DIURNAL VARIATIONS OF TERRESTRIAL MAGNETISM. 63 
such changes as due to the temperature variations in the lower regions of the 
atmosphere. As regards the lunar day, regular temperature variations should be 
almost or quite non-existent, and Sp/p should not alter with height. The relative 
decrease of the solar as compared with the lunar semi-diurnal atmospheric oscillation, 
from a ratio of 16 to one of about 10, may possibly be explained in this way. 
In order to obtain a numerical estimate of the electrical conductivity of the region 
in which the magnetic variations are produced, we will determine the constant K in 
the formula (40) by a comparison of the lunar diurnal atmospheric velocity potential 
(38) with the observed magnetic variation. Considering the equinoctial “ annual ” 
harmonic Q 3 2 , from Table J, we find the amplitude (cf. 17) in C.G.S. units to be 
(78) 5‘5 . 10 -7 R. 
The theoretical value (§ 23) is 
(79) 4X K 2 2 CK pi 
2m +1 \r J 2 ^ 
(where m = 3), and, paying no attention to signs for the present, 
(80) C = f, K 2 2 = 32'4E. O'OIO. 
Substituting these values in (79), and equating the result to (78), we find that 
(81) K=l-92.10- 6 . 
Hence, approximately, 
(82) pe = 2 . 10“ 6 (l + 3 cos w + f- cos 2 «). 
At points directly beneath the sun (w = 0) the value of pe thus given is 12 . 10~ e . 
This calculation applies to years of low solar activity. At times of solar maximum 
(cf. Table R, p. 60) pe would rise to 17 . lO -15 or 20 . 10“ ,; . Moreover, as we shall 
see when we come to take self-induction into account (§ 26), all these values must be 
increased by 30 or 40 per cent., and the probable maximum value of pe which has to 
be explained in any theory of the conducting layer must be, approximately, 
(83) 25. 10- 6 . 
Schuster’s approximate determination of P e was 3 . 10~ ,; .* The larger value here 
obtained accentuates the difficulties in the explanation of the conducting layer which 
have been mentioned in § 21 ; but there seems no reason to suppose that they 
are insuperable. 
* Schuster, ‘Phil. Trans.,’ A, vol. 208, p. 181. 
