468 
PROFESSOR A. SCHUSTER OH THE DIURHAL 
the result should not be doubtful, even if we have only an approximate knowledge of 
the vertical forces. 
Two years ago I showed that the leading features of the horizontal components for 
diurnal variation could be approximately represented by the surface harmonic of the 
second degree and first type, and that the vertical variation agreed in direction and 
jihase with the calculation on the assumption that the seat of the force is outside the 
Earth. The agreement seemed to me to be sufficiently good to justify the conclusion 
that the greater part of the variation is due to causes outside the Earth’s surface. 
Nevertheless, it seemed advdsable to enter more fully into the matter, as in the first 
approximate treatment of the subject a number of important questions had to be left 
untouched. I now publish the results of an investigation which has been carried as 
far as the observations at my disposal have allowed me to do. My original conclusions 
have been fully confirmed, and some further information has been obtained which I 
believe to be of importance. The results of the calculation point not only to an 
external source, but to an additional internal source, standing in fixed relationship to 
the external cause. This we might have expected. A varying potential due to 
external causes must be accompanied by currents induced in the Earth’s body, which, 
in turn, must affect the magnetic needle. The phase of these cun-ents and their 
magnitude lead us to form definite conclusions on the average conducting power of 
the Earth, and it will be seen that there is strong evidence that the average con¬ 
ductivity is very small near the surface, but must be greater further down. Tn this 
part of the investigation I had much assistance from my colleague. Professor Lamb. 
I hope that the results obtained in this paper may induce the heads of magnetic 
observatories to consider the suggestions which T have made at the end of it, as their 
adoption would very materially assist further investigations. 
I had, in the first place, to fix on a year for which we possess as complete magnetic 
records as possible. The phase of the variation of horizontal force changes sign in a 
latitude not far removed from that of Lisbon, and it seemed to me, therefore, essential 
that the excellent observations there made by Sen. Jos. Capello should be made use 
of The observations are published as far as 1872, and I had to take a year therefore 
anterior to this. It seemed also desirable to make use of the St. Petersburg observa¬ 
tions, as it is the most northerly station for which we have records extending over a 
period of years, and as Mr. PI. Wild’s well-known skill gives special value to the 
observations made under his direction. The obseravtions (continued since 1878 in 
Paulowsk) were interrupted in 1871 and 1872, and we have to go back, therefore, as 
far as 1870 if we want to utilise the St. Petersburg and Lisbon observations simul- 
o 
taneously. As far as the horizontal components are concerned, we also possess good 
records of 1870 at Greenwich and Bombay. Four stations are sufficient to find to 
the necessary accuracy the potential on the surface of the Earth, but it would be of 
advantage if in future similar investigations a greater numbei' of stations could be 
utilised. 
