50 Capt. E. H. Hills and Mr. H. F. Newall. 



The adjustment of the axis of the coelostat was effected very 

 quickly by means of the attached declination theodolite. The level 

 attached to the telescope makes it possible to adjust in altitude 

 without any astronomical observation, for the latitude of the place 

 can be taken from the map with sufficient accuracy ; and setting the 

 telescope to the south declination equal to the co-latitude, and in the 

 meridian, the level should indicate horizontality. Index errors of the 

 circle and level are eliminated by reversal of the instrument. There 

 is a slight uncertainty attending the placing of the telescope in the 

 meridian, but this does not seriously affect the adjustment in altitude. 

 If a cross-level were made for the pivots of the telescope, this uncer- 

 tainty could be removed. 



To adjust in azimuth we must have an observation of the sun (or 

 a star) at a distance from the meridian. Observing his declination 

 (in reversed positions of the instrument and taking the mean), the 

 instrument must be moved in azimuth until this observed decli- 

 nation agrees with that given in the * Nautical Almanac.' A very few 

 trials, if the sun can be seen for half an hour, will soon indicate the 

 true azimuth without any calculations, within a minute or two of 

 arc, though if the instrument be moved much in azimuth the altitude 

 observation must be repeated. 



After the initial adjustment of the coelostat it was not disturbed, 

 but the adjustment was re-tested at intervals, with the following 

 results. The individual readings were only taken to minutes of arc, 

 but both limbs of the sun were observed in both positions of the 

 instrument, and the mean of the four set down : 



Coalostat. Observer H. 



Jan. 



From this general watch kept on the instrument ifc is clear that the 

 adjustments remained good within 1' or 2', which is more than suffi- 

 cient for the purpose. The only other adjustment required is the 

 setting of the face of the mirror parallel to the polar axis of the 

 instrument. This was easily effected by reversing the mirror in the 

 YS and observing if the sun's image in the two cases crossed the 

 same position on the ground glass when the mirror was moved in 

 right ascension, 



