194 Mr. W. Ellis. On the Simultaneity of 



photographic registration was not commenced until the year 1885, 

 and it thus became necessary to omit Pola in the comparision. It 

 might be supposed that, in giving the Greenwich time of movement to 

 the nearest hour only, it would be possible in some cases to find on the 

 registers at other observatories several movement sufficiently near, 

 any one of which might be taken, and thus not be a corresponding 

 movement in the sense meant. But the movements in question, so 

 abrupt in character, are neither numerous nor easily mistaken. 

 Their character is well illustrated in the annexed reduced copy of 

 the Greenwich registers for 1884, October 1 (one of the selected 

 days), in which it will be perceived how in all elements, declination, 

 horizontal force, vertical force, and earth currents, a definite and 

 rapid movement occurs just before 22h., and simultaneously in all ; 

 that in horizontal force corresponding indeed to an increase in the 

 earth's horizontal force of about l/200th part. It will be thus under- 

 stood that the really corresponding movements would be readily 

 identified at other places ; indeed the returns received show that no 

 difficulty was experienced, the suddenness of movement being a 

 matter of common remark. For instance, at Toronto it is said that 

 " the initial movement was always very sudden," at Mauritius the 

 movements are spoken of as " remarkable jumps," at Bombay as 

 " abrupt and unmistakeable at the beginning," and at Batavia the 



commencement was "in all cases very abrupt." In the 



Zi-ka-wei curves, so kindly furnished, the same characteristics 

 are to be observed, and the same simultaneous commencement 

 of movement in all elements is shown as in the Greenwich 

 specimen register of 1884, October 1. The certainty with which the 

 selection was made further appears by the circumstance that the 

 stations in addition to Greenwich being seven, and there being 

 seventeen days, 119 identifications had to be made. Not counting 

 six instances in which the corresponding record was not available on 

 account of failure or want of register, it was found that, of the re- 

 maining 113 cases, there were anly four in which there was dis- 

 cordance, and a re-examination of the registers in two of these cases 

 immediately showed that a wrong movement haa been taken. Had 

 there been opportunity for further examination in the two remaining 

 cases, there is little doubt but that the source of the discordance 

 would have been in these also ascertained. All this assures us of 

 the real correspondence of the movements discussed. 



It has been mentioned, in regard to Greenwich and Zi-ka-wei, that 

 the three magnets, decimation, horizontal force, and vertical force, 

 were on all occasions simultaneously affected. The reports received 

 from other observatories show that, at places for which the times for 

 the different magnets are separately given, the magnets were 

 similarly simulti neously affected. This circumstance i;3 peculiar to 



