surveyor's semicircle protractor with diagonal scale and rotating arm
Date: 1700-1750
Inventory Number: DW0656
Classification: Surveyor's Circle
Subject:
Maker: French ?
Cultural Region:
Dimensions:
5 × 15.6 × 20 cm (1 15/16 × 6 1/8 × 7 7/8 in.)
DescriptionA half circle brass protractor with a two-tiered brass cross beam along the diameter. There is a brass plate screwed onto the top of the cross beam in the center where the higher tier drops down to the lower tier. The plate has a half circular hole at the top within which the edge of the step is centered. As such, half of the half is full and the other half is empty. There is a quarter-circle brass extension just above the left-hand side of this brass plate that aligns with the half circular hole creating a three-quarter circle, one quarter of which is full. There is also a rotating brass arm attached to the diameter cross beam with a solid quarter circle at its base. When the arm is placed directly in the center of the protractor, a complete circle is created at the bottom, with two diagonal quarters empty and two full, creating a useful sight for aligning the center of the protractor with a specific point below.
There is a square hole and three small round holes through the diameter cross beam on either side of the central brass plate. A rectangular extension with a rounded top, a rectangular hole cut through the top, and a vertical slit through the center, rises from a brass base plate screwed on to each end of the diameter cross beam. An identical extension rises from the rotating brass arm, near the base. An inverted extension, with a hole near the bottom and the slit above to the top, rises from the free end of the rotating arm. These four extensions act as sights by which the users can align the diameter cross beam and the arm of the protractor with a desired point.
In between the two brass extensions on the arm is a brass sleeve that fits onto the circumference of the protractor. As the arm is rotated back and forth, the sleeve slides along the circumference. There is an adjustment screw on the top of the sleeve that, when tightened, holds the arm in place on the circumference.
The circumference of the protractor is divided into 180° by engraved lines. There is an additional set of engraved lines that run parallel to the circumference; dividing each degree into fourteen small squares (the outermost two squares are slightly smaller). There are two numeric scales engraved on the circumference as well, one along the outermost edge and one along the innermost edge, separated by the degree grid. The outer scale begins with 0 at the leftmost end and increases by tens to 180. The inner scale begins with 180 at the leftmost end and decreases by tens to 0.
There is a square hole and three small round holes through the diameter cross beam on either side of the central brass plate. A rectangular extension with a rounded top, a rectangular hole cut through the top, and a vertical slit through the center, rises from a brass base plate screwed on to each end of the diameter cross beam. An identical extension rises from the rotating brass arm, near the base. An inverted extension, with a hole near the bottom and the slit above to the top, rises from the free end of the rotating arm. These four extensions act as sights by which the users can align the diameter cross beam and the arm of the protractor with a desired point.
In between the two brass extensions on the arm is a brass sleeve that fits onto the circumference of the protractor. As the arm is rotated back and forth, the sleeve slides along the circumference. There is an adjustment screw on the top of the sleeve that, when tightened, holds the arm in place on the circumference.
The circumference of the protractor is divided into 180° by engraved lines. There is an additional set of engraved lines that run parallel to the circumference; dividing each degree into fourteen small squares (the outermost two squares are slightly smaller). There are two numeric scales engraved on the circumference as well, one along the outermost edge and one along the innermost edge, separated by the degree grid. The outer scale begins with 0 at the leftmost end and increases by tens to 180. The inner scale begins with 180 at the leftmost end and decreases by tens to 0.
Signedunsigned
FunctionThe protractor is used for measuring angles or circles in degrees. The center of the protractor is lined up with the vertex of an angle or the center of a circle using the circular sight in the diameter cross bar. The cross bar is then lined up with an angle edge or a diameter of a circle. Angles can then be measured using the engraved scales on the circumference. The rotating arm can help increase the precision of measurements. It also facilitates drawing angles, since it doubles as a straight edge. The vertically extended sights also permit users to align the protractor with points on perpendicular planes, making this instrument useful for surveying.