graphometer
Datecirca 1790
Inventory NumberDW0861
ClassificationGraphometer
Subject
Maker
Jean-François Richer
1743 - 1820
Cultural Region
Place of Origin
Dimensions19.5 × 24.2 × 10.8 cm (7 11/16 × 9 1/2 × 4 1/4 in.)
blue board box: 13.3 × 29.2 × 22.5 cm (5 1/4 × 11 1/2 × 8 7/8 in.)
blue board box: 13.3 × 29.2 × 22.5 cm (5 1/4 × 11 1/2 × 8 7/8 in.)
Material
DescriptionBrass graphometer, or semicircle, with two alidades and attached compass.
The fixed alidade is composed of two slit slights bolted to either end of the straight edge of the semicircle. A second alidade, composed of two slit sights bolted to either end of a brass bar, rotates abouts a brass pin at the center of the fixed alidade.
The circumference of the semicircle is engraved with a scale carrying values from 0 to 180 (degrees), graduated into halves. At each end of the moving alidade bar is another scale. Together the main scale and the alidade scales form a vernier scale with 1-minute precision.
Bolted to the center of the semicircle is a compass with a gold face, and cardinal and intercardinal points labeled.
Bolted to the back of the semicircle above the compass is an irregular brass piece held in place with a large brass screw.
Attached to the graphometer is a ball-and-socket staff mount. The socket is tightened against the staff by turning large brass screw with a butterfly knob.
The fixed alidade is composed of two slit slights bolted to either end of the straight edge of the semicircle. A second alidade, composed of two slit sights bolted to either end of a brass bar, rotates abouts a brass pin at the center of the fixed alidade.
The circumference of the semicircle is engraved with a scale carrying values from 0 to 180 (degrees), graduated into halves. At each end of the moving alidade bar is another scale. Together the main scale and the alidade scales form a vernier scale with 1-minute precision.
Bolted to the center of the semicircle is a compass with a gold face, and cardinal and intercardinal points labeled.
Bolted to the back of the semicircle above the compass is an irregular brass piece held in place with a large brass screw.
Attached to the graphometer is a ball-and-socket staff mount. The socket is tightened against the staff by turning large brass screw with a butterfly knob.
Signedengraved in script on semicircle, top center: Richer Paris
FunctionUsed to measure horizontal angles, in navigation or surveying.
To calculate the angle between two points with respect to the user's position, the user first positions the semicircle on the staff horizontally, then rotates it until he sights the first object through the fixed alidade. Then, holding the semicircle in place, he rotates the second alidade until he sights the second point through it. He reads the angle from the vernier scale, using the compass as desired to find its position with respect to magnetic north.
To calculate the angle between two points with respect to the user's position, the user first positions the semicircle on the staff horizontally, then rotates it until he sights the first object through the fixed alidade. Then, holding the semicircle in place, he rotates the second alidade until he sights the second point through it. He reads the angle from the vernier scale, using the compass as desired to find its position with respect to magnetic north.
ProvenanceDavid Wheatland purchased from Roux-Devillas, Paris, on July 27, 1964 (lot 2/529).
Michael Butterfield
Date: 1675-1715
Accessories: leather case stamped with fleur-de-lys
Object number: DW0663
Benjamin Martin
Date: 1738-1777
Accessories: brass cover; ball-and-socket mount (DW0511c); tripod (DW0511b).
Object number: DW0511a
Edward Nairne
Date: circa 1790
Accessories: mahogany box with clasp closure
Object number: 5228
Andrew Yeates
Date: circa 1840
Accessories: case, second eyepiece, plumb bob
Object number: 5235
