XIU 



probably derived from solar influence. On tbis idea be instituted a series of 

 experiments to determine wbetber a source of beat applied to two substances 

 of different conducting powers in uniform contact, like tbe eartb and tbe 

 atmospbere, would produce pbenomena corresponding to tbe diurnal 

 variation, as tbe source of heat was applied successively to different parts of 

 tbe combined system. Tbe results be obtained were in accordance with 

 tbis supposition, but of course tbeir validity as evidence is subject to tbe 

 question of bow far tbe actual conditions of tbe earth were truly repre- 

 sented in tbe ingenious experimental combination which be adopted. 



Mr. Christie appears to have been tbe first to make use of a torsion 

 balance for the determination of tbe equivalents of magnetic forces ; he also 

 devoted himself to tbe improvement of tbe construction of both tbe hori- 

 zontal needle and tbe dipping-needle ; and be served constantly upon tbe 

 " Compass Committee " formed to assist tbe Admiralty in bringing tbe 

 Compasses of tbe Royal Navy into some accordance with tbe advanced 

 knowledge of tbe day. 



In tbe Report of tbe British Association for 1833, tbe portion which 

 refers to tbe then state of knowledge of tbe magnetism of the earth was 

 drawn up by Mr. Christie, and be therein again maintained that not only tbe 

 daily variation, but also tbe quasi-polarity of tbe earth is most probably 

 due to tbe excitation by the solar heat, of electric currents at right angles, or 

 nearly so, to tbe meridian ; and he suggests that the direction of these 

 currents must be influenced by the form, extent, and direction of the 

 continents and seas over which they pass, and also by tbe height, direction, 

 and geological structure of chains of mountains. 



The Letter of Baron Humboldt in 1835 to H.R.H. the Duke of Sussex, 

 P.R.S., on the establishment of permanent magnetic observatories at widely 

 separated stations within the British territories, was referred by H.R.H. 

 tbe President, to Mr. Christie and ]\Ir. Airy to report upon. Their report 

 was read to tbe Royal Society in November 1836; and upon a further 

 report to tbe same effect from tbe joint Committee of Physics and Meteor- 

 ology in 1838, the President and Council made a representation in favour 

 of the measure to Her Majesty's Government which was successful. 



In connexion with Mr. Christie's career as a teacher, it may be mentioned 

 that be was the author of an * Elementary Course of Mathematics ' for 

 use in tbe Royal Military Academy. In 1837 Mr. Christie succeeded Mr. 

 Children as one of the Secretaries of tbe Royal Society, and retained that 

 office until 1854, when be went to reside at Lausanne upon his retirement 

 from tbe post of Professor of Mathematics at tbe Royal Military Academy. 

 He was one of tbe Visitors of tbe Royal Observatory at Greenwich ; a Vice- 

 President of tbe Royal Astronomical Society ; a Corresponding Member of 

 tbe Academy of Sciences of Palermo, and a member of tbe Societe Phi- 

 lomatbique of Paris. He died at Twickenham, where he had resided for 

 some years, on tbe 24tb of January, 18C5, having nearly completed hi§ 



