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. Ritten- 

 house 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 men- 

 tioned on the Delaware 20^4 miles and there corrected by the following 

 Zenith distances taken at its West termination by a most excellent sector con- 

 structed 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." 

 Giftof 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 — 



145 



