MAGNETIC SURVEY OF GREAT BRITAIN. 



68 



Table IX. 

 Annual Decrease of Dip. 



The differences whicli appear in the progression and rate of 

 the annual decrease at the four stations in this table, are proba- 

 bly attributable in far greater proportion to incidental errors in 

 the observations, than to the actual existence of such differences. 

 We may consequently regard the final column, or the mean of 

 the results at the four stations, as affording, in all probability, 

 a more satisfactory conclusion in regard to the rate of change at 

 any one of the stations than is drawn from the observations at 

 that station only. 



We may proceed to examine how far this rate of decrease cor- 

 responds with the most recent observations made in Britain. In 

 August 1821, I made a series of more than usually careful ob- 

 servations on the amount of the dip in the Regent's Park in 

 London ; employing for that purpose a needle on Mayer's 

 principle, with weights of different magnitudes to obviate the 

 liability to any constant instrumental error, and continuing the 

 observations during several days in order that the general re- 

 sult might approximate the more nearly to the true mean dip 

 at the period. These observations were published in the Phil. 

 Trans, for 1822, Art. I. ; their final result being a dip of 70° 

 02'''9, corresponding to the middle of the month of August 

 1821. To compare with this, we have the observations made 

 in London, at different times and in different localities, by 

 the contributors to this report. It is proper that we should 

 employ for the present purpose only those observations which 

 give entirely independent determinations ; viz. those only which 

 are complete in all the requisite positions of the needle and 

 circle, including the reversal of the poles, and which need no 

 correction for instrumental defects. Of such observations we 

 have those at Westbourne Green, already given in Table III. j 

 those in the Regent's Park, ccmtained in Table V. ; an obser- 

 vation by Mr. Fox, in May 1838, in a field west of Maiden 



