VARIATION OF TERRESTRIAL MAGNETISM. 
487 
subject to in various years. The numbers are copied from the published records of 
the Greenwich Observatory. 
Table XI. 
Coefficients in the series 
sin (^ + a) + Cg sin { 2 t + /3) + Cg sin (3^ + y) + C4, sin {U + y) 
where changes in represent changes in the vertical force, the unit being 
'00001 of the whole vertical force ; t being the time from midnight. 
G- 
a. 
Cj. 
C3. 
7- 
G- 
d. 
0 
0 
0 
0 
1883 
14-3 
148T3 
13-1 
266-58 
5-3 
89-60 
1-3 
293-20 
1884 
14-8 
139-33 
11-7 
272-00 
5-5 
95-52 
2-1 
289-49 
1885 
13-0 
137-50 
11-7 
265-35 
5-1 
88-28 
1-5 
281-04 
1886 
12-5 
160-58 
11-9 
268-38 
4-0 
94-22 
1-2 
297-50 
The principal discrepancy here occurs in the angle a, which would give a difference 
of phase between 1885 and 1886 of about 23°, or about an hour and a half; the 
angle a differs much at Greenwich during different months; the phases of the other 
terms show practically no difference. In comparing the observed and calculated 
values, I have taken the year 1884, as during that year the range of the declination 
needle was greatest, and corresponded most nearly to that of 1870. 
At St. Petersburg, also, the results for 1870 are not corrected for temperature, and 
there is reason to believe that this has affected the observed type considerably. I 
have therefore compared the results of calculation with observation, also, for the year 
1878, in which year the temperature corrections have been taken into account. 
We must remember, then, in the comparison between the observed and calculated 
values, that the greatest weight must attach to the Lisbon observations, and that 
comparatively little value is to be given to St. Petersburg, as I have no records at my 
disposal which would enable me to judge how far the type of vertical force there 
varies from year to year. 
Tables XII. and XIII, give the values of the coefficients a^, a^, 63 , &c., as calculated 
on the hypothesis that the disturbing foree comes from the inside of the Earth. 
Tables XIV. and XV. give their values ealculated on the hypothesis that the 
disturbing force is outside. In both cases the observed numbers are given for 
comparison. I have calculated the values for Lisbon separately for the winter and 
for the summer months ; in the other cases the values for the mean of the year only 
have been taken. The years attached to the observing stations refer to the date of 
the observations ; the calculated values all belong to 1870. 
