“Sector with thin brass strip”

Data:

• This is a brass sector dial in which the two arms have an opening to allow the thin brass strip to fall into them. Both sides feature scales, and the engravings are beautifully executed and clearly visible. Unfortunately, there is no name or other inscription on this sector dial, despite its high-quality finish.
• Length 153 mm (ca. 6"), width 36 mm (ca. 1.4").
• Thickness only 2 mm, and this still accommodates the thin strip!
• Each leg is constructed from two thin brass plates that are engraved. A spacer plate has been partially inserted, but positioned so that the thin brass strip can move in between. To hold the two engraved plates and the spacer together, they are riveted to each other. The round dark brown spots are in fact the rivets that are still visible. These brown spots appear at the outer side of the legs because the spacer plate is attached there.
• The strip hinges at one point on the outermost side of one of the legs, and that is where you can also see a round brown spot from a rivet. The strip can be rotated over at most 90 degrees.

Questions:

1. Who recognizes this sector and knows anything about its maker?
   
   The London Science Museum has a several sectors with a thin internal strip:

   • A 220 mm sector by Charles Whitwell, between 1600–1611. The strip has engraved scales and can slide through the opposite leg. Note the rivets.

   The 1623 dating for the second and third instrument is suspect: 1623 is the publication date of Edmund Gunter's The Description and Use of the Sector

   • A 200 mm Elias Allen sector, 1623. The photos show the strip either parallel or perpendicular to a sector's leg. Faint scales can be seen on one half of the strip
   • A 247 mm sector by John Allen, 1623. The photos do not show if the strip is hinged or detachable, but the strip has faint engraved circular and linear scales. There is a hole in the strip, probably at the center of the circular scale. The hole is close to the sector's hinge.
   • A 9" George Adams sector ca. 1756–1760. The photos do not show if the strip is hinged or detachable, but the strip has a semicircular scratch.
   • The ISRM shows an unseignd sector and claims that the strips are used to “keep the arms aligned when closed” and suggests an attribution to George Adams or Thomas Heath.

     Andries de Man
   
   
   
2. When was it made, approximately?
   The flowery engraving on the hinge looks like that on Folding Level and Sector, Signed: Le Maire Lefils à Paris c. 1750.
   A similar engraving, but more fiery than flowery, can be seen on an 18th century French gunnery sector, with different scales.
   Note that the engraving on Leo's sector runs clockwise on one side and counterclockwise on the other. Andries de Man
   
   
3. What exactly is the purpose of this design with the thin brass strip?
   From the description of a wooden Gunner's Folding Rule, Signed Bengamin Jobson 1680: “To use the instrument as a quadrant, the thin brass arm is set at 90° to the legs. With a plumbline hung from the tip of the arm, readings are taken against the degree scale on the supporting arm.” This layout is also shown for an Elias Allen sector. Andries de Man
   Is an other use, the strip connects the two arms, see e.g. the T. Wright sector This is demonstrated by the Arithmeum and requires that the strip carries a scale!
   See also Nicolas Bion, “Traité de la construction et des principaux usages des instrumens de mathematique”, Paris, 1709, Plate 60 p.188 Andries de Man
   
   
4. 
   Is there a known purpose for the special shape of the end of the brass strip?
   If the instrument is used as a quadrant (see above), the shape of the end of the brass allows for attaching a string. Note that the ends of the strip of Elias Allen sector are plain. Andries de Man