1885.] lerrestrial Magnetism in the Horizontal Plane. 255 



V. ''Results deduced from the Measures of Terrestrial Mag- 

 netic 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, 1885. 



(Abstract.) 



In offering to the Royal Society some results deduced from the 

 systems of magnetic observation and magnetic self -registration 

 established several years since at the Royal 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 Royal 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 Royal 

 Observatory. When I commenced residence at the Royal 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 Royal Navy. But the locality was incon- 

 venient, 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 superintending the Park gave their 

 consent to an extension of the grounds of the Observatory, 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 the building, except in 1864, when the ground 

 below the east, west, and south arms, was excavated, in order to 

 obtain positions for the three fundamental instruments in which the 

 severity of the temperature-changes would be much diminished. 

 Small accidental interruptions of observations occurred in 1847, 

 January, and 1861, July. 



The interest taken in the subject of terrestrial magnetism in the 

 first half of this century was occasioned principally by the enterprise 

 of Gauss and other German philosophers. Magnets were, therefore, 

 established at the Royal Observatory, furnished with apparatus adapted 

 to eye observations corresponding to those of Gauss, and some 



vol. xxxix. s 



