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Doc. 200.] 575
REPLY TO CELSIUS.
accurately observed. The same might also be obtained with
out an observation of the latitude, simply by making repeated
observations of the declination on several distant places on
the way, and looking for them on the Chart: for the magnet
declines differently for every degree in latitude and longitude.
As this, however, requires great care and labour, although I
feel confident of being able to supply the present difficult by
an easier method, it shall certainly not be undertaken with
out encouragement from others.
The only thing that seems to lie in the way of, or to ob
struct, the feasibility of this method is the difficulty of observ
ing the correct declination of the magnet with sufficient
accuracy and exactness at sea and on land: for many and en
tirely different circumstances may cause an observation to be
at fault for a half or a whole degree. Some of these I may
here mention ; if, for instance, the meridian line be not laid
down most accurately, the compass with its breadth or dia
meter, (suppose this to have been most carefully constructed)
would not be placed correctly in accordance with the due
north line : and this error would obtain, even though it should
be placed upon a perfectly horizontal plane, without the least
declivity; and though the needle should swing most freely on
its pivot, without the least friction caused by rust, dust, smoke,
vapour, water, or other thing. Further, it is almost impossible
for a needle to swing with absolute freedom and without any
impediment whatever : for if the point of the pivot and the
small cavity of the needle in which it runs, are examined by
the microscope, great unevenness may be observed, which is
increased by the least particles of vapour or dust which may
be deposited there. Great care too must be taken lest any
effluvia of iron either from a greater or shorter distance
be conveyed to it ; also that no pieces of iron be concealed in
the window -frames, in the room, walls, mortar, or in the floor;
also that they are not carried about by the observer : more
over, all currents of air must be carefully excluded. Any of
these circumstances may cause the observation to vary several
hours or days; yea, according to the experiments of Professor
Celsius, (see Transactions of the Academy for the months of
April, May, and June, 1740,) they may differ from twenty -seven
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