1872.] The Astronomer Royal's Magnetical Observations. 



85 



rejected zinc from Kirchhoff's list, and agreed with him in rejecting alu- 

 minium. It need scarcely be added that these lines are in each case the 

 longest lines in the spectrum of the metal. 



The help which these determinations afford to the study of the various 

 cyclical changes in the solar spectra is then referred to. 



December 19, 1872. 



Sir GEORGE BIDDELL AIRY, K.C.B., President, followed 

 by Mr. BUSK, Vice-President, and Dr. SIBSON, Vice-Pre- 

 sident, in the Chair. 



The following communications were read : — 



I. " Magnetical Observations in the Britannia and Conway Tu- 

 bular Iron Bridges." By Sir George Biddell Airy, K.C.B., 

 P.R.S., Astronomer Royal. Received October 12, 1872. 



(Abstract.) 



The author states that he was first induced to make these experiments 

 by consideration of the perpetual tremor to which the iron of these struc- 

 tures is subjected, and which appeared likely to have made them unusu- 

 ally sensitive to the production of induced and perhaps subpermanent 

 magnetism. The experiments were actually conducted by Mr. James 

 Carpenter (then Assistant at the Royal Observatory), with the friendly 

 cooperation of Captain Tupman, R.M.A. Permission was given by the 

 Directors of the Londou and North- Western Railway Company, and 

 every possible assistance was given by the resident Officers of the Com- 

 pany, with a degree of zeal and liberality which cannot be too highly 

 appreciated. The observations were made in the axis of each tube, a 

 large step-ladder and stage adapted to the circumstances having been 

 provided ; the observations made were those of disturbed magnetic azi- 

 muth, disturbed time of vibration of a horizontal needle, and disturbed 

 dip. The places of observation were : — in each line of the Britannia Bridge, 

 a station on each of its five supporting towers and a station in the 

 middle of each of the four sections of the continuous tube, and also stations 

 iu the prolongations of the axis of the bridge to a considerable distance in 

 each direction, making in all twenty stations ; and in the Conway bridge, 

 stations in each line at the piers and in the middle of the tube, and also 

 distant stations in the prolongation of the axis of the bridge, making 

 eight stations in all. 



The means of the results at each station are given in a Table. For 

 further treatment, the means of those means for collateral stations are 

 used ; and the means of those for the two distant statious at each bridge 

 are adopted as giving undisturbed local constants. By treatment of 



