SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY 1, 1886. 71 
disturbance registered by the Kew curves at the same G.M.T. as that at which the 
observations were taken. 
(5) S (obs.) is the value of the Declination obtained by subtracting the reading for 
the magnetic from that for the corresponding geographical meridian. 
(G) (t is the secular correction to Jan. 1, 1886, obtained as is hereafter explained. 
(7) S (red.) is the mean value of the Declination reduced to the epoch of the survey. 
As an example we take Athlone, where observations were made on two different days. 
On May 8, no back observations were taken, so that /x could not be calculated. 
Declination. Athlone. Observer, T. E. T. Magnetometer 61. 
Geographical Meridian. 
Date, 1887. 
V 
Alt. 
/'• 
G.M. 
]i. m. s. 
0 / 
, 
0 / 
]\Iay 8 
+ 1 40 59 
48 4-2 
338 38T 
» 9 
-3 43 58 
33 51 
- 0-2 
210 31-2 
Magnetic Meridian. 
Date. 
G.M.T. 
t p 
t bo' 
a’. 
V, 
A. 
M.M. 
May 8 
,, 9 
li. m. 
12' 39 
9- 9 
+ 0-2 
-0-4 
1 
00 ^ 
0 
+ 6'3 
-3-9 
+ 1-5 
0-0 
.316 21-6 
188 14-4 
Declination. 
h (obs.). 
Mean. 
(7, 
c (red.). 
2°2 16-5 
22 16-8 
0 / 
22 16-6 
> 
+ 10-1 
0 / 
22 26-7 
In the case of the Horizontal Force observations the deflections were always taken 
when the distances between the centres of the magnets were 0‘3 and 0'4 metre. If 
then Zj and are the common logarithms of the values of m/H obtained at the two 
distances, the factor P used in the corrections may be calculated by the formula 
P z= 0-4737 (Zi - Z 2 ) - 1-947 (Z^ - Uf 
of which the second term is very small. (‘Nature,’ Aug. 13, 1887, p. 366 ; Sept. 29, 
p. 508 ; Dec. 8, p. 127 ; and Jan. 19, 1888, p. 272). 
