DR. S. CHAPMAN ON THE SOLAR AND LUNAR 
42 
As regards the resistivity 274.10 12 , we may note that this is considerably less 
than that of the dry constituents of the outer rocky crust of the earth. Ordinary 
sea water is more conducting ; thus while for distilled water p is about 1'4 . 10 15 , and 
for rain water 6 . 10 13 , Schmidt* found that for North Sea salt water p is 2*5 . 10 10 , 
and Uller f states that water from the Mediterranean Sea may he only two-thirds 
as resisting as this. Uller also finds that the resistivity of moist earth ranges from 
LO 13 to 10 14 , but that for dry earth it is about 10 15 . Lowy| has measured the specific 
resistance of the ordinary constituents of the earth’s crust (rock, stone, and so on) 
and concluded that for the majority of specimens p is greater than 10 16 , though the 
results varied somewhat with the moisture in the stone. It would seem, therefore, 
that apart from the comparatively shallow oceanic or water-bearing strata at or near 
the earth’s surface, the outer crust is from 100 to 1000 times as resisting as, from 
our calculations, the inner core appears to be. 
The above data apply only to the solid crust, which geologists consider, on the 
evidence of seismological and gravity measurements and the study of radio-activity, 
to extend down to a depth of 30 or 40 miles only.§ Very little is known of the 
nature or condition of the underlying substance. The above value of p, 274.10 12 , 
is much greater than the resistivity of metals such as iron at ordinary temperatures 
(for which p is about 10 7 ) ; their resistance, however, increases with temperature, and 
may also be affected by the great pressures to which the interior layers of the earth 
must be subject. 
As regards the depth of the non-conducting layer, 200 to 300 km., it may be 
noticed that Helmert||, and also Tittmann and HayfordII, have concluded that 
the inequalities in the distribution of mass near the earth’s surface extend down 
to a depth of 120 km. ; variations in the electrical properties of the earth seem 
therefore to extend below this region of variation of elasticity. 
Part V.—On Certain Properties of the Earth’s Atmosphere. 
§ 19. The Solar Diurnal Barometric Variation. 
As was remarked in § 6, the atmospheric motions to which the daily magnetic 
variations are to be attributed, on the Stewart-Schuster theory, are the horizontal 
and not the vertical movements. Before proceeding with the study of the magnetic 
variations, a brief statement will be made of the principal relevant facts regarding 
the daily circulation and electrical conductivity of the atmosphere. 
* SCHMIDT, ‘ Jahrbuch d. Drahtlosen Telegraphic,’ vol. 4, p. 636, June, 1911. 
t Uller, ibid ., p. 638. 
j Davy, ‘Ann. d. Physik,’ 36, p. 125, October 3, 1911. 
§ Cf . Sir A. Geikie’s Article on Geology, ‘ Encyc. Brit.’ (11th ed.), vol. 11, p. 654. 
|| Helmert, ‘Encyc. d. Math. Wiss.,’ VI., 1, B, vol. 2, 1910. 
If “Geodetic Operations in the U.S.A., 1906-9.” ‘Report to 16th Conference of the International 
Geodetic Association/ by O. H. Tittmann and T. Hayford. 
