DEPARTMENT OF TERRESTRIAL MAGNETISM. 265 



from widely separated regions show very much greater similarity when re- 

 ferred to the same time basis than when referred to their respective local 

 times. This, too, is in agreement with the 1921 results, which, however, 

 referred to the positive current alone (see reference above). The mean value 

 of the current density from these observations is about 9X10"^ E. S. U. 

 (3X10~^® ampere per square centimeter), and the range of the mean diurnal 

 variation is about 30 per cent of this value. 



On earth currents and polar lights. S. J. Mauchly. Bull. 3, Sect. Terr. Terr. Mag. and 

 Electr., Internat. Geod. Geophys. Union, 152 (September 192.3). 



This is a brief report dealing with plans for stimulating interest in, and 

 increasing the number of reliable observations for, earth-current and polar- 

 light investigations, and is based upon a progress report submitted in 1922 to 

 the American Geophysical Union (see pp. 305, 306 of last year's report). 



The results of potential-gradient registrations at Washington, District of Columbia, for the 

 years 1917 to 1922. S. J. Mauchly. 



In the last annual report (pp. 304, 305) preliminary results were given 

 regarding the diurnal and annual variations of the potential gradient at 

 Washington for the year 1918. As then stated, these registrations were 

 obtained on the deck of the laboratory of the Department of Terrestrial 

 Magnetism, in an observatory maintained primarily for experimental and 

 instructional purposes. 



The reduction of the records for the 6-year period 1917-1922 has now been 

 completed and sufficient progress has been made in the analysis of the data 

 to show that the preliminary results given for 1918 (l. c.) hold also for the 

 entire 6-year period. This is especially true as regards the characteristics 

 of the diurnal variation; however, as might be expected, the positions of the 

 maximum and minimum of the annual variation sometimes occur a month 

 earlier or later than those found for 1918. 



The removal in 1920 of a hill covered with trees, near the observatory, 

 introduced a discontinuity into the series so far as yearly change is con- 

 cerned, the nature of the effect being, as would be expected, to cause an 

 increase in the recorded potentials. Thus, the reduction factors determined 

 during 1922 and 1923 are not applicable for the period prior to 1920. However, 

 of the mean recorded values corresponding to the years 1917 to 1919, inclusive, 

 those for 1918 were the largest, and all data obtained since 1919 show a dimi- 

 nution in the successive annual values ranging from 3 to 5 per cent per year. 

 The annual values, in volts per meter, for the years 1920 to 1922 are 186, 181, 

 and 174, respectively. The annual values of the recorded potentials for 1917 

 to 1919 are 151, 158, and 146 volts, respectively. 



Harmonic analysis of the data shows a larger amplitude for the 24-hour 

 wave than for the 12-hour wave, except in the summer months. It is also 

 found that with the exception of the summer months, the time at which 

 the 24-hour wave reaches its maximum, is in general accord with the results 

 obtained from the ocean observations on the Carnegie (see pp. 262-263). The 

 mean phase angle of the 12-hour wave at local midnight varies with the time 

 of year and is of the order of 160° to 190°. 



Magnetic east-west paths around the earth. W. J. Peters. Terr. Mag., vol. 28, 83-88 

 (September 1923). 



In the Preliminary Analysis of the Earth's Magnetic Field for 1922^ it 

 was found desirable to ascertain how an east-west path around the Earth 

 would close that was everywhere throughout its length perpendicular to the 

 direction of the compass-needle, or to the horizontal component of the Earth's 

 magnetic field. The considerations involved and the method adopted in 

 ucting such paths are given. 



' Chief results of a preliminary analysis of the Earth's magnetic field for 1922, Louis A. 

 Bauer. Terr. Mag., vol. 28, pp. 27, 28 (March-June 1923). 



