63 



EIGHTH REPORT 1838. 



Table VIII. contains observations by Captain James Ross, 

 with a six-inch circle by Robinson, and its -needles R. 4. and 

 R. 6., at Jordan Hill, in September 1838. The dip is here com- 

 puted by the formula, cot -B = cot - i + cot ^ i' ; and in the 

 linal column the dip observed in the ordinary manner, i. e. in 

 the azimuths and 180°, is inserted for comparison. 



Table VIII. 



Observations with Robinson's Needles R. 4. and R. 6., Jordan 



Hill, September 1838. 



Observer, Captain James C. Ross, 



Needle U. 4. 



Annual Alteration of the Dtp. 



The observations of dip included in this report, extend over 

 an interval of four years and upwards. To reduce these to a 

 common epoch, we require to know the amount of the change 

 which the dip undergoes from year to year. In the Reports on 

 the Magnetic Observations in Ireland and Scotland, an annual 

 decrease of three minutes was provisionally assumed ; but we 

 must now endeavour to assign the amount with somewhat 

 greater precision. 



In the 2Ist volume of the Annalen der Physik, M. Hansteen 

 has assembled all the most trustworthy observations of the dip 

 in London, Paris, Berlin, and Geneva during the present cen- 

 tury, and the latter part of the last ; and has computed from 

 them the most probable amount of the annual decrease of the 

 dip at each of those stations, corresponding to every tenth year, 

 from 1780 to 1830. As the results of this investigation have 

 not been published, I believe, in this country, I have subjoined 

 a table in which they are exhibited. 



