I 



* EEPORT ON THE COMPLETION OF 



the observatory, served to render the small room in which it was placed suflB- 

 ciently comfortable for a computing-room. 



The Instruments. — The declination and bifilar magnetometers were made by 

 Mr Grubb of Dublin. The magnets during 1841 and 1842 were enclosed by 

 loosely fitting cylinders, resting upon the pillar tops, covered by glazed lids ; but 

 the junctions were neither pressed together by any contrivance, nor was there 

 any moderate precaution taken to prevent currents of air from circulating in the 

 boxes. In 1843 and 1844, these and various other defects were removed : the 

 joints were lined with velvet and pressed firmly together ; rectangular boxes 

 were placed within the drums ; both cylindrical and rectangular boxes were gilt 

 outside and inside ; and the whole apparatuses were covered by thick cotton 

 hoods. The gilding prevented the effects of radiation in producing currents 

 within the boxes ; the other precautions prevented the entrance of external cur- 

 rents, and secured to a considerable extent a uniform temperature and dryness. 



The suspension threads for the declination magnet broke at short intervals 

 in 1841 and 1842 ; they were succeeded in June 1843 by a thread carefully pre- 

 pared, which is still in use, and apparently as good as ever. In case, however, 

 the suspension thread should have received any torsion, the magnet was removed 

 occasionally, a brass bar was suspended, and the torsion was rendered zero for the 

 magnetic meridian before replacing the magnet ; when little torsion was found A 

 existing the trials were made seldomer, as the mere manipulation was apt to in- 

 troduce torsion of itself. 



Errors due to twist in the suspension wires of the bifilar magnet, were 

 eliminated by the process employed to determine the unit-coefiicient. The 

 interval of the wires was adjusted according to the instructions by the Committee 

 of the Royal Society ; but this rendered the instrument too sensitive, and, during 

 moderate disturbances, the scale went out of the field of the telescope. This was 

 attempted to be remedied at the time by turning the torsion circle till the scale 

 reappeared, generally a difficult and unsatisfactory operation; in 1846, however, 

 I employed a small magnet for the purpose, this being placed at known distances 

 where its effect upon the bifilar magnet was determined. 



Even for the ordinary diurnal movements, however, the instrument was too 

 sensitive ; the reading of the magnet scale was estimated to one-tenth of a division, 

 and of the thermometer to one-tenth of a degree, but a variation of one-tenth of a 

 degree in the temperature of the magnet is equivalent to two-tenths of a scale divi- 

 sion in its motion. The thermometer was placed with its bulb resting on a brass 

 bar, so that it might show as nearly as possible the temperature of the magnet. 



The balance magnetometer was made by the late Mr Robinson of London ; 

 the box was gilt inside and outside, and covered with cloth as in the case of the 

 other instruments. The magnet was never lifted from its support, excepting 



