120 KIGHTH REPORT 1838. 



Collecting in one view the values of ii ajid r at the central 

 geographical positions in England, Scotland and Ireland, as 

 they have been derived from the several series in each country, 

 we have as follows : 



England, Lat. 52° 38'. Long. 2° 07'; ^<=-65°05'; r=0-S75' 

 Scotland, — 56° 49'. — 3° 39'; m=— 56°06'; r = 0-549' 

 Ireland, — 53° 21'. — 8° 00' ; z<=-60°32'; r=0-594' 



Whence it appears that the isoclinal lines do not intersect the 

 geographical meridian at the same angle in the three countries; 

 that they form a greater angle with the meridians in England 

 than in either of the other two countries ; and that the angle 

 is also greater in Ireland than in Scotland. 



It also appears that the distance between the lines is greatest 

 in Scotland, less in England, and least in Ireland ; the number 

 of geographical miles, measured on the perpendicular, corres- 

 ponding to differences of a degree of dip, — being 



109-2 in Scotland; 

 104*4 in England; 

 101-0 in Ireland. 



It follows, from the different values of r, that the assumption, 

 upon which we have hitherto proceeded in these combinations, 

 of parallelism of the lines and their equidistance apart, does not 

 hold good when applied to an area of the extent of the British 

 islands, and not strictly so for any of its three portions ; and 

 that it is desirable to find a method of more exactly represent- 

 ing the observations, by tracing each isoclinal line separately 

 from observations nearly of its own value, and consequently but 

 little removed from it in geographical distance. If we have 

 the approximate values of u and r at any station where the dip 

 has been observed, we may readily compute the latitude and 

 longitude of a point furnished by that observation for the po,- 

 sition of the next adjacent isoclinal line. If the isoclinal lines 

 sought are those of complete degrees {i.e. the lines of 69° 00', 

 70° 00', 71° 00', &c.), and if the observation be also without 

 fractional minutes — say, for example, 69° 00' — the point fur- 

 nished by that observation for the line of 69° 00' is at the 

 station itself. If the observation exceeds or falls short of 

 69° 00' by a few minutes, the point furnished by it for the 

 isoclinal line must be distant from the station a geographical 

 space, equivalent to the value in distance of the fractional mi- 

 nutes, as computed by the value of r, and in the direction of 

 u + 90^. Thus, if D be the degree of dip represented by the iso- 



