MAGNETIC SURVEY OF GREAT BRITAIN. 



53 



Table I. 

 Trials of the Axles of the under-mentioned Dipping Needles. 



The observations having been made in a house, the dip ob- 

 served is not the true dip in London. This is immaterial, as the 

 object of the experiment was solely the agreement or otherwise 

 of the results in the diffei'ent positions of the axle. 



Had the axles been perfect, the same dip should of course 

 have been given in all positions of the axle : we perceive, how- 

 ever, that the differences in the one needle amount to above 40', 

 and in the other from 7' to 11'. The results of these experiments 

 fully impressed Mr. Robinson with the necessity of employing 

 more effectual means for ensuring a true figvu-e to the axles of 

 dipping needles ; and in several which he has since made, and 

 which have been carefully examined, he has proved successful. 



Having exhibited the discrepancies of the earlier needles, it 

 may be satisfactory to show the improvement in some of the later 

 ones ; and for that purpose the following observations are given 

 with needles which were afterwards employed in the general ob- 

 servations of this report. The axles of these needles, being made 



