210 



REPORT 1895. 



the month. That the second object has been very satisfactorily accom- 

 plished, so far as a five years' survey is concerned, will be seen from the 

 following table. The figures it contains are the intervals, in days, from 

 the beginning of the month to noon of its mean 'quiet' day, the 300 

 ' quiet ' days being grouped under the months to which they belong. 



Table I. 



In one of the 300 ' quiet ' days the Ivew record was defective and has 

 been omitted. Its omission reduces the entry in the table under August 

 from 15-6 to 15-0. 



The departures from the theoretically exact figures, 15 "5 for months 

 of thirty-»ne days and 15 for months of thirty days, are so small, con- 

 sidering the uncertainties arising from other sources, that I have decided 

 to neglect them in getting out annual variations. Equal weight has also 

 been allowed to each month. 



§ 3. In 1890 the diurnal variation at Kew was got out from measure- 

 ments at the hours ' 1 to 24, counting from midnight. What has been 

 styled in the Kew • Report ' ' solar diurnal range ' meant in that year 

 the departure of the hourly values from the mean for the day taken as 



{[l] + [2]4- 



+ [24]} /24; 



where by [w] is meant the value answering to the nth. hour after the first 

 midnight. In the subsequent four years measurements were taken at 

 the first as well as the second midnight, and the mean for the day was 

 taken as 



{[0] + [l]+ .... +[23] + [24]}/25. 



This second method of fixing the daily mean is of course not mathe- 

 matically correct, as it attributes double weight to the midnight value ; 

 but the departure of the midnight value from the mean for the day — at 

 all events when inequalities are got out only for summer, winter, and the 

 whole year — is too small to cause appreciable error. The error in defi- 

 nition, if error it can be called, was, I think, fortunate for several reasons. 

 Tor instance it left no additional measurements to be made for the present 

 inquiry other than those at the first midnights of the sixty ' quiet ' days 

 of 1890. 



If on-cyclic Nature of Results obtained from Quiet Days. 



§ 4. During the five years considered, the westerly declination at 

 Kew has been diminishing by about 6''9 annually, the horizontal force 

 increasing by about 195 x 10"'^ C.G.S. units. Thus on an average there 

 occurred in twenty-four hours a decrease of about 0'-019 in declination, 

 an increase of about 53xl0~* C.G.S. units in H.F. (horizontal force). 

 Now at Kew, declination is measured only to 0'"1, and H.F. to 1 x 10~^ 

 C.G.S. units. It is thus obvious that if two sets, each of 150 days, and 



' As in tlie Kew Reports, Greenwich, not local, time is employed. Local time is, 

 however, only 1 min. 15 sec. later than Greenwich time. 



