68 EIGHTH REPORT — 1838. 



We have in this table the dip observed at twenty-nine sta- 

 tions, of which the central geographical position is 52° 45' N. 

 and 2° 49' W. If we desire to express the general result of 

 this series of observations, as to the position of the isoclinal 

 lines, their mean direction, and their mean distance apart in 

 the district of country which the observations comprise, in the 

 manner proposed by Mr. Lloyd in the discussion of the Irish 

 Magnetic lines (British Association Reports, vol. iv. pages 

 151 — 156);— and if we call B the dip at the central posi- 

 tion ; u the angle which the isoclinal line, passing through the 

 central position, makes with the meridian ; r a co-efficient de- 

 termining the rate of increase of the dip in the normal direction ; 

 a and h co-ordinates of distance in longitude and latitude of 

 the several stations from the central position, expressed in geo- 

 graphical miles : and if we make r cos «=.r, and r sin ic = i/ -, — 

 we may proceed to form equations of condition of the form de- 

 scribed in the report on the magnetical observations in Scot- 

 land (British Association Reports, vol. v. pages 4 and 5), and 

 to combine them by the method of least squares. It is unne- 

 cessary to encumber this report with the details of calculation ; 

 and it is sufficient to state, that from the three final equations 

 we obtain .r=+ -2633; ?/=--5154; ?<= — 62"'41 (the direc- 

 tion being from N. 62° '41 E. to S. 62° '41 W.) ; ?- = 0'-580, 

 being the rate of increase of dip in each geographical mile mea- 

 sured in the direction perpendicular to the isoclinal line ; and 

 g = 70° '22*9 the dip at the central position at the mean epoch 

 of the observations, namely, January 1, 1837. 



Mr. Lloyd's Observations. — These observations were made 

 with a 4^ inch circle by Robinson, and two needles, designated 

 as L 3 and L 4, employed also for determinations of the inten- 

 sity. These needles consequently had not their poles reversed ; 

 and the dips observed with them require corrections to produce 

 the true dip. These corrections have been ascertained by Mr. 

 Lloyd, as stated in a subsequent part of this Report, to be as 

 follows : 



Needle L 3. + 5^-3 



Needle L 4. +13''4 



These corrections have been applied in the following table^ in 

 the column entitled Corrected Dip. 



