Declinometee. xiii 



suspension thread : this box also enclosed a copper ring for checking the vibrations 

 of the magnet ; it was removed October 15, 1844, and is not shewn in the figure : 

 a cylindrical wooden drum i, together with two lids (not shewn in the figure), fitting 

 by pegs upon the cross-piece h, enclose the box and magnet. There are two glazed 

 apertures also in the wooden drum, opposite those in the rectangular box ; one to the 

 north, where a small mirror I throws light upon the glass scale ; the other to the 

 south, between the lens and reading telescope. All the joints of the boxes, including 

 those in contact with the marble slab, are covered with velvet, and both boxes are 

 pressed firmly against the marble slab by means of leaden weights, which were pre- 

 viously determined to have no effect upon the position of the magnet. In order to 

 destroy any effect of radiation, both boxes were covered with gilt paper, externally 

 and internally. The suspension apparatus is covered by a wooden cap (not shewn 

 in the figure). In order to prevent the variation of humidity within the boxes as 

 much as possible, the whole apparatus was covered, February 14, 1844, by a thick 

 double cotton hood, tied round the stone pillar, and having only small openings 

 at the glazed apertures of the boxes. The reading telescope is fixed to a stone 

 pillar ; the object glass is 8 feet to the magnetic south of the magnet lens. 



7. The pillar of the azimuth circle, used for determinations of the absolute 

 declination, is between the pillars of the magnetometer and its reading telescope. 

 This theodolite is by Troughton; the circle is 15 inches in diameter, is divided to 

 5 minutes, and is read to 5 seconds with three verniers. The lines of collimation of 

 the theodolite and reading telescopes coincide when the middle wire of the former 

 is seen, in either telescope, coinciding with the vertical wire of the latter. The 

 circle is retained in the same position on its pillar, but the transit telescope is re- 

 moved, excepting when required for observations of absolute declination. 



8. The following are the data used in reducing the observations of the decli- 

 nometer : — 



Values of the declinometer scale divisions in angular measure. 

 The adopted mean value of one division of the long scale of 500 divisions = 0'-6725* 



During the observations of absolute horizontal intensity, the magnet with the 

 long scale, usually in the declination box, was removed to the unifilar box in the 

 intensity house, and a magnet with a short scale was substituted. 



The adopted mean value of one division of the short scale of 300 divisions = 0'-7500* 



From the adopted values of the long and short scale divisions, the coefficient 

 for reducing the divisions of the short scale to the same value as the divisions of the 

 long scale = 1*115 ; the reciprocal = 0-897. 



* For details, see Introduction to the Observations for 1843, pp. xiii. and xiv. 

 MAG. AND MET. OBS. 1845 AND 1846. d 



