82 
ME. C. CHAMBERS ON THE DIRECTION AND INTENSITY 
January and December 49 24 
February and November 49 38 
March and October 49 42 
April and September 49 40 
May and August 49 32 
June and July 49 22 
the large values occurring in the middle of the column near the time of the equinoxes. 
The mean for the months April to September is 49' 31", and that for the months 
January to March and October to December 49' 33", showing a scarcely appreciable 
amount of semiannual inequality. 
14. Probable Error . — The probable error of a single determination of Absolute 
Declination has been calculated separately for each of the years 1868 to 1870. The 
numbers of observations made in those years are 53, 53, and 57 respectively, and each 
one was reduced to the constant reading 35 ’00 of the large Declination Magnetometer. 
If there was no error of observation, and if the large Declination Magnetometer w r as 
perfect, these reduced values ought all to be alike, and the degree of accordance existing 
amongst them will indicate the extent to which these conditions are fulfilled. The 
differences being taken for each year between each individual reduced value of the 
Declination and the means of all the values for that year, the probable error of a single 
determination was found from these differences by the usual method (of least squares) 
to be +22"'5, +22" , 0, and +15 //- 4 for the years 1868, 1869, and 1870 respectively; 
and the probable error of the annual means to be + 3"T, +3" - 0, and + 2"-0 respectively. 
III. Horizontal Force. 
15. Monthly Mean Values . — In Table XIII. are collected the mean monthly values, 
as observed, of the Horizontal Force for the period from July 1867 to December 1873, 
and the corresponding monthly mean readings of the large Horizontal-force Magne- 
tometer, corrected to the constant temperature 82° Fahr. In correcting the magnetometer 
readings 0T5 has been adopted as the decrease of reading produced by an increase of 
1° of temperature. 
Table XIII. 
Month. 
Mean observed Absolute Horizontal Force. 
Corresponding Mean Scale-reading of la: 
Horizontal-force Magnetometer, 
corrected to Temperature 82°-0. 
rge 
i 1867. 1 
1868. 1 1869. 
1870. 
1871. 
1872. 
1873. 
! 1867. 
1868. 
1869. 
1870. 
1871. 
1872. 
1873. 
January 

8-0608 8-0616, 
8-0655 
8-0768 
8-0727 
8-0750, 
24-60 
24-30 
24-77 
25-57 
25-89 
February 
■ • 
8-0556 8-0688 
80670 
8-0791 
8-0743 
8-0811 
24-61 
24-07 
24-72 
25-61 
25-95 
10-47 
March 
8-0597 8'0655 
8-0758 
80774 
80818 
8 0830 
24-48 
24-29 
25-26 
25-64 
25-96 
10-50 
April 
8 0595 8 0616 
8-0697 
8-0743 
80873 
80856 
23-79 
24-34 
24-90 
25-35 
26-15 
10-64 
May 
8-0614 8 0547 
8-0706 
8-0742 
8-0876 
80859 
24-32 
24-09 
25-49 
25-80 
26-32 
10-76 
June 
8-0668! 8-0683 
8-0818 
80790 
80844 
8-0874 
24-54 
24-84 
25-62 
25-78 
25-79 
10-8S 
July 
80494 
8-065 1 8-0695 
80756 
8-0819 
8-0811 
8-0889 
23-53 
2407 
24-84 
25-21 
25-54 
25-67 
10-93 
August 
1 8-0612 
8-0490 8-0633 
8-0768 
8-0808 
8-0813 
8-0847 
23-41 
24 15 
24-25 
2515 
25-42 
25-64 
10.72 
September 
8-0508 
8-0546 8-0710 
80700 
8-0824 
8-0808 
8-0828 
23-37 
23-29 
24-31 
24-35 
25-56 
25-39 
10-83 
October 
8-0569 
8 0637 8 0646 
80687 
8-0799 
8-0753 
80853 
23-32 
24-10 
24-45 
24-56 
25-11 
25-18 
10-99 
November 
8-0523 
8 0659 8 0690 
80680 
8-0757 
8-0769 
8-0841 
2409 
24 51 
24-48 
24-73 
25-37 
25-65 
11-35 
December 
80593 
8-0588 8 0625 8-0787 
8-0710 
80795 
8-0849 
24-27 
24-42 
24-74 
25-59 
25-59 
25-38 
11-61 
