C 1S 9 3 
VI. Observations on the diurnal changes in the position of the 
horizontal needle, under a reduced directive power, at Port 
Bowen, 1825. By Lieutenant Henry Foster, R.N. F.R.S. 
Communicated January 12, 1826. 
The daily variation of the horizontal needle is a subject 
which has, for nearly a century, attracted the attention of 
several accurate observers, whose object was principally 
limited to determining the hour of the day, when its amount 
was the greatest, and the times of the needle’s successive 
easterly and westerly motions. 
From these observations, however, it could not be ascer- 
tained whether the cause of this daily variation proceeded 
from an actual change in the direction of the magnetic axis 
of the earth, or whether it arose from some foreign force, 
acting transversely on the needle, impelling it out of its 
natural direction. To submit this question to the test of 
observation, Mr. Barlow, in 1823, undertook a set of expe- 
riments on the daily variation of a horizontal needle nearly 
neutralized by the application of artificial magnets ; under an 
idea, that if the daily variation proceeded from an actual 
change in the direction of the earth’s magnetism, the needle 
in this case, as when in its natural state, would merely take 
up its new direction without any increase of amount ; but if 
it proceeded from a foreign force acting transversely upon 
it, the needle now having less intensity of direction than 
when in its natural state, it would yield more easily to this 
transverse force and give a larger expression, which would 
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