Deborah Warner, Curator, National Museum of American History
|Tintype of a surveyor and chainman with a vernier compass made by Levi Colton; photo taken around 1870. Cat. no. 1985.0181.01, National Museum of American History|
At the heart of a magnetic compass is a magnetized needle mounted on a pivot in such a way that one end points to the north magnetic pole of Earth, and the other points to the south magnetic pole. The form was known in China by 1111 and in Europe by 1187, and was later used by sailors who ventured far from home.
To make a compass suitable for use on land, a navigational compass was provided with two vertical sights. The form probably originated in England shortly before 1610, when the first description appeared in print. The earliest extant example was made in Ireland and dated 1667. Surveyors and scholars soon recognized, however, that compass readings could vary from place to place and even from one part of the day to another. The explanation for this phenomenon is that Earth's magnetic poles seldom coincide with the poles defined by Earth's axis of rotation--a phenomenon known variously as magnetic variation or deviation--and that the direction of the magnetic axis is not stable with respect to the polar axis.
Because land in England tended to be bounded, settled, and relatively valuable, English surveyors used relatively pricey instruments that measured angles between one landmark and another without reference to the magnetic needle. But in the colonies, both Irish and American, land was plentiful and cheap, so rough-and-ready magnetic compasses were good enough. But they would not be good enough for long. Samuel Moore of Connecticut spoke for many when he noted that imprecise surveys opened "a Door for endless Litigation."
|The earliest vernier compass in the Smithsonian collections is this experimental model marked “David Rittenhouse PHILADELPHIA” that must have been made between Rittenhouse’s move to the city in 1770 and his death in 1796. Cat. no. 1983.0498.01, National Museum of American History|
Americans eventually realized that, because of magnetic variation, a compass survey run one day might not correspond with a similar survey run another day. And so they decided that surveyors must determine variation before setting out for work each day, and that each plat must indicate the extent and direction of variation at the time the survey was conducted. This can be seen in the instructions prepared for New Jersey surveyors in 1746, in an act passed by the Virginia General Assembly in 1772, and in the plans for a federal land office that Thomas Jefferson compiled soon after the Revolution.
For this scheme to work, surveyors had to be able to adjust their compass for variation. A vernier (or variation) compass, which used a screw mechanism to rotate the compass box in relation to the line of the sights, allowed them to do just that. The term "vernier" here referred to the scale that indicated the extent of this rotation. The various components of this type of compass had long been known, but the whole came together when a community of men with similar interests addressed a need that was at once technical, economic, social, and political.
Popular legend names David Rittenhouse, the celebrated astronomer and instrument maker who succeeded Benjamin Franklin as president of the American Philosophical Society, as inventor of the vernier compass, but research reveals a more complex story. The earliest vernier compass in the Smithsonian collections is an experimental model marked "David Rittenhouse PHILADELPHIA" that must have been made between Rittenhouse's move to the city in 1770 and his death in 1796. The earliest dated vernier compass is marked "Benjamin Rittenhouse Fecit 1790," the signature being that of David's younger brother. The earliest advertisement comes from Lewis Michael, a German immigrant who worked in York, Pennsylvania, and who in 1787 offered "all kinds of compasses with or without a nonius to lay off the variation on an old line."
|Benjamin Rittenhouse made this compass in partnership with his nephew, Benjamin Evans, in Worcester Township, Pennsylvania, around 1798-1801. Cat. no. PH*309543, National Museum of American History|
Others must also have been involved in discussions of the vernier compass. One was Robert Patterson, an Irish-born mathematician who taught at the University of Pennsylvania; his explanation of a way of finding the meridian (or true north) by observations of the pole star was published by the American Philosophical Society in 1786. Another was Andrew Ellicott, a geodesist who had worked with David Rittenhouse designing instruments and running several state boundary lines. On his famous 1792 map of the City of Washington, Ellicott noted that he had drawn the meridian by astronomical observations, and "left nothing to the uncertainty of the Compass." In 1796, while Congress was debating the new land office bill, Ellicott published a pamphlet explaining his method for determining the meridian.
The vernier compass reached a stable form in the late 1790s, and was soon being used around the country. It would remain an essential surveying tool until the introduction of electronic instruments in the mid-20th century.
Visit the Museum's website for more on the experimental David Rittenhouse compass.
From Prototype, June 2011