THE EE V. STEPHEN J. PEEET ON MAGNETIC OBSEEVATIONS 
i 
2 
Table II. 
Semiannual inequality of the Horizontal Force. 
Date. 
Correction 
for secular 
variation. 
Mean 
+ secular 
variation. 
Observed 
values. 
Observed— 
Computed. 
April to 
September. 
October to 
March. 
■ July 1, 1863 
-0-0137 
3-5897 
• 3-5925 
+ 0-0028 
-0-0116 
•5918 
•5926 
+ 0-0008 
July 1, 1864 
-0-0095 
•5939 
•5958 
+ 0-0019 
.Tan. 1, 1863 
-0-0074 
•5960 
•5948 
— 0-0012 
July 1, 1865 
-0-0053 
; 5981 
•5979 
-0-0002 
.Tan. 1. 1866 
-0-0032 
•6002 
•5965 
-0-0037 
July 1, 1866 
-0-0011 
•6023 
•5997 
— 0-0026 
Jan. 1, 1867 
+ 0-0011 
•6045 
•6038 
-0-0007 
July 1, 1867 
+ 0-0032 
•6066 
•6085 
+ 0-0019 
Jan. 1, 1868 
+ 0-0053 
•6087 
•6042 
— 0-0045 
July 1, 1868 
+ 0-0074 
•6108 
•6083 
-0-0025 
j .Tan. 1. I860 
+ 0-0095 
•6129 
•6176 
+ 0-0047 
July 1, 1869 
+ 0-0116 
•6150 
•6166 
+ 0-0016 
Jan. 1, 1870 
+ 0-0137 
•6171 
•6191 
+ 0-0021 
Mean differences in the semiannual periods 
+ 0-00014 
-0-00036 
Hence we may conclude that there exists an annual variation whose mean value is 
(P0005 ; but the great difference between the figures for the semiannual periods shows 
that the variation in this particular case is not wholly due to the disturbing action of 
the sun. 
We can now test the accuracy of our assumed values of the secular and semiannual 
variation, and of the observations themselves, by the formation of the following Table. 
Table III. 
Residual errors in the monthly mean values of the Horizontal Force. 
1863-64. 
1864-65. 
1865-66. 
1866-67. 
1867-68. 
1868-69. 
1869-70. 
Mean. 
Semiannual 
mean. 
April 
+ 21 
+ 17 
-17 
-39 
+ 35 
-49 
+ 14 
-0-00026 
h 
May 
+ 25 
+ 58 
+ 37 
-36 
+ 25 
- 6 
+ 103 
+ 0-00294 
June 
July 
— 48 
+ 86 
+ 59 
+ 60 
+ 15 
-66 
-20 
+ 35 
+ 24 
0 
— 8 
-57 
+ 45 
- 16 
+ 0-00096 
+ 0-00060 
V + 0-00016 
August ... 
+ 51 
-23 
O 
— 51 
+ 31 
-55 
+ 20 
-0-00041 
1 
j September 
+ 15 
-75 
+ 3 
— 62 
- 10 
+ 10 
- 84 
-0-00290 
J 
' October ... 
November 
+ 2 
-17 
-58 
0 
-79 
-29 
— 41 
— 1 
+ 26 
0 
+ 56 
+ 52 
+ 31 
+ 41 
-0-00061 
+ 0-00066 
J 
December 
+ 48 
-44 
— 20 
— 4 
- 9 
+ 70 
+ 83 
+ 0-00177 
> + 0*00014 
1 
January ... 
+ 26 
- 1 
-16 
— 1 
-215 
+ 66 
+ 66 
—0-00107 
February 
+ 50 
+ 38 
— 15 
+ 14 
- 29 
+ 92 
- 7 
+ 0-00204 
1 
1 
March 
-48 
0 
-51 
+ 9 
- 27 
— 35 
- 78 
— 0-00194 
J 
Means 
+ 18 
+ 2 
— 20 
-16 
- 12 
+ 11 
+ 18 
This Table shows that the assumption of a semiannual inequality, whose mean value 
