DIURNAL VARIATIONS OF TERRESTRIAL MAGNETISM. 
61 
same relative scale as the results in 
Table 
Q, for 
12-hour and 
24 -hour atmospheric 
movements of the same equatorial amplitudes, tl 
le following 
values are obtained 
(cf. Table I., p. 299, ‘ Phil. Trans.,’ A 
213) 
Q, A. 
Q.3 2 . 
QA 
QA 
f 12-hour wave . 
. 25 
6'5 
0‘65 
0-021 
(77) 1 
(_ 24-hour wave . . 
. 23 
1 
CO 
CO 
0’24 
0 
The ratios in Table S might be reproduced more closely if a 24-hour atmospheric 
wave of about one-third the amplitude of, and in phase with, the 12-hour wave 
is supposed present in the solar diurnal variations. 
The seasonal changes in the amplitudes of the annual harmonics in the solar 
diurnal magnetic variations are similar to those in the lunar variations. The diminu¬ 
tion at the solstices is partly explicable by the similar decrease in the semi-diurnal 
barometric variation (§ 19), which in each case takes place without appreciable 
change of phase. The further reductions in the higher magnetic harmonics, at the 
solstices, is to be referred to the effect of the dependence of conductivity on the solar 
zenith distance «, though the law (73) has been seen to be insufficient to account 
altogether for the observed changes. The necessary modification of (73) would seem 
to be in the direction of a more rapid diminution of p (the conductivity) as w increases 
from 0 degree to 90 degrees. This is probable on other grounds (§ 21), but the 
theoretical discussion of its consequences would be a very laborious task. 
The same modification would also increase the theoretical values of the seasonal 
harmonics Q,”, which for the lunar diurnal magnetic variations were found to be 
three or four times too' small, relatively to the annual harmonics, when compared 
with the observed data. But a more serious difficulty arises here. If we examine 
the solar diurnal seasonal harmonics it is found that they are in fair agreement with 
the theoretical values from (73), except in regard to phase. The seasonal harmonics 
in the lunar variation are, in fact, nearly thrice as great compared with the annual 
harmonics as in the solar variation. This is immediately evident in the initial data 
of this paper (cf Tables III. (a) and Ill. (y) with VI. (c) and VI. (d)), and the 
following ratios of the solar and lunar seasonal harmonics E (1 " show the same thing :—- 
EP. EA E 3 s . Ep. 
7'6 3'4 4U (147) 
These numbers are comparable with those of Table S ; the same preponderance in 
the ratio for the diurnal harmonic is seen here, but the first three of these ratios are 
less than half as great as those for Eq +1 . 
If we suppose the electrical conductivity such that the seasonal harmonics due 
to an atmospheric oscillation Qp have the relative magnitudes shown in the lunar 
