1904- Xo. 2. THE DIURNAL VARIATION OF TERRESTRIAL MAGNETISM. 



this current remain unchanged in space, independent of the rotatorv mo- 

 tion of the earth, in the course of an hour a place O King on the same 

 parallel as Bossekop, but 15° west of it, will come under the influence 

 of this north-easterly current, while Bossekop by that time is being acted 

 upon by a current from X 67° E or EXE. An hour later Bossekop has 

 come under the influence of a current from X 88° E, or almost due E, 

 while at the same time the place O has come under the influence of the 

 east-north-easterly current, and so forth. 



In order to find out whether, in the period of time here treated of, 

 a system of currents could actually be demonstrated that may be assumed 

 to have remained constant for a greater or smaller number of hours, I 

 have proceeded in the following manner. 



From Table 2, taking the 10 stations whose difference in longitude 

 from that of the magnetic pole is greater than 90°. I have taken out se- 

 parately those values of 5 which fall within the 12 hours during which 

 it is night at the magnetic pole, or in other words, bet^\-een 6 p. m. and 

 6 a. m., magnetic-polar time, which correspond with the hours from 2 a. m. 

 to 2 p. m., Göttingen time. In this way I believe I have eliminated what 

 I have called the indirect influence of the sun. 



All these 120 current-directions are graphically shown in Fig. 4. The 

 figure represents a plane above the earth, at right angles to the earth's 

 axis. The plane is supposed to be fixed in space, and the earth revol- 

 ving beneath it in the direction of the two curved arrows from W to E. 

 The central point answers to the earth's geographical north pole, and the 

 straight lines radiating from it are meridians lying at a distance of 15° 

 from one another. The hours are placed according to the position of the 

 sun, this being marked by an arrow. The parallels for latitudes of 

 ever}- lOth degree are projected up on to the plane by orthographic 

 projection. 



Along the parallel of each of the 10 stations mentioned, I have 

 marked with dots the 12 moments of observation, by local time, that 

 occur bet^veen 2 a. m. and 2 p. m., Göttingen time; and through each 

 dot I have placed an arrow, which represents the calculated direction of 

 the current at the moment of observation, projected up on to the plane 

 after the following formula: 



tan S' = tan 5 cosec cp, 

 where S' denotes the azimuth of the set of the current in the above-named 

 plane, 5 the azimuth of the set of the current in the horizontal plane 

 (Table 2), and y the latitude. Lastly, I have expressed by the number 

 of feathers on the arrows the quantitative efi"ect of the current, each 



