154 



RECOEDS OF DISTIKi: ANU-s 



Further, ingenious as the Schmidt machine is, no ordinary operator can produce with it an absolutely 

 perfect copy of the original. It is also not so well adapted for use with the broad trace of the Kew 

 pattern magnetograph as the finer trace of the Eschenhagen instrument. It can deal, moreover, much less 

 satisfactorily with rapid to-and-fro movements than with those of a rounded character. Thus Plates XIV 

 to XXI are more appropriate for general descriptive purposes than for any minute investigation of details 

 with a view to numerical calculations, and no use has been made of them for the latter purpose. Partly 

 for this reason, and partly with a view to avoiding an excessive multiplication of lines, the ordinate scales 

 are shown only at the left-hand margin of the curves. The Horizontal- and Vertical-Force scales varied 

 slightly with the time, but the following values are sufficiently close considering the nature of the 

 curves : 



At all four stations the Horizontal-Force curves were those which were most affected by disturbances, 

 whilst the Vertical Force ones were the least affected. Thus the curves shown are mostly those of 

 Horizontal Force. 



As indicated by the scale figures and by the arrows, there is considerable variety in the procedure 

 adopted at the different stations. At Falmouth and Colaba, as at Kew, movement up the sheet denotes 

 increase in all three elements. At Mauritius, movement up the sheet means increase in Declination, but 

 diminution in Horizontal or Vertical Force. At Christchurch, movement up the sheet means decrease in 

 all three elements. In deciding, as I eventually did, not to invert any of the traces, I was influenced by 

 several considerations. 



In the Antarctic curves, which it was decided to reproduce photographically the difficulty of 

 obtaining reasonably accurate results with the Schmidt tracer appearing excessive movement up the 

 sheet meant increase only for the Vertical Force. The three elements were in this case all on one sheet, 

 and to have retained one unaltered while inverting the others presented serious difficulties. Thus 

 absolute uniformity of procedure was unattainable unless one was prepared to take a great deal of 

 trouble, and to take much trouble appeared justifiable only if it were clear that the result was a decided 

 gain. On the other hand, reflection showed that in any case only a semblance of uniformity was 

 obtainable. Declination is westerly at Falmouth and Mauritius, but easterly at Colaba and Christchurch, 

 thus an increase in the element means movement of the north end to the west at the two former stations, 

 but to the east at the latter two. In the Antarctic Declination was easterly, but exceeded 90 ; thus 

 increase of the element meant increase of easterly Declination, but movement of the N. pole to the west. 

 At Falmouth and Colaba, where -the N. pole dips, increase of Vertical Force means an additional force on 

 the N. pole directed towards the Earth's centre ; but at Mauritius and Christchurch, where the S. pole 

 dips, it means a force urging the N. pole from the Earth's centre. As regards the Horizontal Force, 

 the magnetic Meridians of the various stations varied from 18 W. at Falmouth to 16 E. at Christchurch 

 and 152 E. in the Antarctic. Thus the planes in which the Horizontal Force is measured were related 

 in very different ways to the local astronomical Meridian. 



Finally, there was the consideration that horizontal and vertical, cast and west, are terms of purely 

 local significance, and indicate directions whose relations to any one system of rectangular axes vary 

 with the latitude and longitude of the station. 



57. Plates XIV to XXI follow the chronological order. Plate XIV (compare Plate XXII) shows a dis- 

 turbance which seems to have been felt all over the world. It lasted for practically 8 hours, the termination 

 being nearly as clearly indicated as the commencement. The three lower, Horizontal-Force, curves show 



