142 Royal Society. 



for each day. We are thus enabled to take in a more extended period, 

 amounting to ninety- six consecutive months, or eight years, from 

 which to derive the average rate of secular change at St. Helena. 

 Proceeding as before, we find for this period an average rate of 0'* 661 

 for the increase of West Declination in a month, or an annual in- 

 crease of 7'*93 in a solar year. During these eight years the hori- 

 zontal magnetic direction at St. Helena had consequently changed 

 altogether rather more than one degree. «. 



" When the number of years are few from which an annual average 

 rate of secular change is derived, it is necessary to be particular in 

 regard to the regular distribution of the observations as to months 

 and hours, because observations made at one time of the year or at 

 one hour of the day, are not strictly comparable with those made at 

 other times of the year or at other hours of the day, unless indeed 

 corrections based on a long series of observations at the same spot 

 or in its vicinity are applied for the annual and diurnal variations. 

 But when the periods of comparison include intervals of consider- 

 able length, the comparative influence of the annual and diurnal 

 variations is greatly diminished, and, if the comparison extend over 

 a great number of years, it may practically be disregarded. Now, St. 

 Helena being a naval station, and frequently visited by navigators 

 of our own and other countries, who have had the requisite knowledge 

 and have been at the pains to take the necessary precautions to 

 make trustworthy observations, we are able to collect from the nar- 

 ratives of their voyages a succession of determinations of the Decli- 

 nation, all made at the same spot, namely, at the one anchorage at 

 St. Helena, which extend over a period of 236 years, or from 1610 

 to 1846. The following Table contains eleven such determinations, 

 all from authorities of high repute, which are fortunately so far 

 equably distributed in respect to the years when they were made, as 

 to throw light not only upon the average amount of the secular 

 change of declination during that long period, but also in a consider- 

 able degree upon the regularity, or uniformity with which the change 

 has taken place. By treating these eleven determinations according 

 to well-known methods, we obtain 11° 48' as the west declination 

 corresponding to the middle epoch, the year 1763, and 8''05 as the 

 most probable rate of the annual increase during the 236 years. 



Declinations observed at the Anchorage at St. Helena. 



1610. Davis - 7 13 Calculated- 8 44 Obs.-Calcul.+? 31 



1677. Halley - 40 



1691. Ilalley + 1 00 



1724. Mathews + 7 30 



1775. Wales -|-12 18 



1789. Hunter -f 15 3^ 



1796. Macdonald -|-15 48 



1806. Krusenstern -(-17 18 



1839. Du Petit-Thouars ... -f-22 17 



1840. Ross -j-22 53 



1846. Berard -|-23 11 



Mean Epoch 1763 



Mean Declination -t-ll° 48^ 



Annual Increase of West Declination 8'-05 



