MAGNETIC DECLINATION. 225 
TABLE IV.—Diurnal Variation of Westerly Declination for each Month in 1843. 
Period. 
January 
February 
March 
April 
May 
June 
July 
August 
September 
October 
November 
December 
Spring 
Summer 
Autumn 
Winter 
The Year 
Table IV. has been obtained from Table III. in subtracting the lowest mean in each month from all the other 
means. Spring in the above Table consists of the months of February, March, and April. 
DIURNAL VARIATION OF DECLINATION. 
The least westerly declination occurs in the first three and last four months of the year, at or after 9? 10™ p.m. 
Makerstoun mean time; in the remaining five months at 7" 10™ a.m. Mak. The greatest westerly declination 
occurs in each month at 12 10™ p.m., but in the winter months the maximum probably occurs before this time. 
The westerly declination increases with more rapidity to the maximum at 1> 10™ p.m. than it diminishes after it, 
and in nine months the rapidity of diminution is less after 5" 10™ p.m. than before it. In the winter months, the 
westerly declination does not begin to increase rapidly till 9" 10™ a.m., whereas in summer, it does so at 7° 10™ a.m. 
While this fact seems to shew some relation to sunrise, there is no corresponding one to sunset, but rather the 
reverse ; for in winter the westerly declination diminishes with nearly equal rapidity from the maximum at 
1* 10™ p.m. till 9 10™ p.m., whereas in summer, on the whole, the rate of diminution seems to receive a check 
about 5" 10™ p.m. 
The minimum of westerly declination occurs in spring, autumn, and winter at or after 92 10™ p.m.; in 
spring and autumn a secondary minimum also occurs about 6° 10™ a.m. In summer the principal minimum 
oceursat 7210™ a.m. The principal maximum occurs about 0% 40™ p.m. in winter, and about 1" 10™ p.m. in summer. 
In the mean for the year 
The maximum of westerly declination occurs about 1" 0O™ p.m. Makerstoun mean time. 
THOTHIDIMUHY Scelasee.ccejcasiascesecceveccceans iter 9. lO Pic Aepansnodsdce con onc 
A minimum ---+. wee cen cee ceeceecsc cee ceeces ADOUtL 7 0 A.M. creer cee ceeceecee 
The exact periods of the principal minimum, and of the secondary maximum, cannot be determined from these 
observations ; they are found, however, from the Term-day Observations in the years 1842 and 1843. The fol- 
lowing Table contains the results of the summations of the observations at 0™ on 11 Term-days of 1842 (January 
Term-day being rejected), and on 11 Term-days of 1843 (June Term-day being rejected). 
MAG. AND MET. oBs. 1843. oL 
