AND OK TKREE8TBIAL MAGNETISM. 277 



any great skill to find between pairs of them intervals of 27 flays, or of any other 

 imml>er of days which the individual desires. 



The dithVulties in the way of treating disturbances individually exist in at least 

 equal measure in the case of quiet days. On some occasions a quiet time ends with 

 great precipitancy, hut t" say exactly when it commences would usually prove an 

 impossible task. 



I have referred t<> this aspect of the problem l>ecause it is rather a fashion amongst 

 experimentalists to regard statistical enquiries such as the present with suspicion. 

 They an- unable wholly to purge themselves of the popular superstition that statistics 

 can prove anything which the statistician desires. In the present case, however, the 

 popular view is the exact opposite of the truth. The statistics employed are in large 

 part international data, published before the enquiry commenced, and based on 

 estimates of magnetic " character " made independently, at olervatories Mattered 

 IIMT the world, by individuals none of whom had any suspicion of the purpose to 

 which they would be put. The observational data, on the other hand, are usually of 

 so complex a nature, and so influenced by the latitude and longitude of the station, 

 that the observer does not know what to regard as essential and what to consider 

 secondary. Moreover, the record is in nearly all cases photographic. Except in a 

 few of the better staffed observatories, the fact that a magnetic storm has occurred is 

 not known until a day or two afterwards, when the photographic sheets have been 

 developed. If a continuous succession of solar pictures and contemporary magnetic 

 changes could appear side by side during the actual progress of a magnetic storm, an 

 olwerver would have a better chance of framing the right guess as to the nature of 

 the solar link, provided corresponding events on the sun and earth are nearly 

 simultaneous, or are separated by a constant small interval of time. In the case, 

 however, of sunspots and the amplitude of the daily H range at Kew, during the 

 eleven years 1890 to 1900, the results reached in S.M. indicated a clear lag of aliout 

 four days in the magnetic range, and they were at least consistent with a similar lag 

 in magnetic " character." The results of the present paper do not suggest a lag in 

 magnetic " character," but the rate of change of sunspot area near the time of 

 maximum " character," as shown in Table IX. arid fig. 3, is slow, so that the question 

 of lag in " character " is still an open one. If there is a lag, and especially if the lag 

 is of variable amount as might well be the case if cathode rays or electrified 

 particles are concerned the difficulties in the way of direct observation will be 

 materially increased. 



We have seen that magnetic " character " and sunspots have both periods of from 

 27 to 28 days. In some years the phenomena are, so to speak, in phase, in other 

 years not in phase. The period seems better developed in some years than in others, 

 and the years in which it is l)est developed do not seem to be necessarily the same for 

 tin- two sets of phenomena. 



