VARIATION OF TERRESTRIAL MAGNETTSAI. 
499 
Table XXIII.—Comparison betM^een resultant vertical force, as regards magnitude 
and direction when induced currents are taken into account, and vertical force 
calcidated on the assumption that the whole is due to an outside effect. The 
inducing potential is a solid harmonic of degree 4. 
a. 
Reduction in 
amplitude. 
Cliange of phase. 
!>■ 
1 
•9997 
0 1 
1 06 
3-70 X 1014 
10 
•9723 
10 06 
3-70 X 1013 
100 
•4982 
38 49 
3-70 X 1013 
The observed amplitude of the vertical force at Lisbon is about one half of its cal¬ 
culated value. If the conductivity of the Earth was such as to produce this reduction 
in amplitude, it is seen from Tables XXII. and XXIII. that the phase would be altered 
about 40°, while in reality there is a remarkable agreement in phase. If the con¬ 
ductivity is so small as to leave the resultant phase practically unaltered, as observation 
tends to show, the amplitude also should not be sensibly altered. There is, therefore, 
no uniform conductivity of the Earth which can make the observations agree with 
the calculation. Such an agreement, however, can be easily brought about, as 
Professor Lamb has suggested to me, if the conductivity of the inside of the Earth is 
larger than the conductivity of the ujoper layers. It is extremely probable that this 
is really the case. The bulk of the outside layer of the Earth, except in so far as it is 
water, is made up of material which in its ordinary condition is non-conducting ; but we 
know that some of the silicates begin to conduct at temperatures above 200 ° C., and, 
generally speaking, insulators lose their insulating powers at high temperatures. With¬ 
out regard even to the quantities of metallic matter that may be stored inside the Earth, 
there is nothiug improbable in the supposition that its conductivity increases towards 
the inside. If the bulk of the observed induced effect is due to currents in a fairly 
conducting inner sphere,'the calculated phase would be that due to good conducting 
matter, and would not differ from the observed value, while the reduction in amplitude 
might yet be sufficient to account for the observed facts. In order to give a better 
idea of the kind of conductivity which is required to produce a certain change of 
phase, it may be stated that for the purest distilled water obtained by Kohlrausch 
p would be about 1'4 X 10^®. Such w'ater is, as is well known, a very bad conductor, 
and, according to our Tables, if the whole Earth wms made up of matter which cond ucts 
as badly, there would be no currents in the Earth induced by the diurnal variation of 
sufficient intensity to affect our magnetic needle sensibly. Ordinary rain water, how¬ 
ever, has a specific resistance of about 6 X 10-®. A conducting sphere of the same 
resistance would already produce a retardation in phase of about an hour for the 
diurnal variation if the solid harmonic is of degree 2 . For salt water the resistance 
3 s 2 
