C—lα

16

increase of the horizontal magnetic force of about 254y was recorded, with a simultaneous increase in the vertical component of the field of about 3y, followed, however, immediately by a much larger decrease of 10y. The oscillations in the value of the vertical component were very irregular, and for some part of the time of duration of the storm the recording light spot was carried off the recording-paper. The maximum value of the vertical component recorded during the storm was only 20y above normal at 16 h. 55 m., and the storm ceased at about 21 h. 30 m., lasting about ten hours. In the case of the horizontal force, a maximum value 414y above normal was recorded at 12 h. 80 m. Greenwich time, the force then diminishing 652y, or 0-00652 c.g.s. unit, in the course of an hour, the recording spot being then carried off the paper. The total range of the changes in declination during the storm was slightly over l°-5, a marked maximum being recorded just after 13 h. and others occurring between 16h. 40 m. and 18 h. 30m. During the disturbance I he average value of the declination was greater than normal, of the horizontal componenl probably less than normal, and of the vertical component also less than normal, pointing to an average disturbing magnetic force (at Christchurch) directed (somewhat downward) to the wesl of south. The low value of the horizontal component persisted throughout the following day Milne Seismograph No. 16. This instrument has been kept in satisfactory working throughout the year, and a number of the most important seismograms obtained are reproduced herewith. Records were obtained of eighty-seven earthquakes during the year, about the average number recorded per annum. The earthquake recorded here on the 12th June, 1909. commencing al 20h. 25-5 in. Greenwich time, was also recorded at Sydney ami Perl k ((bseiv atoiies. and all t luce seismograms are given herein. It is evident from a comparison of the Sydney and Christchurch records thai the origin of this quake was equidistant Erom I lies.- two places. The almost perfect agreement between Ihe Sydney and ('lt list.cliuioh times of the various phases of the records is very striking. At Christchurch the beginning occui red at 20 h. 25-5 m., the commencement of larger waves al 20h. 28-1 m., and the maximum motion ai 20h. 34-3 m. Greenwich time. Ai Sydney these occurred at 20h: 25-Bm., 20h. 28-2 m., and and 20 h. 7,|-3 in. Greenwich time respectively. It is evident that in this case the times of occurrence of the different phases of mot ion were not influenced by any differing local geological peculiarities, the short, medium, and long waves reaching both places simultaneously, within the limits of observational error. The longer waves evidently travelled from the origin along a path almost entirely overlain by ocean. A comparison of the three records shows thai the earthquake origin, about 23 distant from Sydney and Christchurch, was under the ocean a. few hundred miles to the south of the Royal ('ompanv Islands. The only records obtained of earthquakes felt in New Zealand were those of the 12th November. 1909, felt at Wellington, and thai of the 29th March, two slight shocks being then felt at ('ln istchiirch. For the Milne seismograph a new recording apparatus has been ordered from the makers. This will give a much more open time scale, and yield records of a much more modern type. It has also been decided to instal gas at the Observatory, which will be conducive to even better working ofthe self-recorders. A table of earthquakes recorded during the year is appended. Vectok Diagrams of Mean Horizontal Disturbing Forces at Christchurch. There are appended to this report four Vector diagrams illustrating the changes thai occur in the direction and magnitude of the mean diurnal horizontal disturbing forces, as deduced from the magnetograms for 1902. 1903, and 1904. The curves are drawn for the mean day from all days for the three years, the first curve being for the summer months of November, December, January, and February for the combined three years. The second diagram is the corresponding curve for the four equinoctial months of March, April, September, and October. The third is for the winter months—May, June. July, and August. The fourth is for the year. The curves are affected with diurnal non-cyclic inequality, but as this is of small magnitude, the effect is slight. They are drawn from the hourly values for Greenwich civil hours, as obtained from the magnetograms for those years, and only one stormy day of very excessive disturbance viz.. the 31s1 October, 1903 has been omitted. Investigation showed that the exclusion of other disturbed days did not materially affect the diagrams. The period covered is practically thai of the period of antarctic research of the "Discovery" and other expeditions, and the diagrams will prove of much interest in connection with the study of the published magnetic results of those expeditions. The diagrams are all drawn to the same scale, the line W. E. orN.-S. being taken as 20y, or 0-00020 c.g.s. unit, and the origin is taken at the point of bisection of these mutually perpendicular lines. Greenwich civil hours are indicated by the spots in the curve, numberd oto 23 (hours), and the magnitude and direction of the horizontal disturbing force at any time in the day is given by the straight line drawn from the origin to that point of the curve corresponding to the given time. It is evident from the curves that the maximum westerly horizontal disturbing force occurs here in summer at 21 h., and in winter at 23 h., Greenwich time, or at 8.30 a.m. New Zealand time and 10.30 a.m. New Zealand time respectively ; while the maximum easterly occurs at 3h. in summer and somewhat later in winter- that is. at 2.30 p.m. New Zealand time in summer, and 2.50 p.m. New Zealand time in winter. The3e maxima occur during the equinoctial months at the hours of 9.30 a.m. and 2.30 p.m. respectively. It is noteworthy that the change of seasons practically only affects the time of the maximum westerly disturbing force, which occurs here in the mornings, and leaves i he time of the afternoon maximum easterly practically unaltered.