WASHINGTON OBSERVATIONS. 347 



Rev. 

 From November 1, 1850, tillJnnuarj' 10, 1851 . . . 12 = 30.319 



2 3 = 29.870 

 1 -3 = 60.089 



From January 16, 1851, (111 March 1,1851 12 = 30.179 



3 3 = 29.936 

 1 3 = 60.115 



After December 18, 1851, the wires were unchanged, having the following intervals: 



Rev. 



1 5 = 60.149 ' 

 14 = 42.997 

 1 3 = 30.091 

 12 = 17.113' 



In observing, the inner side of the wire was brought into contact with the limb of the planet, 

 so that the measured diameters are too great by the thickness of the wire. This thickness may 

 be taken for all the wires at 0".432 ; a value determined by the interval between the centres of 

 two wires brought to touch each other, using the highest magnifying power of the telescope. 



At the commencement of the observations of Mars in 1849, the declination wires were 

 brought into contact with both limbs of the planet at each comparison ; and that a similar result 

 might be obtained from the transits, the preceding limb was observed on A and 0, and the 

 following on B. This practice was discontinued in the subsequent observations. 



The power of the eye-piece used, unless where otherwise noted, was 120. 



The headings of the different columns in the record of observations sufficiently explain their 

 contents. In reducing the observations, the mean differences of right ascension and declination 

 have been taken as the true differences of these quantities at the mean of the chronometer 

 times. For this epoch, the correction to the chronometer and the correction for differential 

 refraction (//?) have been given. In the final discussion of the observations, it will doubtless 

 be necessary to select from the comparisons, and consequently to adopt other means than those 

 used in this preliminary reduction. The corrections to the observations of Venus, necessary on 

 account of defective illumination, have not been computed. 



Very respectfully, your obedient servant, 



J. FERGUSON. 



MATTHEW F. MAURT, LL. D., 



Lieut. U. S. N. , and Superintendent of the Naval Observatory. 



