[Illustration: Figure 79.--Surveying compass made by David Rittenhouse for Gen. George Washington, inscribed "Rittenhouse, Philadelphia." This instrument was used by Washington in making a complete survey of his estate at Mount Vernon, 1796-1799. The survey was assisted by Capt. Samuel Duval, surveyor of Frederick County, Maryland. Washington gave the instrument to Captain Duval, from whom it descended to the Hon. Anthony Kimmel, who donated it to the U.S. National Museum. USNM 92538.] 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