SANTIAGO OBSERVATIONS. Ku 



work follow each night's observations, and thea the results derivable prior to a final discussion 

 of the series. These latter embrace the means of all the observations of each limb of the planet 

 combined in groups according to the periods of time through which they extend ; the chrono- 

 meter corrections and true siderial times of observation at Santiago, and, finally, the diameters 

 deduced from the transits and rnicrometrical measures. In the latter, the variation of the 

 i ., f Declination, ) 



/ Right ascension \ Curing tne interval between the means of the times of observation 



of the two limbs has been computed from the " variation of \ g . t ascension > I j n one hour 



( Declination, ) 

 of longitude" given in the Nautical Almanac. 



Finally, to facilitate comparison with corresponding observations at other stations, the differ- 

 ential refraction (A^) for each of the means of the times has been computed from the formulae in 

 Vol. I, Bessel's Untersuchungen. 



Tan N = Cos r cot <f> p Latitude, 



Sin M = Sin r cos <p r = Hour angle, 



Cos Z = Cos M sin (N+<J ) <J o Declination, 



Z = Zenith distance ; 



and to this end, special tables applicable to each series were calculated, in which the intervals 

 of the arguments r and <J were respectively 10 and 10'. 



All the observations were made by myself, except those duly credited to Lieutenant MacRae, 

 between the 12th and 17th of December, 1849. The most usual magnifying pow.er was 150. 



The transcripts and reduction of the Mars series, 1849-'oO, and from February 28 to the 

 close of the series, in 1852, have been made by Lieutenant T. H. Patterson, U. S. N., from the 

 commencement to February 27, 1852, of the same planet, by Lieutenant Francis Winslow, U. 

 S. N. All the observations of Venus have been prepared for publication by Assistant Daniel G. 

 Major. The computations of Messrs Patterson and Winslow have been revised by Lieutenant 

 R. N. Stembel, U. S. N., and Assistant Major, and, to insure greater accuracy in this publica- 

 tion, the proof sheets have been compared with the original observations. 



THE MERIDIAN CIRCLE AND ITS ADJUSTMENTS. 



I 



The observations at length, and a detailed description of the instrument with which they were 

 made, are given in Vol. IV. In pages 309-332 of the present volume there will be found only the 

 means of the times of transit over the wires, and the means of the microscope readings, together 

 with the corrections which have been applied for reduction of the observations to apparent right 

 ascensions and declinations. 



The instrument consists of two minutely divided circles reading to half seconds of arc by four 

 micrometer microscopes supported by horizontal bearers enclosed in wood, and a telescope of 

 51 lines aperture, 73 inches focal length, and transverse axis 39 inches between the bearing 

 surfaces of its steel pivots. 



The eye end of the telescope contains one horizontal and seven vertical stationary wires, and 

 two micrometer systems. One screw moves a single vertical wire, the other, seven horizontal 

 wires. A revolution of the latter screw equals 27". 93. The intervals of the systems (vertical and 

 horizontal) rather exceeds four minutes of arc, and the eye-piece may be moved in its dove- 

 tailed slides by racks and pinions until perpendicular to any desired wire. In addition to the 

 usual suite of magnifying lenses, a colliraating eye-piece accompanied the telescope. 



For horizontal adjustment of the axis, there is a hanging level, whose glass tube, enclosed 

 within a cylinder of brass covered with cloth, is read through a glazed aperture in the upper 

 portion of the cylinder. One division of the tube equals I'M; the level hung within the 

 bearing points of the pivots. 



