LATITUDE AND LONGITUDE OF CAMPOS RODRIGUES. 395 



Note. — Commander Gago Coutinho mentions that these ob- 

 servations are not the most precise to be found amongst the de- 

 terminations of Latitude efifected by him on 25 vertices of trian- 

 gulation. 



In my observations I employed the Horrebow-Talcott 

 method, availing myself of the transit instrument of the Ob- 

 servatory, which possesses two very perfect levels for this pur- 

 pose, and a revolving micrometer to serve in Right Ascension or 

 Declination. This instrument, which was made by A'lessrs. Bam- 

 berg, of Berlin, bears the number 2 :836 ; the telescope has an 

 opening of 70 millimetres and 65 centimetres of focal distance, 

 and possesses oculars which magnify from 44 to 86 times. 



All the stars observed belong to the " same system," namely, 

 that of the "Preliminary General Catalogue of 6,183 Stars for 

 the Epoch 1900," by Lewis Boss. 



Li the reduction for the year, the data supplied in this cata- 

 logue were used, and for. the day the data of the Berliner Jahr- 

 huch. 



The observing room was opened with due precaution, in 

 order to avoid as much as possible rapid variations of tempera- 

 ture. 



The stars were bisected with the movable thread at sym- 

 metrical points of the field of view on each side of the middle 

 thread, in order to eliminate the defect of inclination of the 

 movable thread, which could not be determined all at once 

 through the observations of the stars. The reticle of this in- 

 strument has 15 threads placed in three grou]>s ; I tried to make 

 the observation at half distance between the threads I — II and, 

 symmetrically, XIV — XV, and in the same way IV — V and 

 XI — XII, VI — VII and IX — ^X, neglecting any observations 

 which had not been made symmetrically. 



In the reduction to the meridian, a table in parts of micro- 

 meter was employed and calculated according to the following 

 formula : 



i (15^ sin 1" i^ tan S) x ^--1-- 



o". 78897 being the value of one part of the micrometer in arc. 

 Of course, I took as value for / the mean of the distances of the 

 two threads between which the observation to the middle thread 

 was being made. The two levels used in this observation have 

 the following values for one part : 



" A " Level i}:) = i".3634 



" B " Level ijo = i".i6io 



The table is calculated to reckon with the total of the differ- 

 ences obtained by reading of the four extremes of the two levels : 



3^g (A + B)=o".i5796 



