PART I. ON MAGNETIC STORMS. CHAP. II. 63 



THE POSITIVE EQUATORIAL PERTURBATION. 

 THE PERTURBATION OF THE 26th JANUARY, 1903. 



PI. 'XIV. 



27. For the study of this perturbation, there are magnetograms from all the stations. As the 

 curves show, only the latter half of the perturbation has been obtained at most of the European stations. 

 The perturbation appears quite suddenly upon a quiet day. It begins at 8 h 52, and lasts until 

 14'' 2O m . (The time, when not otherwise stated, is Gr. M. T., o 1 ' = midnight). 



It is particularly well developed and well defined in the equatorial regions; its effect is not con- 

 fined to any single district, but it appears all round the equator. If, for instance, we look at the curves 

 for Dehra Dun, Batavia and Honolulu, we see that at these three places the perturbation agrees down 

 to the smallest details. We further notice immediately that it appears only in the horizontal intensity, 

 and in such a way that all the time the perturbing force is directed northwards, i. e. in the direction 

 of the magnetic meridian. 



If we pass from the equator towards the poles, we see that the character of the perturbation is 

 maintained, the only difference being that the deflections become a little smaller. As far south as 

 Christchurch, which is our most southerly station, and as far north as Toronto in America, and Stony- 

 hurst and Pawlowsk in Europe, the perturbation preserves in the main its character of appearing only 

 in the horizontal intensity. When we come, however, to our most northerly stations, we find that it 

 also appears in the declination, which means that here in the north the direction of the perturbing force 

 is no longer along the magnetic meridian. At the same time, the average deflection becomes con- 

 siderably less for these stations. This, together with the more disturbed course of the curve, makes it 

 difficult to measure the perturbing force. The perturbation here acquires to some extent the character 

 of marked oscillations about the mean line. 



In glancing at the curves, we also notice at once their jagged character during the perturbation, 

 answering to a great variability in the strength of the perturbing force. If we compare the serrations 

 in the curves for the various stations, we find them to a great extent repeated from place to place. 

 We further notice that as we approach the poles, the serrations become more acute and larger, and of 

 a somewhat local character. A sudden change in the curve answers to a great change in the pertur- 

 bing force, which again must be produced by a great change in the perturbing impulses. 



It might now be asked whether these perturbing impulses reach the various parts of the earth 

 simultaneously, or whether they require an appreciable time to be transmitted from one station to 

 another. 



The very fact that the serrations can be distinctly identified in the different curves, makes it 

 natural to expect that they appear simultaneously, as it would be difficult to imagine that an impulse 

 of this kind during a comparatively slow motion, could preserve its character unchanged. 



In order to throw light upon this circumstance, we have reckoned the times at all the stations, 

 for a series of points that allow of easy identification. The result is given in the Table below, where 

 the points are indicated by the numbers i, 2, etc., and will be found marked on the curve for Dehra Dun. 



The following table shows that the time varies so little with the geographical position, that it 

 would be premature to draw conclusions from it.. The slight differences that appear rather irregularly, 

 may be ascribed to inaccuracies in the determinations of time on the magnetograms; for we see that if 

 a difference in time for a certain point appears between two places, this difference is maintained for all 

 the points, a circumstance which seems best to be explained by an inaccuracy in the statement of the 

 time. We may conclude from this that the serrations appear simultaneously, or rather, the differences 

 in time are less than the amount that can be detected by these registerings. Characteristic serrations 

 such as these may therefore often be of great use in controlling the time of the magnetograms. 



