CIIKISTCIHTRCH TERM DAY o|;SKl;V. \TIOXS. 231 



two last significant figures. Three were necessary only when, as sometimes happened, II was rising 

 or falling in the immediate neighbourhood of the value -22700. In the case of D the value always 

 lay between 16 and 17 east, thus minutes only are recorded except at the beginning and end of 

 each hour. 



Before drawing conclusions from the term-day data it will be convenient to consider the nature of the 

 regular diurnal variation. 



5. Christchurch is a comparatively new magnetic station, and particulars of the regular diurnal variation 

 there do not seem as yet to have been published. There is thus no direct evidence that the hourly 

 readings on term days, pp. 177-179 of the " Physical Observations," can be regarded as representative 

 of undisturbed conditions at Christchurch. This is, however, the conclusion to which the character of the 

 results points, and it seems sufficiently confirmed by the character of the synchronous results from 

 Greenwich, Kew, Falmouth, Pola, Bombay, and Mauritius, stations whose general characteristics are 

 known. Considering the limited number of days, perfectly smooth diurnal inequalities could not be 

 expected from the Christchurch data, even for the year as a whole. There being only two days a month, 

 diurnal inequalities for individual months would clearly have been too uncertain. Examination of the 

 data showed, however, that, as is usual in corresponding northern latitudes, the variation in the type 

 of the diurnal inequality throughout the year is not large, and that sub-division of the year into Midwinter 

 (May to August), Equinox (March, April, September, October), and Midsummer (November to February) 

 would combine months in which the type was closely similar and the amplitude not widely different. 

 Diurnal inequalities for D and H were thus derived for the three seasons and the year. In the case 

 of V the apparent range was so small that, taking the discontinuities into account, the results appeared 

 too uncertain for publication. 



Referring to "Physical Observations," pp. 177 and 178, it will be seen that data are lacking for the 

 second midnight of November 1, find for hours 10 p.m. to midnight on September 15. These missing 

 values were replaced by interpolation, in such a way as not to influence the conclusions one would have 

 derived from the remaining days as to the diurnal variation during the hours of the day concerned. This 

 seemed preferable to omitting two entire days from so small a number. As is generally the case when 

 diurnal inequalities are derived from a limited number of selected days, there was an appreciable non-cyclic 

 element. This was eliminated in the usual way. 



The diurnal inequalities resulting for D and H are given in Table IA, p. 238. It will be noticed that the 

 hours are numbered 1 to 24. The reason for this is as follows : The usual practice is to refer diurnal 

 inequalities either to local mean time or to the standard mean time of the country where the observatory is 

 situated. Thus the natural thing would have been to have employed standard New Zealand time, counting 

 hours 1 to 12, a.m. and p.m. But standard New Zealand time is 11^ hours fast on Greenwich, so that 

 what is a.m. in the one is p.m. in the other. And as G.M.T. with hours 1 to 12, a.m. and p.m. was 

 employed in the Christchurch tables in the " Physical Observations," a similar nomenclature with New 

 Zealand time substituted would have been extremely apt to create confusion. I accordingly decided 

 to employ Greenwich time, but to count the hours 1 to 24, which was really the course proposed in the 

 international term-hour forms. Thus in the tables of this Appendix, hours 6 and 18, for example, are 

 equivalent, respectively, to 5.30 p.m. and 5.30 a.m. New Zealand time. In the discussion a.m. and p.m. 

 mean forenoon and afternoon in New Zealand. 



6. The principal features disclosed by Table IA in the case of Declination are as follows : At all seasons 

 there are two distinct maxima and two distinct minima distinguished by figures in heavy type. There 

 is thus a distinct double daily oscillation, but the oscillation during the night (say from hours 7 to 19 in 

 Midwinter, and 8 to 17 in Midsummer) is trifling compared to that during the day. 



To bring out minute details of the night movements, records would lie necessary from a much larger 

 number of days. The extreme westerly position of the needle is reached from 2 to 3 hours before noon. 

 As is usual in the southern hemisphere, the most conspicuous movement during the day is the swing over 

 to the cast. The extreme easterly position is reached 2 or 3 hours after noon, and so later than is usual 

 for the corresponding westerly maximum in Britain. As at Kew, the inequality derived from the four 

 equinoctial months closely resembles both in type and range that derived from the whole year. The 



