222 



SCIENCE. 



[N. S. Vol. XV. No. 371. 



of astronomy. Its style is clear and attractive 

 and the illustrations, some excellent, are in 

 the main adequate although many of the dia- 

 grams are disagreeably crude. A familiar liter- 

 ary device, that of prefixing a brief metrical 

 introduction to each chapter, has here been so 

 felicitously applied as to deserve especial men- 

 tion. An excellent table of contents and index 

 greatly facilitate the use of the work as a 

 book of reference. George C. Comstock. 



EARTH-CURRENT OBSERVATIONS IN THE GERMAN 

 lELEGRAPH SYSTEM.* 



The origin of these important observations 

 dates back to 1881, when a committee was 

 called together by Werner Siemens, to study 

 the phenomena of earth-currents. Through 

 their efforts, two underground cables were 

 provided by the Imperial Telegraph System, 

 one running in an easterly direction from 

 Berlin to Thorn, 262 km., the other nearly 

 due south from Berlin to Dresden, 120 km. 

 The present work deals chiefly with the con- 

 tinuous observations of earth-currents from 

 these two lines, from 1884 to 1888. The 

 Prussian Academy of Sciences assisted, in 

 part, in the maintenance of the observations. 



The assumption is made at the start that 

 the observed currents are due to potential 

 differences between the ends of the lines; that 

 is, they are derived from currents that flow 

 in closed circuits within the earth, parallel to 

 its surface. Of course vertical differences of 

 potential have to be left out of consideration. 



The attempt to express the intensities in 

 the two lines by trigonometrical formulse ac- 

 cording to Gauss, using the latitude and 

 longitude as variables, leads to equations 

 whose constants are too difficult to be deter- 

 mined. Assuming the validity of Ohm's law, 

 however, the intensity of the earth-current 

 components in the two directions may be 

 given by the equations 



W W 



J=A^i, J' = A-rji, 



* Die Erdstrome im deutsehen Eeichstelegraph- 

 engebiet und ihr Zusammenhang mit den erdmag- 

 netisdien Erscheinungen, bearbeitet und heraus- 

 gegeben von Dr. B. Weinstein. Braunschweig, 

 Friedrieh Vieweg & Sohn, 1900. 



where 4 is a constant and W, W are the 

 resistances, L, L' the lengths and i, i' the 

 observed current strengths in the two lines re- 

 spectively. 



We thus obtain for the total earth-current. 



E 



^.4 



u 



L'2 



The value of the constants was computed for 

 each of the two lines. The results are only 

 relative, however, as no reductions to absolute 

 units were made. 



The most characteristic feature of earth- 

 current variations is their dependence upon 

 the position and condition of the Sun. The 

 diurnal and annual variations are especially 

 marked. In view of this, the attempt is made 

 to modify the trigonometrical representation 

 in such a way as to use, instead of the latitude, 

 the angle with the Sun's declination, and for 

 the longitude, the local time or the right 

 ascension of the Sun. The results indicate, 

 however, as was to be expected, that this is 

 not sufficient, but that other factors have to be 

 considered. In general there can be distin- 

 guished a constant component of the current, 

 due to terrestrial and local conditions, and 

 a variable component, depending chiefly upon 

 the Sun. The four years of observations were 

 not enough to make the derivation of accurate 

 formulae possible. As approximations, how- 

 ever, expressions for the components in the 

 two directions were derived, as functions of 

 the local time and its multiples, from which 

 the diurnal variation is made evident. 



The self-recording instruments were of two 

 different types. In the Berlin-Dresden line a 

 Siemens 'Eussschreiber' was used, in the 

 other line a mirror-galvanometer reflected a 

 beam of light on to photographic paper. The 

 sensitiveness of both instruments was fre- 

 quently determined, and though the results 

 were not reduced to absolute measure, still it 

 is always possible to get accurate relative 

 values between the two lines. 



The magnetic records, which, as the title in- 

 dicates, formed an essential part of the work, 

 were obtained chiefly from the observatories 

 at Wilhelmshaven and Vienna, but to a lesser 

 extent also from the observations during the 



