ig04. Xo. 2. THE DIURNAL VARIATION OF TERRESTRIAL MAGNETISM 29 



for Pawlowsk, 90 a. m. and y2 p. m. 



» Vienna, 9*4 » » yj > 



» Tiflis, 90 » » 3'4 * 



» Zi-Ka-Wei, 78 » ■ y6 » 



The antemeridian hours for Pawlowsk, Vienna and Tiflis, lie ver}- 

 close together, their mean being 91 a. m., while the hour for Zi-Ka-Wei 

 differs from them by more than an hour. This may be in some wav 

 connected with the fact that the sun does not set below the magnetic- 

 polar horizon before 8'5 a. m.. Zi-Ka-Wei time, while at the three other 

 stations it has already done so at, respectively, 24. v^. and 3'4 a. m.. 

 local time. 



In the afternoon, the hours for //Z = o lie more regularly bet«-een 

 3 and 4; but it should be remembered that the sun has risen above the 

 magnetic-polar horizon at 24 p.m. at Pawlowsk, and r; p.m. at Vienna, 

 and at Tiflis is just rising at 3'4 p. m., but does not rise at Zi-Ka-Wei 

 until 8"5 p. m. 



If, therefore, regard is to be paid only to those obsen^atioDS which 

 are absolutely uninfluenced by the circumstance that the sun is above the 

 magnetic-polar horizon, we may from the above, in connection with what 

 Fig. 4 shows, draw the following conclusion: 



On its way northwards, the current passes at 11 '8 a. m., Zi-Ka-Wei 

 time, through the zenith of a place situated east of Zi-Ka-Wei on the 

 same parallel of latitude, with a difference in longitude of 38 hrs. = •;-°. 

 At the same moment, that part of the current travelling southwards, takes, 

 in the tract between the parallels of Pawlowsk and Tiflis, more or less 

 of a meridian direction, which has a westerly difference in longitude from 

 that of Zi-Ka-Wei of 27 hrs. = 40-5°. 



The height of the current above the surface of the earth may then 

 be calculated for the following 4 points at ir8 a. m., Zi-Ka-Wei time: 



Difference in Longitude 

 ^^^^^ from Zi-Ka-Wei. 



A. 31° 12' (Zi-Ka-Wei) S7° E 



B. 41° 43' (Tiflis) 40-5° ^V 



C. 48° 15' (Vienna) 40*5° W 



D. 59° 41' (Pawlowsk) 40-5° w' 



Calling the arc of the great circle between Zi-Ka-Wei and one of 

 these places b, the vertical height of the current above that place h, and 

 the radius of the earth ^ = 6370 km., we obtain 



h^ R sec Ö — R. (i) 



