MAGNETIC SURVEY OF GREAT BRITAIN. 



57 



work in jeweled holes. By means of the " deflectors " which 

 make a part of Mr. Fox's apparatus, the dip may be deduced 

 from readings at various parts of the circle, and there is there- 

 fore the same opportunity of discovering errors caused by mag- 

 netism of the circle, or by imperfection in the bearings of the 

 axle, as the azimuthal and Mayer's methods furnish in needles 

 of the ordinaiy construction : the jewel-plate itself is also made 

 to revolve, so that the resting-places of the axle in the jewels 

 may be changed at pleasure. The performance of these needles 

 sufficiently indicates the great care bestowed on their workman- 

 ship. As the different observations in Table III. include an in- 

 terval of eighteen months, they have been rendered more strictly 

 comparable by the addition of a column, in which they are re- 

 duced to the common epoch of the 1st January, 1838, by ap- 

 plying a proportional part of the annual rate of decrease of the 

 dip in London at this time, which, from reasons that will be 

 assigned hereafter, is considered to be 2'*4. 



Table III. 



Observations of Dip at Westbourne Green in 1837 and 1838, 



with approved Needles. 



The subjoined tables, IV., V., VI., VII., VIII., exhibit in de- 

 tail the azimuthal examinations which have been made of some 

 of the instruments employed in the observations contained in this 

 report ; it has appeared the more desirable to give these tables, 

 because the practice of this method is new in this country. 



