34 



LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 



Declination. — I have collected in the subjoined Table all the recorded observations 

 of the declination at Matavai Bay with which I am acquainted, from the earliest dis- 

 covery of the island to the present time. 



Table XV. — Declination observed at Matavai Bay, Otaheite. 



Year. 



Month. 



Observer. 



Month. 



Observer. 



Declination. 



(East.) 



1765 

 1767 

 1769 

 1773 

 1774 

 1777 



June 



July 



June 



August 



Apnl and May 

 December . . 



Byron 



WaUis 



Cook 



Wales and Bayley 

 Wales and Bayley 

 Cook 



January . 



May 



March . . . 

 April . . . 

 November 

 May 



Vancouver 

 Duperrey . . 

 Kotzebue . . 

 Beechey . . 

 FitzRoy .. 

 Belcher . . 



^ 12 

 6 40 

 6 



7 

 7 

 6 



50 

 33 

 34 

 30 



A first glance at these observations shows that the easterly declination has been in- 

 creasing at Otaheite from the time of the first discovery of that island. It is scarcely 

 probable that the progression has been strictly uniform throughout the whole period, 

 but the deficiency of determinations in the years that form the middle portion of 

 the interval, renders the data that we possess unsuitable for deducing the variation in 

 the rate of the secular change ; and we must be content with that approximate repre- 

 sentation which may be given by an uniform rate. Assuming, therefore, the change 

 of declination to be proportional to the time, I have computed by the method of least 

 squares from the data contained in the Table, the following formula for the declina- 

 tion S at Otaheite : 



^ = 6° ll'-85 -f l'-656^, 



t being the interval of time elapsed since January 1, 1800, expressed in terms of a 

 year. 



The declinations computed according to this formula^ and the diflferences from the 

 observed declinations, are as follows : — 



It will be seen that the discordances with each other of the observations of recent 

 date are as great, and even greater, than those of the earlier observers ; which ought 

 to be an indication that the larger discrepances are occasioned rather by local 

 disturbing influences than by errors of observation. The probable error of a single 

 determination, as resulting from the tabulated differences, would be about fifteen 

 minutes. 



