158 RECORDS OF DISTURBANCES 



but it was much less uniform and was interrupted by numerous oscillations of considerable size. Again, 

 while the force at Falmouth continued to fall on the whole until 8.30 p.m., a marked rise commenced at 

 about 6.30 p.m. at the other two stations. At Mauritius and Colaba the most rapid and striking moments 

 are those occurring from 8 to 10 p.m. 



In this case the correspondence is much closer between the Colaba and Mauritius curves than between 

 either of these and the Falmouth or Kew curves. At first sight the Colaba and Mauritius curves seemed 

 to present a number of opportunities for comparing amplitudes at the two stations, but on further 

 investigation this did not prove to be the case, the different elements not showing convenient synchronous 

 turning-points. 



ANTARCTIC DISTURBED CURVES. 



63. In the Antarctic, as already explained, all three elements and the temperature were recorded on 

 one sheet. Thus at times of more than usual disturbance there was apt to be a good deal of intercrossing 

 of curves, especially those of Declination and Horizontal Force. As a rule, however, the traces from the 

 different elements differed in appearance to some extent, one being thicker or less exactly in focus than 

 the others. Thus it was seldom that any serious doubt existed as to the identity of any particular 

 portion of trace. The Vertical-Force trace was so much less open than the others, i.e., the sensitiveness 

 was relatively so low, that it usually showed very little range of movement. Even at times of high 

 disturbance changes of ordinate exceeding 2 cm. occurred very rarely. Thus the Vertical-Force trace, 

 VV in the plates, was almost always easily distinguishable. It bore at times a resemblance to the 

 temperature trace TT which had the same base line V V but the movements in these two elements at 

 times of magnetic disturbance were of such absolutely different types that confusion was impossible. 

 One feature that often served to distinguish between the Declination trace DD and the Horizontal-Force 

 trace HH was that the former was shown freely from one edge of the sheet to the other, whilst the latter 

 became invisible at a considerable distance below the top edge of the sheet. This was due, as already 

 explained, to the light being eclipsed by the Declination instrument. Exactly where the trace became 

 invisible depended on the intensity of the light and the photographic conditions. 



Whenever a trace died out gradually near the top of the sheet one knew it was that of the Horizontal 

 Force. In the reproductions in the plates the letters D, H, and V appear whenever there seemed any risk 

 of confusion. The suffixes 0, as before, distinguish the base lines. 



In the originals the base lines were interrupted for a short interval every hour. These breaks indicated 

 hour intervals with the usual precision, but the eclipsing arrangement was not usually so set as to act 

 exactly at the hour of local (or Greenwich) time. To determine the absolute time one had to refer to the 

 times of starting or stopping the registration. Mr. BERNACCIII always entered these times in a notebook, 

 and he transferred them with his estimate of the error of the watch with which they were taken to the 

 sheets themselves. Suppose, for instance, the exact local time of starting to be 10.30 a.m., and 15 mm. 

 trace to be recorded before the first break began, and suppose the interval between the left- (or between 

 the right-) hand sides of the hour breaks to be 20 mm. (i.e. 1 mm. to 3 minutes). Then the time to which 

 the left-hand side of the first break corresponded was lOh. 30m. + i& x 60 minutes, or llh. 15m. 



Supposing the magnetograph clock to have a negligible rate, the breaks in the above case would all 

 commence at 15 minutes after the hour throughout the 24 hours, and the cross lines indicating the exact 

 hour would all be drawn at 15/3 or 5 mm. to the left of the left-hand side of each break. 



Usually regard was had mainly to the time of commencement of registration. The clock rate was 

 normally very small, but on a few occasions when it was large, or when there was some special cause of 

 uncertainty, use was also made of the time of stopping. Of course the position of the last hour mark 

 relatively to the end of the trace was always noted, but very exact measurements were not made unless 

 there was some apparent inconsistency. 



What accuracy can be claimed for the hour marks it is a little difficult to say. Whether owing to 

 variations of temperature which were unusually large in the magnetograph room, or to differences in 

 the shrinking or stretching of the photographic paper, the interval between the hour marks varied slightly 

 from day to day, and was seldom exactly 20 mm., usually somewhat less. There were even sometimes 



