ON THE MAGNETIC SURVEY OF ENGLAND. 



Table II. 



257 



Long. =/i 



18 W. 



33 E. 

 45 W. 

 28 W. 

 14 W. 



30 w. 

 32 w. 



iiW. 



07 w. 



iiW. 



18 w. 



19 w. 

 sow. 



Stations. 



Jardine Hall ... 

 Scarborough ... 



Cambridge 



Llandovery 



Stackj3ole Court 



Cawston 



Margate 



Folkestone 



Cleethorpc 



Lowestoft 



Cawston 



Cromer 



Kew 



Lat. = X. 



55 10 



54 17 



52 13 

 52 01 



51 38 



52 47 

 51 23 



51 05 



53 32 



52 30 



52 47 

 52 56 



SI 29 



l.ong.=/t 



24 W. 

 23 W. 

 06 E. 

 45 W. 

 55 W. 

 12 E. 

 23 E. 

 10 E. 

 00 



45 E. 

 12 E. 



17 E. 



18 W. 



Dip = 9. 



70 

 69 

 6S 

 69 

 68 

 68 

 68 



67 

 69 

 68 

 68 

 68 

 68 



43'4 

 58-0 



42-7 



I 1*2 



59"9 

 55*3 

 077 

 51-0 



33*4 

 42-5 

 56-1 

 59'3 



21'2 



Mean epoch ist January, i860. Mean latitude, 52° 2o'=X,. Mean longitude 1° 41' W. 

 Mean dip at the central station 68^ 59''2 = 0i. 



= /^i. 



The stations and dips contained in tlio preceding Table require to be 

 combined according to the method described in the 'British Association 

 Report' for 1838, p. 68 (and adopted in the British Magnetic Survey for 

 1837), in order to determine («) the angle which the isoclinal lines in England 

 make with the meridian, and (r) the distance between them corresponding 

 to differences of 1° of dip measured on the normal or perpendicular to tlic 

 direction of the isoclinal lines themselves. Thus, if Ave make a and b co- 

 ordinates of distance, in geographical miles, of the several stations in lon- 

 gitude and latitude from the central position, and if we put rcosM=a?, and 

 rsinw^y, we have from Table II. 26 equations of condition of the form 



Q—d^=ax-\-by ; 



combining these by the method of least squares, we find 



a;= 4-0-1993; y=-t-0-5911; m=-71°22'; and r=0'-624. 



The most probable dip at each station will therefore be given by the 

 formula 



0= -1-68° 59''2-f0-1993a+0-591 1 b, 



a and b being the distances in longitude and latitude, expressed in geogra- 

 phical miles, from the central position in 1°41' W. longitude, and 52° 20' N. 

 latitude. 



Table III. contains in columns 2 and 3 the values of the coordinates 

 a and b for the stations named in column 1 ; in columns 4; and 5 are 

 placed the values of (0 — 0j), in column 4 as observed, and in column 5 as 

 calculated ; in columns 6 and 7 the dips at each station are shown, viz. the 

 observed dips in column 6 and the calculated dips in column 7 ; and in the 

 final column the differences are stated between the observed and calculated 

 dips. From these differences we obtain +3''85 as the probable error of the 

 observed dip at a single station in this survey. This small amount of pro- 

 bable error will doubtless contrast favourably with the results in countries 

 where igneous rocks are of more frequent occurrence than they are in 

 England ; it includes both station anomalies and the effects of magnetic dis- 

 turbances, as well as observational and instrumental errors. 



1861. s 



