History Corner: The Zenith Sector In Colonial America

The first zenith sector to reach the American Colonies was imported from England by Thomas Penn in 1763, and was used extensively for more than a decade. After serving the English astronomers Charles Mason and Jeremiah Dixon in 1763-1768 for surveying the boundary between Pennsylvania and Maryland, which became known as the Mason and Dixon Line, it was borrowed by the American Philosophical Society in 1769 for making observations of the forthcoming transits of Venus and Mercury. It was used again on surveys by David Rittenhouse between 1769 and 1774. When it was sought in 1786 for the survey of the Pennsylvania-New York boundary, however, it was no longer to be found.

Following the American defeat by the British at the Battle of Brandywine on September 11, 1777, the American wounded were taken to Philadelphia and brought to the State House, where the upper floor was hastily converted into a temporary hospital. Meanwhile the imminent threat of capture and occupation of Philadelphia increased daily, and became a reality as the British Army began its march on the rebel capital. Frenzied Philadelphians hurried to remove supplies and materials that would be useful to the enemy. Bells were dismantled and, together with books and public papers, money and court records, shipped to safety outside the city. The astronomical instruments stored in the State House and elsewhere were taken to Easton, Pennsylvania for safekeeping.

Residents Fled With Valuables

The roads out of the city were crowded by residents who carried their most valuable possessions piled high into every possible means of conveyance as they fled to the countryside. For some reason, the Bird transit and equal altitude instrument appears to have been separated from the other instruments and was overlooked when the others were removed, but apparently during the last moments was hastily hidden under floor boards in the State House tower, where it remained forgotten until 1912. Following the departure of the British months later, the returning residents found the city in shambles. Plans that were made to restore the observatory in the State House Yard never materialized, and it was eventually sold for the value of the lumber.

In 1786, when the boundary between New York State and Pennsylvania was to be established, David Rittenhouse and Andrew Ellicott were appointed surveyors for the Commonwealth of Pennsylvania, while the State of New York was represented by James Clinton and Simeon De Witt, the Surveyor General of New York. The commissioners agreed that the only instrument sufficiently accurate to trace the required parallel of latitude was the zenith sector, but the Bird instrument belonging to the Penn Proprietors apparently was no longer available, and Rittenhouse and Ellicott assumed it had been shipped to Thomas Penn in England. Consequently, they were forced to provide their own instruments.

Accordingly, Rittenhouse and Ellicott, both experienced instrument makers, set to work to construct a sector of their own, basing its design on that of the Bird sector. In a communication concerning the status of the survey to Governor John Dickinson of Pennsylvania, Rittenhouse commented that De Witt, "… says they must depend on us for the necessary Instruments. I believe there is no Instrument fit for the purpose in this part of America excepting the 6 foot Sector belonging to Mr. Penn. But I have been for some time employed in making one which will be much more portable than that of Mr. Penn, and, I doubt not, equally accurate. It might soon be finished if I were not obliged to go to the westward."

5.5 Feet In Radius

Ellicott and Rittenhouse working together, directed their efforts to the design and construction of a zenith sector 5.5 feet in radius. While the work was in progress, Ellicott was summoned away, and the principal part of the work was executed by Rittenhouse, to which Ellicott later added modifications. In the course of his work, Rittenhouse realized that he required an achromatic lens of 5.5 feet focus in order to observe stars of the second and third magnitudes as they passed near the zenith any time of the day.

Astronomical lenses were not yet being produced in the United States and had to be imported from England or France; consequently they were costly and available only after considerable delay. Work stopped on the project until Ellicott recalled that a lens having the required specifications was owned by Thomas Pryor (also Prior), a maker of mathematical instruments with a shop on Chestnut Street in Philadelphia. An early member of the American Philosophical Society, Pryor had participated in observations of the transit of Venus in 1769 from the State House Yard observatory and was the owner of an accurate clock that had been used. During the American Revolution Pryor's loyalist inclinations caused him to fall somewhat into public disfavor by advertising in a Philadelphia newspaper published in 1778, during the British occupation, that he made, sold and repaired navigational instruments, items that would be useful to the occupying enemy. He subsequently retired from business in the 1790s; the city directory for 1795 listed him merely as a "gentleman."

Rittenhouse Borrowed The Lens

When approached for a loan of the lens, Pryor agreed that he had no need for it at the time, and gave it to Rittenhouse on condition that a similar one should be imported for him. Rittenhouse used the lens for his zenith sector and, as Ellicott later reported, "from some cause, or other, not being attended to by M.r Rittenhouse, a similar glass was never procured for M.r Pryor." The new zenith sector was used by both Pennsylvania and New York surveyors to obtain the latitude of points on a random line to run the eastern section of the northern boundary through a dense wilderness.

At about the same time Ellicott also arranged for Rittenhouse to construct a modified version of the zenith sector for him, one having a focal radius of 19 inches. It was considerably smaller than the larger instrument, and consequently less accurate. It had the advantage of greater portability, however. Ellicott used the portable sector along the line of survey for observing the aberration of the stars and the nutation of the earth's axis.

Rittenhouse and Ellicott employed their new large zenith sector successfully in the New York and Pennsylvania boundary survey. The instrument had become Ellicott's property and he used it again between 1791 and 1793 during the survey of the Federal Territory for establishing the national capital of Washington. For the first several months during the latter survey, from February into April, the sector was tended nightly in the field observatory tent by Benjamin Banneker, who served as Ellicott's assistant for several months until Ellicott's brothers, employed on a survey in upper New York State, arrived to assist him.

Ellicott planned to use the large zenith sector again in 1796 for the survey of the southern boundary between the Spanish possessions and the United States, and it was then that a problem arose. Ellicott was in Philadelphia and had already packed all the instruments and other gear for his departure to the south, including the zenith sector, which in Ellicott's words, "was indispensably necessary," when Pryor called upon him and inquired whether it was his intention to take along the large sector.

As Ellicott later reported the meeting in a letter to Albert Gallatin, Secretary of State, Pryor ". . . was told we could not do without it to which he replied, ‘you shall not take my object glass.' I then referred him to M.r Pickering [then Secretary of State], who offered him any reasonable price for it but it was to no purpose. - he returned to me, and demanded the glass, which I refused to give up, he then prepared to attach the Sector:- being averse to the opposition of law, and knowing that the object glass was really the property of M.r Pryor I called upon M.r Pickering and proposed offering my Telescope to M.r Pryor in exchange for his object glass, provided my Telescope should be replaced by one of equal value at the expense of the United States.- My Telescope was new and had never been unpacked but three times:- it was made agreeably to my own directions by the late M.r George Adams mathematical instrument [maker] to the present King of Great Britain.- M.r Pickering approved of the proposal, and M.r Pryor after a deliberation of two days acceeded to it."

Pennsylvania Paid More For Sector

Now lacking a telescope that he would require for the survey to determine the longitude of different points in the line, Ellicott applied to the American Philosophical Society and borrowed the Society's telescope until he could replace his own. The telescope imported as a replacement for Ellicott's instrument was made by W. & S. Jones, successors to George Adams, and was of the same size and magnification and worksmanship as that which he had surrendered to Pryor. It was procured for him by Rufus King, ambassador to the Court of St. James. Ellicott's letter to King with the instrument's description and price were submitted to Pickering and forwarded by him to London. Ellicott then noted, "From the time that Mr. Pryor received my Telescope, I conceived the United States to be entitled to the value of it in the Sector, and so I reported to the late Secretary of State Mr. Marshal [sic], in the month of September 1800, which I presume will be found in Mr. Madison's office … It was from this impression, that I left the Sector, with all the instruments belonging to the United States in the Continental Store then kept by Mr. Samuel Hodgden in the City of Philadelphia." He assumed that the remainder of the value belonged to the State of Pennsylvania. The contemporary evaluation indicated that Pennsylvania had an investment of £76 and 5 shillings, while the United States had an investment in the sector of £43 and 15 shillings.

Price of Instrument was $160.00

At some time after 1786 Rittenhouse produced another zenith sector, at the request of De Witt, which the Surveyor-General described many years later, in a letter to Stephen Van Rensselaer,

I was so pleased with the ingenious construction and extreme accuracy of the instrument [the Rittenhouse zenith sector] that I expressed a wish to have one made like it, but of a more portable size, if such a one could be procured, whereupon M.r Rittenhouse promised to furnish me one, which he afterwards did. It is called a 30 Inch sector, but it appears that the distance from the centre to the graduated arch is only a little over 28 Inches. I cannot conceive that an instrument of the kind can be more accurately executed. The division on the arch are points hardly visible to the naked eye. M.r Rittenhouse explained to me his method of making them. A method I suspect entirely peculiar to himself, and observed that he had been so fortunate as to obtain a superior achromatic object glass for my sector; such glasses I suspect were not used in Mason & Dixons time … . The price of the instrument was $160.00.

Meanwhile, in 1798 the entire contents of the State House including furniture, the Bird sector, and other instruments that had remained in storage there, were transferred to a temporary state capital in Lancaster, Pennsylvania. There the sector was displayed in the library until 1812, when a permanent capital was established in Harrisburg. Once again the sector was moved, and displayed in the old court house building. In 1878 the instrument was turned over to the Bureau of Internal Affairs and deposited in the State Library with other Mason and Dixon survey materials. Early in 1897 the sector was removed from exhibit and disassembled into its separate components in preparation for cleaning. Shortly thereafter, on February 2nd, a great conflagration swept through the building, destroying all contents and burning the structure to the ground. None of the sector's parts were recovered, nor are there any contemporary illustrations of it.

On Display at Smithsonian

The disposition of De Witt's sector is not known, and the only zenith sectors known to have survived are the two made by Rittenhouse for Ellicott. They remained in Ellicott's possession during his lifetime, and together with his other surveying instruments were subsequently inherited by descendants. In the late nineteenth century, they were deposited in the Smithsonian Institution; the deposit converted to a gift in recent times. They are on permanent display in the National Museum of American History.

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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