DIURNAL VARIATIONS OF TERRESTRIAL MAGNETISM. 
11 
It may also be noticed that the comparison between the calculated and observed 
variations in his paper showed a much more satisfactory agreement for the West 
component than for the other two. This seems to be partly a consequence of 
greater freedom from local irregularities in this element, and is confirmed by the 
results of the present investigation. 
§ 4. G. W. Walker’s Investigation (1913). 
The investigation by Walker was confined within narrower limits than 
Fritsche’s, both in respect of the data used and consequently also in their analysis. 
The data consisted of the annual mean solar diurnal variations of the North, West, 
and vertical components of force at nine observatories, and the components having 
periods of 24 and 12 hours were alone considered. The nine stations ranged in 
latitude from 60° N. to 6° S. While in every case the observations were of recent 
date, they did not all refer to the same year. It appeared that a potential function 
of simple type (Qd and Q 3 2 for the 24- and 12-hour periods,* respectively) could be 
fitted fairly well to either the West or North force data separately, but that the 
same numerical coefficient would not apply to both. This was taken to indicate that 
the assumption of a potential function, at any rate of one depending solely on local 
time, was invalid. Schuster and Fritsche, using the two components together, and 
including a considerable number of harmonics in their analyses, did not notice such a 
discrepancy (cf., however, the last paragraph of § 3). 
Walker tried to overcome this difficulty by introducing harmonic functions not 
dependent solely on local time, and in this way he obtained a better representation of 
his data. But the crucial test of the existence of such additional harmonics must 
consist of the examination of data from stations of widely different longitudes, and, 
unfortunately, seven out of the nine stations used by Walker lay between 3° W. 
and 31° E. The data of the present paper, which had been collected before the 
publication of Mr. Walker’s paper, were chosen with a view to a decision upon this 
question,! and are from fairly widely distributed stations. While the simple 
potential functions of type Qd, Q 3 2 do not well represent the North force data, 
the evidence for the existence of any considerable harmonics not depending on local 
time does not appear to be strong. For the components of period 12 hours or less, 
I am inclined to attribute the North force discordance to local irregularities, while 
leaving the question open in the case of the 24-hour component. 
The terms in Walker’s representation which depend on local time and are 
symmetrical about the equator are compared in § 10 with the corresponding annual 
terms obtained by other writers. The phases agree well, and the amplitudes found 
* These are the main annual terms of these periods which were found also in the other investigations 
summarized in Table D. 
f In consequence of a suggestion made by Schuster in his second memoir, p. 172. 
c 2 
