1868.] Mr. MvY— Comparison of Magnetic Disturbances ^c. 249 



February 6, 1868. 



Lieut.-General SABINE, President, in the Chair. 



The following communication was read : — 



" Comparison of Magnetic Disturbances recorded by the Self-register- 

 ing Magnetometers at the Royal Observatory, Greenwich, with 

 Magnetic Disturbances deduced from the corresponding Terres- 

 trial Galvanic Currents recorded by the Self-registering Galva- 

 nometers of the Koyal Observatory." By George Biddell 

 Airy, Astronomer Boyal. Received December 20, 1867. 



(Abstract.) 



The author, after adverting to the origin of this branch of science, as 

 commencing (with himself) in communications with Messrs. Edwin and 

 Latimer Clark, but more particularly with Mr. Charles V. Walker, and 

 alluding to the important labours of Mr. W. H. Barlow, Mr. Walker, and 

 Dr. Lamont, proceeds to give the official history of the establishment of 

 the wires and other apparatus necessary for its prosecution at the Royal 

 Observatory. In 1860 and 1861, the author submitted to the Board of 

 Visitors of the Royal Observatory proposals for extending wires from the 

 Royal Observatory in two directions nearly at right angles, — on the second 

 occasion, specifying Croydon and Dartford as terminal points. The Board 

 in 1861 recommended this to the A^dmiralty, who immediately gave 

 their sanction. The author then applied to the Directors of the South- 

 Eastern Railway for permission to place his wires on their poles, which 

 was granted, at a merely nominal rent. All the wires and labour in 

 mounting them were provided by the Railway Company at cost price, and 

 the insulators were furnished by Messrs. Silver without profit. The wires 

 communicate with the earth at both ends of each by solder-attachment to 

 water-pipes. 



The author then describes the apparatus made by Mr. Simms for the 

 record of the currents. For each wire the current acts on a galvanometer 

 whose needle-carrier also supports a small plane mirror ; and, by proper 

 adjustment of cylindrical lenses, neat spots of light are formed upon a 

 rotating ebonite cylinder, covered with photographic paper and made to 

 revolve (by clockwork) in twenty-four hours. With angular motion of 

 the galvanometer, the spot of light moves. The zero of measure is 

 obtained by interrupting the wire-circuit. The zero of time is obtained by 

 interrupting the light and observing the corresponding clock-time. Other 

 adjustments have received great attention. 



Many delays occurred in the establishment of the apparatus, and finally 

 from the discovery that the earth-currents were very much stronger than 



VOL. XVI. 2 A 



