38 REPORT ON 



the telescope, a special table was provided, which was mounted upon a tripod stand, 

 and which carried both the declinometer and theodolite in a fixed and invariable 

 position relatively to each other — the object-glass of the telescope being about 

 three inches from the south end of the magnet. 



Poclcet Chronometer, Fletcher, No. 906, was always used to note time. Its errors 

 have been already given in detail in Section III. 



General remarks on the method of using the instruments. When observations 

 were to be made the tripod stand was set up, and the table, having been placed 

 upon it, was approximately levelled by the eye, and set, by means of a pocket com- 

 pass, so that its longest side was nearly in the magnetic meridian, the end destined 

 to carry the declinometer being to the north. In packing the declinometer for 

 travelling, the glass suspension tube was never unscrewed from the magnet-box, 

 but when the collimator magnet was lifted from the stirrup a cylinder of wood of 

 the same size was at once substituted, and two pieces of wood, provided for the 

 purpose, were slipped in, one from each side of the magnet-box. These pieces of 

 wood completely filled up the box, and at the same time held the wooden cylinder 

 securely between them in such a manner that it could neither break the suspension 

 fibres, nor allow them to twist in the slightest. With this packing, after the 

 suspension fibres were once thoroughly freed from torsion, they remained so, . 

 and it was not necessary to examine them whenever the instrument was used, 

 but only at considerable intervals, thus saving much time in the field. The 

 brass carriers for the deflecting magnet having been screwed, one on each 

 end of the wooden bar, and the bar in its turn having been screwed to the 

 bottom of the magnet-box, the declinometer was placed upon the table in such 

 a position that its three levelling screws fitted into the cavities provided for 

 their reception. Then the packing blocks were taken out of the magnet-box, 

 and the wooden cylinder having been removed from the stirrup, the collimator 

 magnet was put in its place, and left free to assume its proper direction. The 

 magnet-box was next levelled. For that purpose the suspension fibres were 

 used as a plumb line, and the box was assumed to be level when they were seen to 

 hang in the axis of the suspension tube throughout its whole length. Finally, the 

 magnet was made to hang nearly level by moving it a little endwise in its stirrup; 

 its scale was placed horizontal, with the figures erect; it was shaded from the direct 

 rays of the sun by covering the glass top of the box; the mirror was screwed to the 

 back of the box and adjusted so as to illuminate the magnet scale properly; and a 

 thermometer was placed inside the magnet-box. The theodolite was next placed 

 in its proper position on the other end of the table and levelled; particular care 

 being taken that the horizontal axis of the telescope was truly level — especially if 

 the altitude of the sun was considerable. The telescope having been turned 

 towards the magnet and adjusted so as to obtain distinct vision of its scale, the 

 horizontal circle was firmly clamped in such a position that the vertical wire in the 

 field of the telescope cut the magnet scale as nearly as possible at the magnetic 

 axis. By means of the vertical circle the optical axis of the telescope was then 

 placed truly level, and the final adjustment of the magnet for horizontality was 



