326 
MR. A. W. RliCKER ARD DR. T. E. THORPE OH A MAGHETIC 
The local disturbance at any place may be estimated from tbe disturbances at 
neighbouring stations as given in Plates IX., X., and XII. It is evident that this 
correction must be somewhat uncertain, but the maps will at all events give informa¬ 
tion as to whether the disturbance is likely to be large or small. As an example of 
the use of the tables and maps, we calculate the Declination for lat. 52° 30' N. and 
long. 1° 30' W. on July 1, 1889. 
From Table VIII. it was 
18° 29'-0 + 8'-2 + 15'-2 = 18° 53'-4 on January 1, 1886. 
From Table XII. the secular change is 
- (6-5 -f -1 + -15) X 3-5 = — 23'-6 
which reduces the value to 18° 29'‘8. 
From Plate IX. we see that the station is in a region of negative disturbance, and 
that the true Declination is probably less than the calculated amount by 15' or 20'. 
It is obvious that for this purpose much rougher methods of calculation would 
suffice, but the main reason for making the process as accurate as possible is that the 
values of disturbing forces can only be determined if the rates of secular change are 
carefully discussed and accurately known. As we hope that these will be further 
investigated, we give the fullest data at our disposal for the calculation of the 
undisturbed values of the elements. 
