6 



INCLINATION AND DECLINATION 



- A theodolite was set up, and the telescope directed to the declination 

 instrument, which was placed as nearly as could be estimated, so that its 

 plane should be at right angles to the axis of the telescope. The teles- 

 cope being then made to pass through the 0" and the 180° divisions of 

 the exterior limb was found to form an angle with the line of north and 

 south in the compass-box, the wire of the telescope passing to the west 

 of the north end, and to the east of the south end. 



If! It was not easy so to adjust the two instruments that the line 

 described by the cross wires should exactly fall on both 0° and 180". It 

 was, in fact, found that more satisfactory results were obtained by bring- 

 ing them on the upper point or 0° by means of the tangent screw, and then 

 estimating the deviation on each of the other three points. As the 

 telescope has a considerable magnifying power, and as the declination 

 circle was within five feet, such an estimate it was found could be perform- 

 ed with tolerable accuracy. To make this clearer, I shall give the detail of 

 one observation : 



Cross wires of telescope, on 0° . 0' of outer circle, 



passes to West of North, . . .30 inner circle. 



East of South, . . .12 inner circle. 



East of 180° . . . . .10 outer circle. 



Here then it is evident, that the cross wires of the telescope, descxibed 



a line forming an angle pf ^ + — 5/^ with the line joining 0° and 180* 



on the outer circle. While it formed an angle of — 21', with the 



line of north and south in the compass-box. Consequently, the latter 

 must have formed an angle with the former of 16', and by that quantity 

 must the declinations determined by this instrument be erroneous. A 

 second observation gave 15J, a third 13^, a fourth 12J, — mean 14' 4". 



