181 



On Magnetic Disturbances. 



[Feb. 3, 



Florence in 1829, which had belonged to the Accademia del Cimento, and 

 corresponds with the small thermometer used for determining atmospheric 

 temperatures, of which a figure and description are given in the Memoirs 

 of the Academy. 



The following communications were read : — 



I. " Note on an Extension of the Comparison of Magnetic Dis- 

 turbances with Magnetic Effects inferred from observed Terrestrial 

 Galvanic Currents ; and Discussion of the Magnetic Effects in- 

 ferred from Galvanic Currents on days of tranquil magnetism." 

 By George Biddell Airy, Astronomer Royal. Received De- 

 cember 22, 1869. 



(Abstract.) 



The author, after referring to his paper in the Philosophical Transac- 

 tions for 18G8 on the comparison of Magnetic Disturbances inferred from 

 Galvanic Currents recorded by the Self-registering Galvanometers of the 

 Royal Observatory of Greenwich with the Magnetic Disturbances regis- 

 tered by the Magnetometers, on 1/ days, states that he had now under- 

 taken the examination of the whole of the Galvanic Currents recorded 

 during the establishment of the Croydon and Dartford wires (from 1865 

 April 1 to 1867 October 24). The days of observation were divided 

 into three groups, — No. 1 containing days of considerable magnetic dis- 

 turbance, and therein including not only the 17 days above mentioned, 

 but also 36 additional days, No. 2 containing days of moderate dis- 

 turbance, of which no further use was made, and No. 3 containing the 

 days of tranquil magnetism. 



The comparisons of the additional 36 disturbed days were made in the 

 same manner as those of the preceding 1 7 days, and the inferences were 

 the same. The results were shown in the same manner, by comparison 

 of curves, which were exhibited to the Society. The points most worthy 

 of notice are, that the general agreement of the strong irregularities, Gal- 

 vanic and Magnetic, is very close, that the galvanic irregularities usually 

 precede the magnetic, in time, and that the northerly magnetic force 

 appears to be increased. The author remarks that no records appeared 

 open to doubt as regards instrumental error, except those of western de- 

 clination ; and to remove this he had compared the Greenwich Curves 

 with the Kew Curves, and had found them absolutely identical. 



The author then proceeds with the discussion of the Galvanic Current- 

 Curves on days of tranquil magnetism, not by way of comparison with the 

 magnetic curves, but for independent examination of the galvanic laws. 

 The method was explained of measuring the ordinates and connecting the 

 measures into expressions for magnetic action, at every hour, and group- 



