1904- No. 2. THE DIURNAL VARIATION OF TERRESTRIAL MAGNETISM. 25 



and the chart of the equipotential lines which Von Bczold has constructed 

 upon the basis of Schuster's calculations," is so striking as regards the nor- 

 thern half, that we may well assume the existence in this case of a fully- 

 developed circular system of currents, of which the centre is to be found 

 at a little above 30° N. Lat. If the values of JX and J Y for Zi-Ka- 

 Wei, given in Table 2, be set out graphically, we find that both the hori- 

 zontal components simultaneously have the value o at 11 "8^ a. m. This 

 indicates that the centre should be situated upon the parallel of Zi-Ka-Wei 

 itself, 31° 12' N. Lat. 



Those current-arrows on which the number of feathers is 3 or more, 

 mark, as will be observed, a maximal zone for the quantitative influence 

 of the current; and this maximal zone forms, as it were, a central axis 

 in the current in very much the same manner as the maximal zone for 

 the velocity of the wind in an atmospheric cyclone. The situatfon of this 

 maximal zone in the current-system illustrates very well those peculiarities 

 in the diurnal variation of the deflecting force A in various degrees of 

 latitude, that were pointed out above (pp. 20 & 21). 



If we now consider those parts of Fig. 4 which lie on each side of 

 the primary system of currents here described, we receive the impression 

 that the current, both in the early morning and antemeridian hours, and 

 in the late afternoon and evening hours, has — to continue the same 

 meteorological figure — an anti-cyclonic course. It must however be 

 remembered that the numerical value of JX and JY, as shown by 

 Table 2, in the case of all the stations here under discussion, is very 

 small at that part of the day when the sun is low or below the horizon, 

 and that therefore the determination of the angle a and of the current- 

 direction deduced therefrom, becomes more uncertain. 



The system of currents here graphically represented might then, in 

 my opinion, be regarded as a form for the direct influence of the sun 

 during the 12 hours when its beams do not reach the magnetic north 

 pole. What modifications the system would undergo, both as regards the 

 direction of the current and its quantitative influence, from the moment 

 when the sun rises above the horizon of the magnetic north pole, might 

 be studied by taking the stations on that portion of the hemisphere which 

 I have called the northern polar section, and comparing the current-direc- 

 tions there deduced from the observations, with the current-directions for 

 the corresponding hours on the parallel of the station in question, taken 

 from Fig. 4. I have, however, in the mean time left this part of the sub- 

 ject untouched, as the existing material in the way of observations is 

 scarcely adapted for an exhaustive treatment of the question. 



