398 DR. C. CHREE; ANALYSIS OF RESULTS FROM THE KEW lAIAGNETOGRAPHS 
or in one mouth of the 11 years, irrespective of sign, and divide them by the sum of 
the intervals in minutes between successive readings. The resulting quantity, 
measured in seconds of arc, I have called the “ mean change per minute.” The 
results so found for tlie several years, and the several months of the year, are given 
in Table XXXIT. 
Table XXXII. — Variability of the Declination. 
Year. 
Mean change 
per minute. 
Month. 
Mean change 
per minute. 
1890. 
T'oi 
January '. . . . 
3 ''39 
1891. 
4-44 
February.... 
4-97 
1892 . 
5-83 
March. 
4-46 
189.3. 
3-91 
April. 
4-21 
1894 . 
4-95 
May. 
3-03 
189.5. 
4-46 
June. 
3-29 
1896. 
3-24 
July. 
3-68 
1897 . 
3-43 
August .... 
3-34 - 
1898. 
2-93 
September . . 
4-28 
1899 . 
2-95 
October .... 
4-27 
1900. 
2 * 22 
November 
4-35 
December 
3-67 
Mean. 
3-93 
§ 40. If we divide the sum of the changes of reading during the vdiole eleven years 
by the sum of the intervals between the successive readings, we find for the “ mean 
change per minute” the value 3"‘927, which is in practical agreement with the 
arithmetic mean of the means for the eleven individual years in Table XXXII. 
No observation has been omitted owino- to abnormal disturbance, thouo'h some 400 
to 500 observation days are included in the table. At a polar station no doubt there 
would lie much greater uncertainty on this ground. 
The mean values vary on the whole in a fairly regular way from year to year. 
They have certainly been smaller since 1896 than in previous years ; but, as we shall 
see later, this is only what we should anticipate from sun-spot considerations. The 
value for 1893 is certainly somewhat conspicuously low compared to the values in 
1892 and 1894, while the value for 1890 appears somewhat large. On the whole, 
however, I think we may conclude that the phenomenon is sufficientlv definite to 
form a satisfactory basis for intercomparison of stations and years. There is a 
well-marked annual variation proceeding with very fair regularity. There are two 
minima, near midwintei' and midsummer, with maxima near the beginning and end of 
winter. This is similar to the phenomena observed in declination disturlxmces,"^ more 
* 8ec Mascart’s ' Magiietisme Terrestre,’ Art. 108. 
