21

C.-Ia

Magnetic Survey. The correction of the field observations obtained since 1899 to the same epoch has now been practically accomplished by me, the method used being, in the case of field-work since 1902, to compute the actual values at the Christchurch base at tbe time of the field observation, thus establishing the true station difference and enabling the values to be corrected by means of the known secular or annual change at Christchurch. In the case of older stations, made when the Observatory was not yet operating, a simple correction for annual change and mean diurnal change alone could be applied. The work of discussion of the results obtained is now being carried out with the collaboration of Professor Farr and Mr. D. B. Macleod, and we expect to obtain valuable results from the discussion of disturbance values. Diagrams. —There are published herewith twelve magnetograms exhibiting the most pronounced magnetic storms of the year. It has been found impossible to publish the vertical-force tables for the year, as my time has been more than fully occupied with survey data and in getting the H and D tables ready and in computing data for the vector diagrams, which work could not be undertaken until the latter tables, on which they are based, were finished. From tbe diagrams it is easy to see the times of maxima and minima of declination and horizontal force and of the northerly and easterly components. The hours on the curves are Greenwich civil time, Oh. corresponding to 11.30 a.m. here, local noon being at Oh. 30m. on the curves. From the declination diagrams it appears that there is an afternoon principal maximum occurring at about 2 p.m., New Zealand, and a principal minimum in the early morning at about 8 a.m. in summer, but not until 10 a.m. in winter. There is a secondary minimum shortly before midnight, about 11 p.m., which is most evident in winter-time. A secondary maximum occurs at 3 a.m. in equinoctial months, but becomes later in winter, being then at about 4.30 a.m. There is much more resemblance between the summer and equinox curves than between the winter curves and either. Winter variations are thus peculiar, probably owing to the great northerly zenith distance of the sun at midday. The student will find that the horizontal-force diagrams given exhibit similarly maxima and minima and amplitudes varying with the season of the year. The vector diagrams show very clearly the greater range of variation in summer and the smallness of changes during the nighthours. A complete theoretical discussion of these curves and of those of former years is contemplated. Had time permitted, range curves for the hours would have been made out, and would have proved instructive. I wish to return thanks to the donors of many and various scientific publications received, and I have also to acknowledge the willing assistance rendered me by Mr. Maben in the work of the year. Henby F. Skey, Director.