[VI 



INTRODUCTION TO THE 



Stars with which Mars was compared in 1851- 52. 



Column 13 contains the initials of the observers names : Gr Grilliss ; M MacRae ; P 

 Phelps. 



The transcripts and reductions were made by Lieut. R. N. Stembel, U, S. N., by whom, 

 also, the proof sheets were collated with the original records. 



Horizontal and vertical diameters deduced from the observations and compared with the Nautical 



Almanac tables, pages 333-341. 



The horizontal diameter of Mars, from observations with the equatorial, is found by obtaining 

 the mean interval between the transit of each of the limbs and the comparing star for the night, 

 together with the mean of the times of transit of the limbs, and applying to the arc equivalent 

 of the difference of the former the variation of the planet in right ascension during the period 

 embraced by the latter. As the observations were made only near the opposition of Mars, no 

 correction has been applied for defective illumination of either limb. 



In the same manner the vertical diameters are deduced by comparison of the differential 

 declinations of the planet s limbs and star, increased or diminished by the variation of declina 

 tion in the interval between the means of the times at which the measures were made. To the 

 result, a further correction is applied for thickness of the micrometer wire. 



The mean apparent error of the Nautical Almanac, resulting from a comparison of each night s 

 observation with its tabulated diameters, is : 



