AT THE ROYAL OBSERVATORY, CAPE OF GOOD HOPE. 467 



The excentric co-ordinates of the south polar pivot, x= 11". 7 towards the zenith, y=z 2". 3 

 towards the west. 



Declination circle reading for the instrument at polar point, 270 0' 17". 2. 



The hour circle verniers, on November 1, were supposed to read Oh. or I2h. exactly, when 

 the telescope was in the meridian east or west of the pier. However, on the 29th of December, 

 they appeared to show the hour angles 7s. too small; or telescope west, 6s., excluding the colli- 

 mation error. It is not likely that the cradle bearing would change its position, or the pier 

 sink towards the west. Both verniers gave the same reading. There is no right ascension 

 wire other than the parallel wires placed perpendicular to the meridian by turning the position 

 verniers through 90. But this is of little consequence ; for all practical astronomers know 

 that the bisection of an object at a given instant is merely a matter of'chance. For this reason 

 the hour circle readings were held to represent the hour angles. The object was bisected or 

 touched on entering the field, and continued to be corrected as to bisection or contact until it 

 reached the centre of the field. The clock times noted in general for Mars are considered in 

 no other light than checks on the hour angles ; but they will serve also for investigating if any 

 change took place in the position of the verniers, or whether it was constant from the 21st of 

 November to the 17th of January, bearing in mind that they are liable to a fluctuation of 2s. 

 or 3s. with respect to the meridian passing through the centre of the field. 



PARTICULARS RELATING TO THE OBSERVATIONS. 



The telescope was usually on the west side of the pier, and the polar axis always clamped 

 during each comparison. On 35 of the nights, the comparisons were made as follows: 



The declination axis being clamped at a convenient reading, viz: at the reading which corre- 

 sponded with the middle in declination between the objects, on the preceding object entering 

 the field it was observed with the micrometer wire. If there was a sufficient interval in right 

 ascension, the hour circle was next read. The micrometer wire was then slowly driven to the 

 parallel of the following object, which, on entering the field, was observed with the micrometer 

 wire. If the hour circle had not been read between the observations, it was now read off. The 

 polar axis was then undamped, and the telescope moved in advance for another comparison ; 

 and so on. 



On December 7, 8, 16 (2d series,) 17, 18, 20, 21, 22, the declination axis was not clamped. 

 On the preceding object entering the field, the zero wire (usually at or near the 30th revolution) 

 was made to bisect or to touch the object by gently tapping the telescope, (the declination tan- 

 gent screw is not sufficiently delicate for this purpose,) and the micrometer wire was placed on 

 the following object. The reading of the hour circle was made according to the circumstances 

 before stated. When one-half of the comparisons contemplated were thus made, the position 

 circle verniers were turned 180, and "the micrometer wire brought to the opposite side of the 

 zero wire, in which position the remaining half of the comparisons were made as before. Re- 

 versing between each comparison would have absorbed too much time ; the verniers are incon- 

 veniently placed, and the divisions minute. By this proceeding, the coincidence of the wires' 

 reading is not necessary when the observations in reversed positions are combined. Much 

 delicacy is required in moving the micrometer when the declination circle is not clamped, and, 

 indeed, when it is clamped ; for there is considerable play. On December 6, the verniers were 

 not reversed. For that occasion the coincidence of the wires is given. 



On December 24, * H. C. 10669 could not be observed until it escaped from the limb. It 

 was then compared witli the planet's centre, using the driving clock as in measuring a double 

 star. I suspect the declination of H. C. 11946 (comparison star Nov. 25, 26) is erroneous. 

 The two stars nearest to the ephemeris position were observed with the planet. But Mr. Childe 

 has observed a star which agrees in right ascension with the ephemeris. This led me to 

 examine with the equatorial on Monday night, when I was again unsuccessful. Last night 



