FROM ORDINARY DAYS OF THE ELEVEN YEARS 1890 TO 1900. 
197 
The fall of temperature usually continued throughout January, but the readjust¬ 
ments of the magnetograph were usually made in that month leading to special 
uncertainties in the n.c. changes. 
Diurnal Inequalities. 
§ 5. The diurnal inequality of a magnetic element is in continuous variation with 
the season ol the year ; it also varies according to the development of sunspots, and 
it depends on the more or less disturbed character of the day. There are most likely 
other causes of variation ; for instance, it seems unlikely that the diurnal variation at 
a particular station remains wholly unaffected by the secular change in the earth’s 
magnetism. In deciding on the amount of detail advisable in the presentation of the 
facts, several conflicting considerations have to be allowed for. There is a great deal 
which in the present state of our knowledge must be regarded as accidental in the 
magnetic changes on any individual day. If we derive inequalities from the combi¬ 
nation of a very limited number of days, a good deal of this “accidental ” element will 
remain uneliminated. If, on the other hand, we combine a large number of days from 
the same year, the inequality is inevitably a blend of more or less conflicting 
characteristics. The extent to which this is the case differs at different seasons of the 
year. There is, for instance, much greater variation in the type and amplitude of the 
diurnal inequality in the five months November to March than in the five months 
April to August. If we have at our disposal the data from a large number of years, 
we can get smooth diurnal inequalities for individual seasons of the year a good deal 
shorter than a calendar month. This is what we should naturally do if our object 
were to examine in very minute detail the mode of variation of the diurnal inequality 
throughout the year. It is, however, open to the objection that it would produce a 
mass of detail which few readers if any could digest. The object in view moreover 
might be to some extent defeated by the influence of the sunspot relationship and 
secular change effects. Considerations of space must also be borne in mind. Diurnal 
inequalities for each of the 132 months of the eleven years—let alone shorter periods 
of the year—for H, V, N, W, T and I would have entailed printing an immense mass 
of figures. As a matter of fact, diurnal inequalities were calculated for H and Y 
from each of the 132 months, but not for the other elements. The H and Y ranges 
from these 132 inequalities are given later, but it was decided to publish for each of 
these elements only four tables of diurnal inequalities. Thus, in the case of H, Table A . 
gives the 12 diurnal inequalities obtained by combining all the months of the same 
name in the 11 years, three diurnal inequalities for the seasons winter (November to 
February), equinox (March, April, September and October) and summer (May to 
August), and finally the mean diurnal inequality for the whole year. Table YI. 
differs from Table Y. only in that it is confined to the four years 1892 to 1895 repre¬ 
senting large sunspot frequency ; for brevity, it is described as referring to sunspot 
maximum. Table YII. is similarly confined to 1890, 1899, and 1900, and described as 
2 e 2 
