ARTICLE XIII. 



On the longitude of Washington, computed from the moon-culminations observed during the 

 years 1839 — 1842 inclusive, by Lieut. J. M. Gilliss. Read April 20th, 1849. 



The observations at Washington which form the basis of the subjoined computations, 

 were made at the naval observatory by order of the honourable Secretary of the Navy, 

 and were intended for the benefit of the exploring expedition. Having been printed at 

 length by the U. S. Senate in 1846, and very generally distributed, it will scarcely be con- 

 sidered necessary to reprint any portion of them in this place. 



It may, however, be proper to state: 



The observatory was situated nearly twelve hundred feet N. 5° W. from the centre of 



the capitol-dome, and its approximate geographical position as follows: — Latitude by 



twelve meridian altitudes of north and south stars, observed with circles of ten inches 



diameter. 



38° 53' 33" North. 



Longitude from the solar eclipse of September, 1838; eight corresponding immersions, 

 and thirteen corresponding emersions of stars with other observatories, and two vanishings 

 of meteors observed also at Philadelphia, 



5" 08"' 04' .6 West. 



The observations were made with a five feet transit instrument, having an object glass 

 of 3.75 inches aperture, constructed under the supervision of Mr. Ilassler, for the Coast 

 Survey, by Troughton, and by the former gentleman very kindly loaned to the Navy 

 Department. It was very substantially mounted on granite piers. There was but one 

 observer during the whole period, and each object was noted over five wires almost with- 

 out exception; variations from the rule occurring only when the moon and stars followed 

 each other too closely. 



The differences of longitude have been calculated for the use of the U.S. Coast Survey, 

 under the direction of Prof. A. D. Bacbe, by whose permission the principal dala have 

 been transcribed from the volumes which they fill. The formula adopted is that detailed 



