DIURNAL VARIATIONS OF TERRESTRIAL MAGNETISM. 
41 
given of the values of sixteen observational quantities (eight values of the amplitude 
ratio and eight of the phase difference). Naturally, however, the hypothesis of a 
non-uniformly conducting earth such as we have considered must be regarded as 
giving only a convenient idealized representation of the real facts. 
The theoretical values of f calculated for the lunar diurnal variation are given in 
Table J; they differ but little from those of Table N. The observed values in 
Table J are somewhat irregular, but their mean (2'3) is in satisfactory agreement 
with the calculated mean (2*5), when the accidental error of the lunar data is 
considered. 
§ 18. The Electrical Conductivity of the Earth as Deduced from the 
Diurnal Magnetic Variations. 
In § 17 the following two quantities were determined, in connection with the 
theory that the earth lias a conducting nucleus of radius Ji c and specific resistance p, 
surrounded by a non-conducting layer :— 
(25) p/Vf = 7*31 . 10~ 6 , R/R„ = U04. 
Here It denotes the radius of the earth (2x11 = 4 . 10 9 cm.). 
The thickness of the outer layer is given by 
(26) R-R c = R(l-R c /R) = 245 km., 
or about 160 miles. The specific resistance p of the inner core is similarly found to 
be as follows :— 
(27) p = 7*31 . 10- 6 . R c 2 = 274.10 12 C.G.S. 
These values may be compared with those deduced by Schuster in his second 
memoir. # The calculation there made was intended to give only a rough estimate, 
and in order to explain the apparent absence of phase difference between the external 
and internal fields (cf § 2) it was necessary to assume a high—practically infinite— 
conductivity of the inner core. Hence no comparison with (27) is possible. On this 
basis, however, the deduced value of the thickness of the non-conducting layer was 
1000 km., in place of the present value 245 km. The difference seems altogether 
beyond the probable limit of error in the latter result, and 1000 km. must be 
regarded as definitely too large. The estimate 245 km. can hardly be liable to so 
much as 50 per cent, error, so that the outer layer is probably from 200 to 300 km., 
or 100 to 200 miles in depth. It should not be forgotten, however, that we have 
no evidence for a sharp line of demarcation between the outer non-conducting and the 
inner conducting matter. 
* ‘Phil. Trans.,’A, vol. 208, 1907, p. 169. 
VOL. CCXVIII.-A. G 
