dip circle
Date: 1765
Inventory Number: 0026
Classification: Dip Needle
Subject:
Maker: Edward Nairne (1726 - 1806)
User: John Winthrop (1714 - 1779)
User: Samuel Williams (1743 - 1817)
Cultural Region:
Place of Origin:
City of Use:
Dimensions:
35 x 32 x 26.5 cm (13 3/4 x 12 5/8 x 10 7/16 in.)
circle diam: 28.2 cm (11 1/8 in.)
Accessories: Small magnifying glass on end of box to assist in aligning the dip needle.
Bibliography:
The Apparatus of Science at Harvard, 1765-1800
Description:
This instrument is made of a brass circle mounted on vertically on a round brass circular base. The base has a copper level and is supported by three feet, which have each a leveling screw. The whole instrument can rotate on its base with the help of a tangent screw.
There is an engraved scale around the vertical circle, marked by single degrees and subdivided every 30'. It is numbered by tens in the following way: 30°-0°- 90°-0°-30°, the zeros matching up at horizontal diameter. The index arm with vernier pivots on center of circle. The magnetic needle is mounted on knife-edge pivots and is enclosed in a rectangular brass box with copper edges, silvered base and glass cover.
A small copper magnifying glass is attached at the upper end of the box to assist in aligning the needle. Some screws have small pre-determined punched dots for dismantling purposes. A very uncommon instrument by Nairne.
There is an engraved scale around the vertical circle, marked by single degrees and subdivided every 30'. It is numbered by tens in the following way: 30°-0°- 90°-0°-30°, the zeros matching up at horizontal diameter. The index arm with vernier pivots on center of circle. The magnetic needle is mounted on knife-edge pivots and is enclosed in a rectangular brass box with copper edges, silvered base and glass cover.
A small copper magnifying glass is attached at the upper end of the box to assist in aligning the needle. Some screws have small pre-determined punched dots for dismantling purposes. A very uncommon instrument by Nairne.
Signedon base: Nairne London
FunctionThis kind of instrument is used to measure the inclination (or dip) of the earth's magnetic field. A compass needle has two motions: horizontal (to determine the magnetic north) and vertical, towards the center of the earth, what is called inclination or dip of the needle. Since the magnetic poles shift gradually and irregularly, these two motions varies with time and according to where you stand at the surface of the earth. For an accurate picture of the earth's magnetism, these two motions need to be taken into account. The dip circle can measure precisely the magnetic inclination on a day-to-day basis.
Historical AttributesThis instrument was the fourth magnetic instrument bought by Harvard from Nairne in 1765. It was also the most expensive, costing £18.3.0. It came in a mahogany box lined with green cloth, lock and key. It has been lost since then.
At Harvard, Samuel Williams, the Hollis Professor, assisted by Stephen Sewall, the Hancock Professor of Hebrew and other Oriental Languages, published readings taken on this instrument (and the variation compass, 0025) in the first volume of the Memoirs of the American Academy of Arts and Sciences (1785).
Before the fire destroyed Harvard's apparatus in 1764, Harvard had received a dip needle from John Hancock of Boston in late 1761 or early 1762. Thanks were given by the Corporation on January 5, 1762. (see related references)
At Harvard, Samuel Williams, the Hollis Professor, assisted by Stephen Sewall, the Hancock Professor of Hebrew and other Oriental Languages, published readings taken on this instrument (and the variation compass, 0025) in the first volume of the Memoirs of the American Academy of Arts and Sciences (1785).
Before the fire destroyed Harvard's apparatus in 1764, Harvard had received a dip needle from John Hancock of Boston in late 1761 or early 1762. Thanks were given by the Corporation on January 5, 1762. (see related references)
Curatorial RemarksXRF analysis of the metals was done by Katherine Eremin and Sara Schechner in August 2007.
Published ReferencesDavid P. Wheatland, The Apparatus of Science at Harvard, 1765-1800 (Cambridge: Harvard University Press, 1968), 160.