Part 19

Zenith Sector for measuring the angle between a star at its zenith and the vertical. Made of brass, with focal length of 6 ft. and an aperture of 2-1/2 in. The original lens was made in London about 1780. The instrument was made in the old pattern with brass tube and mountings and a wooden supporting post. The tube is suspended by trunnions at the top and swings against a graduated arc extending north and south for measuring zenith distances in the meridian. It is adjusted in the vertical by a plumb line whose errors are eliminated by reversing the whole mounting about the supporting post. Constructed principally by David Rittenhouse, with some modifications by Andrew Ellicott.

In the Journal of Andrew Ellicott its author referred to this sector as follows:

The boundary line to the North of Pennsylvania was fixed by Dr. Rittenhouse and Captain Holland in the year 1774 and completed in 1786 and 1787. We commenced operations by running a guide line west from the point mentioned on the Delaware 20-1/4 miles and there corrected by the following Zenith distances taken at its West termination by a most excellent sector constructed and executed by Dr. Rittenhouse.

The zenith sector is again mentioned in the appendix of the Journal: "One Zenith Sector of nearly six feet radius similar to the one made by Mr. [George] Graham for Dr. Bradley and Mr. Molyneux, with which the aberrations of the stars and mutation of the earth's axis were discovered, and the quantities determined."

Gift of Andrew Ellicott Douglass, Tucson, Ariz., in 1931. USNM 152078. FIGURE 11.

Zenith Sector, made of brass, original lens broken. Constructed by David Rittenhouse with some additions made by Andrew Ellicott. In The Journal of Andrew Ellicott the instrument is described as a Zenith Sector of 19 inches radius to be used when the utmost accuracy was not necessary, and where the transportation of the large one could not be effected without great expense and difficulty. These instruments were principally executed by my late worthy and ingenious friend, Mr. Rittenhouse, except some additions which I have made myself. The plumb lines of both Sectors are suspended from a notch above the axis of the instruments in the manner described by the Rev. Dr. Maskelyne, the present Astronomer Royal at Greenwich, in the introduction to the first volume of his Astronomical Observations. A particular description of those instruments is rendered unnecessary by being accurately done in a number of scientific works, particularly by M. de Maupertius in his account of the measurement of a degree of the meridian under the Arctic Circle--The Sector is of all instruments the best calculated for measuring zenith distances which come within its arc. The large one above mentioned [large Zenith Sector] extends to 5 degrees North, and South of the Zenith. Stars when so near the Zenith are insensibly affected by the different refractive powers of the Atmosphere arising from its different degrees of density. Add to this that the error of the visual axis is completely corrected by taking the Zenith distances of the stars with the plane, or face of the instrument both East and West.

USNM 152079. FIGURE 80.

RITTENHOUSE & EVANS, Philadelphia, Pa., 18th century.

Surveying Compass, about 1780, made of brass, overall length 13-3/4 in., diameter of dial 5-1/4 in., silvered bubble level, vernier on alidade. The glazed dial, engraved "Rittenhouse & Evans," is fitted with a brass cover.

This instrument was made during a brief partnership between David Rittenhouse and David Evans, a clock- and watchmaker of Philadelphia and Baltimore. It was one of several owned and used by John Johnson in 1818 for surveying the boundaries between Canada and Maine.

The survey, made in compliance with the Treaty of Ghent, is described in The Collections of the Maine Historical Society (Portland: Hoyt, Fogg & Donham, 1881, vol. 8, p. 20):

Thomas Barclay, of whom we have heard more than once before, as a Commissioner under the treaty, on the part of Great Britain, and Cornelius P. Van Ness, on the part of the United States, were appointed Commissioners to ascertain and run the line. An actual survey was arranged, and surveyors appointed, to wit: Charles Turner, Jr., on the part of the United States, and Colin Campbell on the part of Great Britain. About twenty miles of the line was surveyed, then the work was discontinued, never to be resumed; but an exploring survey was commenced by Colonel Bouchette, on the part of Great Britain, and John Johnson, on the part of the United States. These gentlemen made an exploring line in 1817, extending ninety-nine miles from the monument at the head of the river St. Croix, and made separate reports of their doings. In 1818 Mr. Johnson, with Mr. Odell, who had taken the place of Col. Bouchette, finished running the exploring line to the Beaver or Metis River....

Gift of John Johnson Allen of Burlington, Vt., in 1927. USNM 309543. FIGURE 81.

THOMPSON, Captain SAMUEL ROWLAND (18th century); Lewes, Del. Octant made of dark wood and with lignum vitae; brass fittings. This harbormaster's instrument, used by Captain Thompson during the second half of the 18th century, is without numerical designations on the arc. The eighth part of a circle is connected to an apex by two side pieces with a swinging arm hinged at the apex, with a blade at its end that moves along a checkered scale on the arc.

Gift of George Andrews Thompson of Baltimore, Md., in 1926. USNM 308473.

VOIGHT, HENRY (1738-1814), Philadelphia, Pa.

Equal Altitude Telescope of brass, 17 in. long, on wooden tripod about 46 in. high. Objective lens is missing. Signed "Henry Voigt." Made about 1790 and used for determining meridian lines and time observation of the sun's noon transit. This form of instrument was originally invented about 1716 by Roger Cotes, professor of astronomy at Cambridge, as a simple instrument for the determination of time.

Deposited in the U.S. National Museum by the Smithsonian Institution in 1939. USNM 311772. FIGURE 31.

WASHINGTON, GENERAL GEORGE (1732-1799), Mount Vernon, Va.

Compass Sundial described by the donor as having been presented to Gen. George Washington by General Braddock on the retreat through Paris Gap, Fairfax County, Va. Gift of Samuel Keese in 1902. USNM 9842.

Field Glass, brass tube in three sections, length closed 9 in., opened 22-1/2 in. Diameter of object lens 1-3/4 in., of ocular lens 1-1/8 in. With original case of russet leather, which is 9-1/2 in. long and 2-1/2 in. in diameter. Maker not known. Stated to have been used by Washington during the Revolutionary War at the campaign of Valley Forge.

According to related correspondence, when not in use the instrument was carried by the General's body servant, Billy Lee. The General presented the field glass to Major Lawrence Lewis, his favorite nephew, in 1799, the last year of his life.

Purchased by the U.S. Government from the Lewis heirs in 1878 and transferred to the U.S. National Museum in 1883. USNM 92424, 92425. FIGURE 82.

Spyglass or Telescope, made of wood, 9-sided, wrapped throughout with twine, 62 in. long. Brass mountings for object and ocular lenses made by "Cole, Fleet Street, London." Diameter of object lens 2-3/4 in., diameter of ocular lens 1 in.

The maker, Benjamin Cole (1725-1813), was the third generation of instrument makers of the same name. Other instruments by this maker are in the National Maritime Museum and the Whipple Museum, Cambridge.

This telescope, used by General Washington at Mount Vernon, "was kept behind the hall door and his favorite amusement was to look out over the river with it." According to Mrs. Lewis, the General used it to observe life on the river and especially to discover guests approaching Mount Vernon, as many of their visitors arrived by boat. Benjamin Latrobe, the architect, on a visit to Mount Vernon made an amusing sketch of his host looking anxiously up the stream for some belated dinner guests.

Part of the collection purchased from the Lewis heirs in 1878 by the U.S. Government and transferred to the U.S. National Museum in 1883. USNM 92423. FIGURE 83.

Survey of Land, drawn and documented by George Washington on April 2, 1751 for Thomas Loftan of Frederick County, Va. Paper, 12 in. wide by 7-3/4 in. high.