, England, 1808-1847 Three lead discs 1808-1847
Quintant made by John Dennett Potter, London, about 1885. Anodized brass diamond-pattern frame, wooden handle. Signed on the limb: J.D. Potter; Poultry & Tower Hill London. Marked at the bottom of the index arm: H O [broad arrow] 10. Inlaid silver scale from -5° to 163° every 10', measuring to 138°. Silver vernier to 10". The tangent screw and clamping screw are on the back of the index arm. Four shades (three grey, one green); three horizon shades (two grey, one green). Index-glass adjustment by screw; adjustment of the horizon glass by capstan screws and a square-headed screw and a detached key. Magnifier on a 90 mm swivelling arm, a frosted glass shade. Threaded telescope bracket in two parts, fitted for correcting collimation error; perpendicular adjustment by rising-piece and a milled knob. Telescope (82 mm) erect image; telescope (184 mm) inverted image, two parallel cross-wires; telescope (155 mm) star finder; extra draw tube (80 mm) inverted image, four cross-wires. Sight-tube (80 mm); three shaded eyepieces (all green); detached threaded telescope bracket for the star finder; adjusting key; adjusting pin. Mahogany keystone box, containing in the lid three certificates of examination of the National Physical Laboratory, for 1885, [illegible, stuck over] and for 1910. The lid also contains a trade label for Cary, Porter Ltd, and a Notice by them explaining how the star finder should be attached to the framed; it was an addition, made by Cary in April 1889. Formerly the property of the Hydrographic Office of the British Admiralty. Diamond- pattern frame quintant with case. 1860-1889
Surveyor's pocket box sextant with leather case, made by John Dennett Potter, Poultry, City of London, London, England, 1855-1865. Consists of two circular lacquered brass plates with lid that acts as a handle when used. Fitted with Inlaid silver 134° scale (-5° to 160°) with 30' divisions and a silver vernier (1' div.). The index arm is rotated by a milled knob on the upper plate with two filter shades (red & green) operated by lever. The index glass is fixed with adjustment for the horizon glass. Fitted with magnifier (single lens) on hinged 40 mm swivel arm and choice of a large and a small pinhole sight but telescope missing. Marked ‘H O’ the property mark of the Hydrographic Office of the British Admiralty. Box type pocket sextant with case. 1855-1865
Repeating circle (Borda circle) made by George Dollond, London, abut 1850. Circular anodized brass frame, polished limb, a wooden handle; a detached U-shaped bracket holds a second handle. Signed on the bottom of the index arm: Dollond | London. Inlaid silver scale from 0° to 720° every 20' (the digits are engraved in brass). Two silver verniers to 20", zeros at the centres. The second scale is to 180° every 5° on either side of 0°, engraved in brass. The index arm carries the index glass; the tangent screw, working against a spring-loaded stop, and the clamping screw are on the back of the index arm. Three index arms, moving with a brass scale and the second vernier on the inside of the limb; they carry the telescope and the horizon glass. One arm has a tangent screw, working against a spring-loaded stop, and a clamping screw on the back of the arm. Index-glass adjustment by a screw; adjustment of the horizon glass by screw and capstan screw. Three index shades (two red, one green); three horizon shades (two red, one green). Two magnifiers on 65 mm swivelling arms. Threaded telescope bracket with perpendicular adjustment by rising-piece and a milled knob. Telescope (96 mm) erect image; telescope (158 mm) inverted image, four cross-wires; extra draw tube ( 50 mm) inverted image, four cross-wires; extra draw tube (48 mm) inverted image, four cross-wires. Sight tube 84 mm); shaded eyepiece (red). Square fitted mahogany box containing in the lid a trade label for J.D. Potter, 31 Poultry, London. Borda type repeating circle with case. 1845-1852
