History Corner: The Zenith Sector In Colonial America

As its colonies became firmly settled in New England, the English Crown developed a plan to establish a series of plantations along the North American seacoast to form an Anglo-American frontier. Although every effort was made to support the plan, with promotional materials extolling the riches of the new lands widely distributed in England, the Crown failed to entice prospective investors and emigrants in sufficient numbers.

Even where the promotional effort was more successful, the descriptions incorporated many errors, occurring particularly on the maps of the new regions to be populated. In Lord Baltimore's pamphlet describing his colony, for example, a large section of Virginia's former Prince William County was shown as part of Maryland. There were many such misrepresentations, which eventually contributed to violent disputes not only between individual settlers in the regions but also between provincial governments. In general, grants made by the Crown to colonial proprietors were ill-defined, often with haphazard and inexact boundaries. It was not unusual for grants to occasionally overlap one another, so that numerous disputes arose not only between land owners but between provincial proprietors as well.

Advanced Instruments and Skills Required

Whereas the local surveyor in a community could provide satisfactory property definitions with simple instruments produced by colonial instrument makers, professional instruments and skills were required for the development of larger areas due to the ever-increasing population and assignments of land. Special regional commissioners were appointed by provincial assemblies to meet and jointly run boundary lines between colonies, but even though they had greater experience and skill than local surveyors, they rarely had sufficient knowledge of advanced surveying practices and instruments required to establish latitude and longitude. Another contributing factor was that land records were often not compiled until decades after settlement.

Notable among the disputed boundaries of the early eighteenth century were those between Virginia and North Carolina, the northern boundary between the Massachusetts Bay Colony and the Province of Rhode Island and Providence Plantation, and between the colonies of New York and East and West Jerseys.

After several abortive attempts had been made to define the line between Virginia and North Carolina, a joint commission was appointed in 1710, but neither side had instruments adequate for the purpose. Furthermore, one of the North Carolina surveyors had secretly staked out claims of land in the region for himself, and the controversy over availability of suitable instruments continued to delay the survey for a number of years.

A provincial boundary that was a source of constant dispute was the line between Massachusetts Bay Colony and the Province of Rhode Island. Although the line had been first established in 1649 and extended by mutual agreement in 1719, three decades of quarrels followed over provincial jurisdiction of property before it was resolved. Rhode Island, after appointing commissioners in 1748 to run the line once more, urged Massachusetts to do the same, but at the appointed time the Massachusetts commissioners failed to appear. The Rhode Island commissioners nonetheless ran the line, but the following year it had to be undertaken once more, even though Massachusetts still failed to participate. In 1750 the Rhode Island Assembly commissioned Benjamin King, Newport mathematical instrument maker, to make "an instrument to determine the latitude in order to run the dividing line between this colony and the Province of Massachusetts Bay," for which he was paid the substantial sum of £300 in 1756. The repeated failure of the Massachusetts governor to respond finally led the Rhode Island Assembly to petition the Crown to have the boundary established in accordance with the line then being run by the Rhode Island commissioners.

The same fate was suffered by the survey of the boundary between New York and East and West Jersey, which also was postponed and delayed due to conflicts between the commissioners concerning the availability of suitable instruments.

Two-Degree Overlap in Latitude

The most prolonged and publicized of the early boundary controversies was the one fought over the line between Pennsylvania and Maryland. In 1632 Cecil, second Lord Baltimore, received a grant of the entire peninsula east of the Bay of Chesapeake and all the land "not yet husbanded or planted" as far as 40º north. Fifty years later, in 1682, King Charles II awarded a grant to William Penn that included the territory lying west of the Delaware River from 12 miles north of the town of New Castle as far as the 42º parallel, unaware that the two grants overlapped.

The resolution of this boundary conflict, by means of the Mason and Dixon Line, as it came to be known, remains one of the most important and successful scientific endeavors attempted in colonial America. After being disputed for more than 80 years, no progress in the line's final delineation was made until after the middle of the eighteenth century. Finally, in response to appeals from the governors of both provinces, and at the urging of the King's Council, the Proprietors of both provinces, the aristocratic Penn family and the Barons Baltimore, both residing in London, jointly took positive action. Realizing that accurate definition of provincial boundaries could be achieved only by means of advanced surveying techniques and instruments, they consulted some of the foremost men of science in London for advice on surveying procedures and equipment. They then set about to implement it, expending considerable effort and expense in this joint endeavor. They arranged to borrow a few of the required instruments and commissioned other recently developed instruments to be made for them by the foremost London makers. Finally, they hired the experienced mathematician/astronomers Charles Mason and Jeremiah Mason to use the instruments in the field.

Zenith Sector Most Important Instrument

The most important of the instruments recommended was the zenith sector. A relatively new instrument, designed for astronomical observation and used in surveying, it was among the most sophisticated instrumentation of the eighteenth century. The first example of the zenith sector had been made in 1727 by the eminent London clockmaker and instrument maker George Graham (1673-1751) for the amateur astronomer Samuel Molyneux (1687-1728). Molyneux, accompanied by James Bradley (1693-1762), Savilian Professor of Astronomy at Oxford University, used the sector to observe the annual parallax of the star Draconis and led to the discovery of apparent stellar motions due to the effects of aberration of light and of nutation. In 1729, by means of another sector made by Graham, Bradley was able to calculate the speed of light in space. Graham also produced the sector used in 1736 by the party of French astronomers led by mathematician Pierre Louis Moreau de Maupertuis (1697-1759) to measure the length of the meridian at Torneo in Lapland, which provided the first reliable value of the elliptical nature of the earth's surface.

The zenith sector is a fixed vertical telescopic instrument designed for measuring the zenith distances that come within its arc, and serves also for discovering the aberration of stars and the nutation of the earth's axis. It is used for determining the parallels of latitude by repeated observations of a number of fixed stars near the zenith as they cross the meridian at differing hours. Because stars near the zenith are free from refraction, they are observed in preference to others for determining latitude. Six or seven stars are observed at varying times, with the observations repeated on a number of nights. The long focal length of the instrument's objective made the slightest deviation in a star's zenith distance readily perceptible as it culminated. The instrument consists of a brass tube between 5-1/2 and 6 feet in length which is suspended by means of trunnions of a framework erected within an observatory or in a field observatory tent in such a manner that its upper end projects through an opening. The tube could be moved like a pendulum on a horizontal axis near the object lens.

Zenith sectors were produced also by two other contemporary English makers of mathematical instruments, John Bird (1709-1776) and Jonathan Sisson (1690-1740). Prices for the instruments varied according to the radius, from £60 to £160. Sisson produced a zenith sector for Cecilius Calvert, who was acting on behalf of his nephew Frederick Lord Baltimore, and Thomas Penn commissioned one to be made for him by John Bird. As Penn then communicated to the Reverend Richard Peters, his secretary in Pennsylvania, "We are advised to send a Six feet Sector to mark points for running a parallel of Latitude: Lord Baltimore has sent one by Sisson and I have bespoke one of Bird… ." The Bird sector made for Penn was the first example of the instrument to incorporate a modification in the suspension of the plumb bob that had previously been suggested by the astronomer Nevil Maskelyne.

Chose Instrument Supplied by Penn

Arriving in Philadelphia on November 15, 1763, the astronomers Mason and Dixon immediately set to work, first meeting with the commissioners and five days later removing their instruments from the vessel on which they had sailed. Before proceeding further, they set up both zenith sectors for comparison. After these had been inspected first by themselves and then by the commissioners for both provinces, the astronomers declared their preference for the Bird sector over the one made by Sisson. The reason for their choice was described by Mason in the Journal he maintained of the survey, "Set up Sector brought by the commissioners from Maryland [made by Sisson] and found the Nonius would not touch the middle of the Arch."

The Bird sector was used constantly during the course of the survey, "to measure the angle between the zenith and a star as it crossed the meridian. The instrument was mounted to rotate about both vertical and horizontal axes. A telescope attached to a sector of a vertical circle with a graduated limb was trained upon the star as it came to the meridian. A fine plumb-line hanging past the center of the sector and its limb served as a reference for the reading of angles in the plane of the meridian."

Survey Required Five Years to Complete

The survey required five years to complete, after which Mason and Dixon departed for England and the instruments were returned to their respective owners, the Sisson sector to Maryland and the Bird instruments to the Pennsylvania Proprietors. Most of the latter were stored in his own house by Joseph Shippen, a Penn employee, while the larger equipment, including field tents and the sector, were stored in the Pennsylvania State House (now Independence Hall).

It was not long before the Bird sector and the transit and equal altitude instrument were required again, for making observations in Philadelphia of the transit of Venus scheduled to occur on June 9th, 1769. Early in the year a committee of members of the American Philosophical Society appointed for the project began to assemble instruments for making observations from three different sites, David Rittenhouse's farm in Norriton, a station selected at Cape Henlopen, and the yard of the State House in Philadelphia, where an observatory was erected for the purpose. Astronomical instruments were few and hard to find in Philadelphia, and finally they applied to Thomas Penn for permission to use the Bird instruments, a request he graciously granted. The Society's Minutes subsequently confirmed that, in addition to four telescopes and a timepiece with which that group was equipped, "they had a transit and equal altitude Instrument and a sector belonging to the Hon.ble Prop.r."

The Proprietor's instruments were used at the State House yard observatory. These served the same committee in November, five months later, to make observations of the transit of Mercury. The latter phenomenon was observed by many throughout the country, and accounts of the various observations appeared in the public press, while those made by the Society were published in its Transactions.

Used in State Boundary Surveys

With the conclusion of the celestial observations, the Bird instruments were returned to temporary storage in the State House yard observatory. The Bird sector was borrowed by David Rittenhouse in the late summer of 1769 and used in establishing the New York-New Jersey boundary to determine the latitude at either end of the line. It was used again by Rittenhouse in October 1774 while he was engaged with Samuel Holland in establishing the beginning of the 43rd degree of latitude on behalf of the province of Pennsylvania. He returned it when the survey was completed and, as darkening clouds of war were fast approaching during the next several years of troubled political unrest, it remained with the other instruments in the State House.

Silvio Bedini is a historian emeritus with the Smithsonian Institution in Washington, D.C. and a Contributing Editor for the magazine.

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