550 
MR. A. W. RUCKER AXD DR. T. E. THORPE OX A MAGXETIC 
Station. 
1886. 
1891. 
Difference. 
Difference 
(reduced to mean 
iso magnetics). 
Lairg. 
O 
- 79 
o 
-135 
+°56 
+°46 
Golspie. 
- 
-120 
+ 28 
+ 14 
Thurso. 
- 53 
- 70 
• + 17 
0 
Wick. 
-146 
-150 
+ 4 
0 
Kjle Akin. 
Gairloch. 
+ 154 
+ 165 
-11 
- 8 
±180 
-134 
— 46 
-44 
Soa. 
-166 
-166 
0 
0 
IMablethorpe .... 
+ 113 
+ 139 
— 26 
-20 
Lincoln ..... 
- 63 
— 72 
+ 9 
+ 5 
Gainsborough .... 
- 36 
- 17 
-19 
— 5 
Oxford. 
-171 
-163 
- 8 
- 1 
Salisbury. 
- 29 
- 83 
+ 54 
+ 46 
Armagh . 
+ 27 
+ 5 
+ 22 
+ 15 
Enniskillen. 
+ 160 
+162 
_ 0 
+ 1 
Sligo. 
-126 
-131 
+ 5 
+ 9 
'Carrick-on-Shannou . 
+ 72 
+ 88 
-16 
-•23 
Swansea. 
+ 4 
- 8 
+ 1-2 
+ 16 
Cardiff. 
— 35 
- 29 
- 6 
+ 1 
Milford. 
- 23 
- 33 
+ 10 
+ 18 
Killarney. 
+ 42 
+ 32 
+ 10 
- 8 
Rantry. 
+ 52 
+ 18 
+ 34 
+ 2 
Valeutia. 
+ 42 
+ 23 
+ 19 
- 1 
It was sometimes impossible or inconvenient to repeat the observations exactly on 
the spot originally selected. This was the case at Lairg, Gairloch, Gainsborough, 
and Valenti a. 
The results are in accord with our previous estimates of the accuracy of our results, 
viz., that the directions of the disturbing forces as given by a single observation was 
correct to “ within (in unfavourable cases) 15° or 20° ” (“ 1890 Memoir,” p. 269). 
The difference of a single result from the mean of the two oidy exceeded these 
limits in three cases out of twenty-two when the same set of terrestrial isomagnetics 
were used, and only in four cases when the two independent sets of isomagnetics 
were emplo^md. In two of these cases (Lairg and Gairloch) the positions occupied 
are known to have been different. 
On the whole then, it appears that when the direction of the Horizontal Disturbing 
Force at any place is determined by means of two independent experiments, the 
difference between the results is of the same order of magnitude whether the same 
or two independently determined sets of terrestrial isomagnetics are employed. In 
other words, the error of the isoma.gnetics is not important as compared with the 
error of experiment. 
As to the errors of experiment, we have adopted 0‘0030 metric or O'OOOSO C.G.S. 
unit as the limit to the magnitude of the Horizontal Disturbing Force, below which 
the calculated value of its direction cannot be relied on, and shall hereafter refer to 
it as the limit of the accurate determination of direction. 
