168 



REPORT — 1859. 

 Table I. 



In this Table the resulting dips are calculated by the formula cot 2 8 = 

 cot 2 i+ cot 2 i', where 8 is the true dip, and i and i' the positions of the needle 

 in azimuths 90° apart. 



These results are satisfactory, and show that any errors due to the axle or 

 to local magnetism in the circle are inappreciable throughout the range of 

 observation : otherwise we should have had greater differences in the result- 

 ing dips. Now the portion of the circle so tested comprehends that used 

 during the magnetic survey; we may therefore with safety suppose the circle 

 to be free from magnetism and error of axle as far as the results of the 

 survey are concerned. It will also be observed that the dips given by both 

 needles are very nearly the same ; and although this amount of agreement 

 did not always hold throughout the survey, yet the average difference be- 

 tween the needles is exceedingly small. It has therefore been thought 

 unnecessary to apply any correction in the case of those stations (very few 

 in number) where only one needle was observed. 



The following Table exhibits the results of a comparison made at Kew 

 Observatory between the Survey dip-circle (No. 23) and other reliable in- 

 struments, in March 1859: — 



Table II. 



