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Doc. 200.] 579
CELSIUS REPLY.
magnet on the largest ocean of the world, and he has had
the good fortune to see these lines, which were constructed
mostly on the basis of the observations taken during his
voyages, confirmed more and more by later experiments. As
I had, moreover, observed that Assessor Swedenborg’s theory
differs considerably from that of Dr. Halley, I was the more
anxious to see how it compared with my observations taken
at Torneå and Upsal, especially as I saw that his computa
tions tallied so well with the observations made in London and
Paris.
Besides, as Assessor Swedenborg is still pleased to persist
in the objections to the hourly changes of the declination of
the needle, which he had previously made in his philosophical
work to the observations of Messrs. Muschenbrock and Graham ,
I cannot refrain from uttering my frank thoughts on the
subject.
I, too, am of opinion that it is a very difficult and delicate
matter to observe the declination of the needle so exactly, as
to be always sure to have this value expressed in degrees and
minutes ; so that the circumstances alluded to by the Assessor
may cause an observation to be faulty, if their demands are
not in every possible manner complied with. But these causes
operate so much the less in observing the hourly changes of
the needle in one and the same place, because in this case
the mere differences of the declination are taken into con
sideration, without troubling oneself about the absolute deter
mination of its direction. It does not matter, therefore, whether
the needle is placed strictly in agreement with the meridian
line, but a certain degree is marked down ; further, it does
not matter if the whole arc is not divided with perfect ac
curacy, provided one half-degree is pretty accurately marked.
Neither is it of any consequence if some iron objects are im
bedded in the window -frame, in the door, in the wall, or in
the earth, as they can only vitiate the absolute quantity of
the declination, without, however, being able to produce a dis
turbance in the differences caused by the same force operat
ing all the time at the same distance. Further, no draught
or commotion in the air can influence the direction of the
needle, after the compass has been enclosed with a glass,
37*
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