260 REPORT— 1861. 



northern and southern parts of England which is implied thereby may be 

 similarly shown, by comparing the dips in 1837 and 1860 at two geographical 

 positions, one iu the extreme north, and the other in the extreme soutli of 

 England. Taking as the northern station the intersection in the map of 

 January 1837 of the isoclinal line of 71° 30' with the parallel of 55" N. 

 latitude, which takes place in the longitude of 3° 00' W., — and for the southern 

 station the intersection of the isoclinal line of 69° in the same map with the 

 parallel of 51°. which takes place in 0° 07' East longitude, — and comparing 

 these values with the values computed for January 1860 by the formula cor- 

 responding to that epoch, we have — 



North Geog. Position. South Geog. Position. 



Lat. 55^ Long. 3° OO'W. Lat. 5P. Long. 0° 07' E. 

 Dip in the map corresponding to 1 ,^,o oa'-o «no aa'-n 



January 1,1837 J ' ^^ ^ ^-^ "" " 



Dip on January 1, 1860, computed 1 



by the formula I 70° 42'-8 67° 58'4 



0=6S°59'-2 + O'-1993a + O''5911«> 



Secular change in 23 years 47'*2 1° 0l''6 



Annualchange 2'-05 2'-68 



The comparison shows that the mean annual secular change in the interval 

 of 23 years between the surveys was 2''05 on the northern border of England, 

 and 2'-68 at a station on the south coast. Tlius it is seen that the annual 

 rate of decrease of dip has varied in different parts of England in proceeding 

 from east to west, from 2'-36 at Lowestoft to 3'-09 at the Land's End ; and 

 in proceeding from north to south from 2'*05 at a position in 55° to 2'-68 at 

 a position in 51° N. latitude. 



In viewing the map in which the isoclinal lines for 1837 and 1860 are 

 represented in comparison with each other (PI. VIII.), it is seen that there 

 are three points where the amount of secular change in the interval must have 

 been the same, viz. the three points where the lines of 68°, 69°, and 70° in 

 1837 intersect respectively with those of 69°, 70°, and 71° in 1860; since at each 

 of these points themean annual change mi:st have been (60' -r 23 years =)2'*62. 

 These three points are seen to be in a curved line which crosses England 

 from the vicinity of Folkestone to the Irish Channel, and would impinge 

 upon the east coast of Ireland a few miles north of Dublin. Kew, also, where 

 the mean annual decrease of the dip in the same interval has been 2''63, is as 

 nearly as may be on the same line. At all stations north and east of the 

 line the mean annual secular change in the 23 years has been less than 2'-62, 

 and at all stations south and west of the line greater than 2'-62. In a pre- 

 ceding page we have the mean annual change at four stations (Lowestoft, 

 Margate, Lew Trenchard, and Land's End) situated at points on the east' 

 and west sides of England, and at two geographical positions {55° N. and 

 3° W., and 51° N. and 0° 07' E.) at north and south points. An intercom- 

 parison of the respective values of annual change at these six localities with 

 2'-62, and of their geographical distances from the aforesaid line of 2'-62 

 measured in every case on a perpendicular to that line, shows that an increase 

 of 0'"1 in the annular secular change for every 30 geographical miles towards 

 the N.E., and a decrease of 0'*1 for every 30 geographical miles towards the 

 S.W., will represent very approximately the observed values. We are thus 

 furnished with a scale by which the variation in the mean rate of the secular 

 decrease of the dip in different parts of England in the interval between the 

 two surveys may be approximately assigned; the limits being an annual 

 decrease of 3''1 at the Land's End, and of 2'-0 at Berwick. If we should per- 



