780 Mr. Charles Ghree [March 4, 



exceedingly small. The mean range from the days made up of the 

 10 days a month of largest sun-spot area exceeds the mean range 

 from all days of the 11 years by 0''17, and the mean range from the 

 days composed of the 10 days a month of least sun-spot area falls 

 short of the mean range from all days by 0'-32. The algebraic 

 diiference, 0''49, between these two quantities represents only :V7 per 

 cent, of the mean daily range, while the mean sun-spot area for the 

 one group of days was more than 4 times larger than that for the 

 other. 



To provide for the possil)ility that the solar influence takes one or 

 more days to travel to the earth, mean declination ranges were formed 

 not merely for the 10 days of largest or smallest sun-spot area, but 

 also for the 10 days immediately following these, for the 10 days 

 separated by 2 days and yet again for the 10 days separated by 8 days 

 from the days constituting the sun-s])ot groups. The results appear 

 in Table YIIL, and are somewhat more favourable for an association 

 between the magnetic phenomena and the solar phenomena 2 or 8 

 days previously, tlian for an association between the plienomena on 

 the same day. The I'csults in some individual years seem quite 

 favourable to a considerable influence of the kind sought for, but 

 other individual years actually associate large diurnal ranges with 

 small sun-spot areas. The contrast between 1894 and 1895 in this 

 respect is very striking. 



§ 12. In the preceding discussion declination has been chiefly 

 referred to, partly because it is the most familiar element, and 

 partly because numerical data for it are the most numerous. In 

 some respects, however, declination records during magnetic storms 

 are inferior in interest to those of horizontal force. This latter 

 element exhibits several curious phenomena, one of which is illustrated 

 by a slide showing the traces obtained at several stations on April i^)-C), 

 1903. After a quiet time lasting several hours, the horizontal force 

 curves all show a sudden rise of force, taking only a few minutes to 

 accomplish. There were corresponding sudden changes in the other 

 magnetic elements, but they were smaller. These sudden movements 

 ushering in magnetic disturbances are not uncommon. They can 

 sometimes be traced all over the earth, wherever observatories exist. 

 So far as can be judged, they occur simultaneously wherever recorded. 

 An interesting illustration of this fact is due to Dr. Yan Bemmelen. 

 Comparing the records of 53 sudden commencements of storms at 

 Batavia and Greenwich, and taking the mean result from his 

 measurements of the local times shown by the curves, he found for 

 the difference of local time at the two stations 7h. 7m. 15s., 

 representing a difference in longitude of lOG"" 48' 45 ', while the 

 true difference deduced from ordinary astronomical observations is 

 106° 49' 45". 



The next slide shows the record of this same disturbance (April 

 5-6, 1903) obtained at the 'Discovery's' winter quarters. The 



