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XIII. Results deduced from the Measures of Terrestrial Magnetic Force in the 
Horizontal Plane, at the Royal Observatory, Greenwich, from 1841 to 1876. 
By Sir G. B. Airy, K.C.B., F.R.S., late Astronomer Royal. 
Received June 24,—Read November 26, 1885. 
[Plate 74.] 
In offering to the Boyal Society some results deduced from the systems of magnetic 
observation and magnetic self-registration established several years since at the Boyal 
Observatory, Greenwich, during a portion of the time in which I presided over that 
institution, I think it desirable to premise a short statement on the origin of the 
Magnetic Department of the Boyal Observatory, and on the successive steps in its 
constitution. 
It appears to have been recognised many years ago, that magnetic determinations 
would form a proper part of the business of the Boyal Observatory. When I 
commenced residence at the Boyal Observatory, at the end of 1835, I found in the 
garden a small wooden building, evidently intended for the examination of compasses, 
perhaps of the size of those used in the Boyal Navy. But the locality was 
inconvenient, and the structure was totally unfit for any delicate magnetic purpose; 
for instance, the balance-weights of the sliding windows were of iron. For some 
preliminary experiments a small observatory was borrowed from Captain Fitzroy, 
but no real progress was made in magnetism. 
In the beginning of 1836, a scheme for the erection of a Magnetical Observatory 
was brought before the Board of Visitors. The Board approved the plan, and 
recommended it favourably to the Admiralty. The Government Department super¬ 
intending the Park gave their consent to an extension of the grounds of the Obser¬ 
vatory, and the ground was inclosed in 1837. The Magnetic Observatory was built, 
from my plans, in the spring of 1838. Since that time, no alteration has been made in 
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