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Inventory no. 38251 - Former Display Label

English SURVEYOR'S SECTOR 1597

Signed and dated: * Robertus * Beckit * fecit * 1597 *

Brass, with iron points. Radius, including points, 329 mm.

The brass limbs are engraved on one side with "Power of Lynes", and "Coards of a
circle"; on the other side is a line of numbers (0 to 112) for calculations made with a
pair of dividers. One limb slides over a wide, curved arc of brass. The two sequences
under "Power of Lynes" bear the following numbers, which represent ratios:
1 (at ends) 1/2, 1/3, 1/4, 1/5, 1/6, 1/7, 1/8, 1/9 1/10
With the limbs open, the line between the pair of points 1:3 (for example) will, when
squared, give an area a third of the square of the line between the points of the limbs.
The two sequences under "Coards of a circle" bear the numbers 3 to 12. These give the
lengths of the chords of a circle, relative to a diameter equal to the opening at the
points, subtending such a portion of the whole circumference as is signified by the
numbers. For example, the distance between the 5s will give the chords for an
inscribed pentagon.

On the brass arc are points numbered 3, 4, and 5, which give the angles for 3, 4, and
5-sides figures, i.e. 60°, 90°, 108°. On the same side of the arc are three engraved
scales with their ends in common, calibrated from 0-180, 0-144, 0-108, and subdivided
to 1/2 units in each case. These represent 18 inches divided into tenths, eighths and
sixths, the measure being taken from the tips of the iron points at the ends of the limbs.

On the side of the instrument is the line of numbers which is used, with a pair of
dividers, for calculations based on the principle of similar triangles. The brass arc
is engraved with one scale of degrees, 0°-110°, numbered in 10s, and divided to 1/4°.
Holes in the limbs at the ends are for sights, and at the centre for fixing to a post and
for a sight.

The sector was invented earlier than the slide-rule, and persisted in use until the middle
of the nineteenth century. The popularity of the sector owed much to Galileo, who pub-
lished a description of his compasso geometrico e militare in 1606. However, there were
sixteenth-century precursors of the Galilean instrument, one of which was the sector
described by Thomas Hood (fl. 1582-98) who, following a Privy Council recommendation
that citizens should be instructed in military matters, was appointed to a mathematical
lecturership in London. This sector resembles the one described by Hood, as may be seen
from an illustration in Hood's book The Making and Vse of the Geometricall Instrument,
called a Sector ... , London [? 1598], and it is dated the year before the probable date of
that publication.

The Museum also possesses a smaller and undated sector of the Hood type, which
differs in several respects from this one and probably represents an earlier stage in
the development of the instrument. Little is known of the maker of this instrument,
Robert Beckit, except that he engraved some five maps for the London edition, 1598, of
Jan Huygen van Linschoten's Discours of Voyages into ye Easte & West Indies ...
Historically, and also because of the quality of the workmanship and engraving, this
instrument merits an honoured place in a collection of early mathematical instruments.

[80-8] Purchase

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