22 
MR, A. W. RtiCKER AND DR. T. E. THORPE ON A MAGNETIC 
Date. 
1 
Survey instrument. 
No. of 
observations. 
Observers. 
No. 60. 
No. 61. 
No. 70. 
No. 73. 
Sept.-Oct., 1887 
- 2 
3 
R. 
Oct., 1887 . 
+ 1 
•• 
3 
T. 
•Tune 2-6, 1890 .... 
-h 15 
9 
G. 
June-July, ,, .... 
+ 13 
•• 
17 
B. 
April 16-24, 1891 . . . 
+ 47 
12 
W. 
Nov. 12, ,, ... 
+ 1 
. • 
1 
G. 
Oct. 12-14, „ . . . 
+ 43 
5 
W. 
Feb. 1-3, 1892. . . 
+ 13 
7 
G. 
2-3, „ . . . 
+ 19 
7 
G. 
March 23—25 ,, . . . 
f « 
. , 
+ 29 
9 
G. 
April 21-22 „ . . . 
. , 
+ 38 
9 
W. 
Oct. 7-10 „ . . . 
+ 38 
6 
W. 
11-13 „ . . . 
. , 
, * 
+ 30 
6 
W. 
„ 20-21 „ . . . 
+ 29 
4 
G. 
,, 24-25 ,, . . . 
+ 20 
• • 
• • 
4 
G. 
From this Table we draw the following conclusions with respect to the years 1889 
to 1892. 
Instruments 60 and 61 appear to have changed between 1887 and 1890. There 
does not appear to have been any further important change during 1890 and 1891, 
and we, therefore, take the means of the values obtained in those years as applicable 
to them. 
The observations made at the end of the year 1892 give higher values than those 
made in February of that year, though the fact that the apparent change in the 
indications of the two instruments is the same in each case, asfain suDfo;ests that the 
main error in these comparisons is'due to the uncertainty of the zero line of the self- 
registering instruments rather than to the absolute instruments themselves. This 
argument is in turn weakened by the fact that no such change was observed in the 
case of the comparisons of Instruments 70 and 73 undertaken during the same year, 
at intervals of 6 or 7 months, as against the 8 months which elapsed between the 
earlier and later comparisons of Nos. 60 and 61. 
We have therefore taken the mean of the corrections determined at the beginning 
and end of 1892 as applicable to that year. 
The following Table gives the corrections actually applied to the results of the 
difterent instruments for each year, expressed in terms of O'OOOl metric unit. 
