39

o.—la

Mean Monthly Curves of Diurnal Change, 1914, with Seasonal Curves. These curves are shown plotted for declination and for horizontal magnetic force. They are of the same character as those for 1913, and those for declination call for little comment; indeed, their resemblance to the declination curves for 1,913 is startling, when one considers that all days are included, stormy or calm. The declination curve for July, 1914, is evidently not so smooth as the one for 191.3, printed in the previous report, and the same remark applies to the July curves for horizontal force. This want of smoothness has made itself evident to some slight extent in the mean winter vector diagram of horizontal disturbing forces. The tendency to an invagination of the curve at 19 h. in the mean winter curve still persists, and this slight effect not being masked gives one confidence in the reliability of the mean winter curves and the deduced winter vector diagram. In the corresponding H diagram also a slight peak persists at 20 h. In 19.1.4 the February curve in H ends with a downward trend from 23 h. to 24 h., whereas in 1913 the trend is upward between these hours, corresponding in appearance rather with the termination of the 1914 January H curve. In HF also, in February, 1914, a decrease is shown from Oh. to 1 h., whereas in 19.13 an increase is shown in February between those hours, as has happened in January in both years. In May, June, and July, from Oh. to 1 h. G, the decrease of II shown in the 1913 curves lias diminished in the 1914 curves It was small for May, 1913, and has become a slight increase of Hin May, 1914. Curves oe Mean Diurnal Ranges. These two diagrams show the mean daily amount of change of declination, and of horizontal magnetic force, for each month of the years 1913, 1914. For convenience in reading off values the ordinates of the 1914 curves are drawn in. The diagrams are instructive, showing clearly the great difference in diurnal range between summer and winter, and the continuity of this phenomenon from year to year. The curves for the two years roughly approximate to each other, more especially the two declination-range curves. The minimum mean diurnal range of declination occurs in June, and the maximum in December, or at the depth of winter and the height of summer respectively. In the HF curve, however, it is remarkable that the minimum range seems to occur in May, with a secondary minimum in July and a third in September. It will be interesting to find whether this phenomenon will persist in future years, for if it persists it will show May, July, and September are less stormy months than June and August in horizontal force, although in regard to declination the coincidence of the two curves at the June ordinate seem to indicate a probable " quietness " in June in declination. It requires many years' results, however, to yield reliable evidence upon such questions, and it requires consideration not of declination and HF ranges, but, of force-ranges in the N, E, and V components. The large peak in HF range for November, 1914, indicated in the diagram is possibly due to greater storminess in November, 1914, than usual, combined with a less than usual degree of storminess in December, 1914. The general agreement of the curves for the two years is very satisfactory, and would not be evident unless the instruments were functioning well. Magnetograms reproduced. Twelve magnetograms are reproduced, showing records for tAventy-four days. Some of these are shown simply as examples of stormy days during the year ; others are typical examples, showing remarkable features not confined to these days. HF, 21st and 22nd February : Two almost " calm " days. The mean value for the 21st is 0-22123. On the 21st the curve is that of an undisturbed day, but the 22nd shows a sudden commencement of disturbance just before 12 h. ; the HF has been lower than on the 21st for the preceding six hours, and the sudden increase, though it tends immediately to diminish, persists for several hours afterwards, gradually diminishing. The mean for the day is 0-22421, very slightly lower than the mean for the month. Shortly after I4h. on the same day a similar sudden stop in the opposite direction is noted, followed by a uniform, increase of force, and similar phenomena recur in a less marked degree at 17 h. to 18 h. Dec, 3rd and 4th March : These exhibit the occurrence of pronounced peaks of minimum declination occurring within some few minutes at the same hour on successive days. Dec, 6th. and 7th April; HF, 6th and 7th April: The most stormy day of the year, involving a range of about |° of declination, and 1.00 T in horizontal force. This storm commences and ends in a gradual manner. The rapid change of compass-direction, over more than |° in about a quarter of an hour, occurring at 16| 1)., is especially remarkable. Dec, Bth and 9th April : Remarkable peaks of small declination occur between 5 h. and 9 h. on the Bth April, followed seven hours after by a sudden considerable increase of declination, which gradually disappears in the course of a few hours. Dec, sth and 6th July : On the sth July a large sudden decrease of declination occurs, which gradually disappears in the course of three hours. It is possible that a displacement in this direction is annulled more rapidly than in the case of a sudden increase of declination. Dec, 29th and 30th July : On the 29th is apparent a storm of comparatively short duration, with a large range, for most of the time in one direction. On the 30th another sharp commencement of a disturbed period, commencing with a phenomenon similar to that on the sth July. Dec, HF, 2nd and 3rd August : These curves exhibit remarkable sharp peaks. On the 2nd these approximately coincide for D and H, but on the 3rd a rapid diminution of easterly force alone