[Illustration: Figure 6.--Astronomical clock made by David Rittenhouse for his observatory at Norristown, Pa., and used by him for the observation of the transit of Venus in 1769. Unembellished pine case 83-1/2 in. high, 13-1/4 in. wide at the waist with a silvered brass dial 10-5/8 in. diameter. Photo courtesy the American Philosophical Society.] [Illustration: Figure 7.--Orrery built by David Rittenhouse for the University of Pennsylvania. The center section shows the motions of the planets and their satellites and the right-hand section the eclipses of the Sun and Moon. The case, considered to be an outstanding example of colonial cabinet-work, was made by John Folwell.] [Illustration: Figure 8.--Brass surveying compass inscribed "Made by Benjamin Rittenhouse, 1787." Photo courtesy Ohio State Museum, Columbus, Ohio.] [Illustration: Figure 9.--Portrait of Andrew Ellicott (1754-1820) by unknown artist.] Andrew Ellicott A name closely associated with that of the Rittenhouse brothers was that of Andrew Ellicott (1754-1820) of Solebury, Pennsylvania, and Ellicotts Mills, Maryland. Andrew was the son of Joseph Ellicott, the clockmaker and pioneer industrialist who founded Ellicotts Mills. Although a Quaker, Andrew (fig. 9) served in the Revolution, and he became one of the most distinguished engineers of the new republic. He worked as a clockmaker and instrument maker from 1774 to 1780. In 1784 he ran the boundary between Virginia and Pennsylvania and in the following year he was a member of the survey that continued Mason and Dixon's line. In 1785 and 1786 he served on the Pennsylvania commissions that surveyed the western and northern boundaries of the state, and in 1789 he served on the commission that fixed the boundary between New York and Pennsylvania. Between 1791 and 1793 he surveyed the site of the city of Washington, D.C., and redrew L'Enfant's plan for the city. In early 1793 Ellicott was appointed commissioner by the Commonwealth of Pennsylvania for the project of viewing and locating a road from Reading to Presque Isle, now Erie. It was an extremely difficult undertaking, but Ellicott completed the work by the autumn of 1796, including laying out the towns of Erie, Warren, and Franklin. In May 1796 Ellicott was commissioned by President Washington to survey and mark the boundary line between the United States and the Spanish Province of Florida in accordance with the provisions of the Pinkney-Godoy Treaty of October 27, 1795. This line was to begin at the point where the 31st parallel of north latitude intersected the Mississippi River, and to proceed thence along that parallel eastward to the Appalachicola River for about 400 miles. In 1801 Ellicott was offered the position of surveyor general of the United States by President Jefferson. Ellicott declined, but subsequently accepted the secretaryship of the land office of Pennsylvania, a post he held until 1808.