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XI. Note on an Extension of the Comparison of Magnetic Disturbances with Magnetic 
Effects inf erred from observed Terrestrial Galvanic Currents ; and Discussion of the 
Magnetic Effects inf erred from Galvanic Currents on days of Tranquil Magnetism . 
Dy George Biddell Airy, Astronomer Boyal, F.R.S. 
Received December 22, 1869, — Read February 3, 1870. 
In a communication to the Royal Society, which was honoured by publication in the 
Philosophical Transactions for 1868, I described the methods and gave the results of 
comparing the Magnetic Disturbances which might he expected as consequent on the 
Terrestrial Galvanic Currents recorded by the self-registering galvanometers of the Royal 
Observatory of Greenwich, with the Magnetic Disturbances actually registered by the 
self-registering magnetometers. The comparison was limited to seventeen days (1865, 
October 5 and 31 ; 1866, October 4; 1867, April 4, 5, 7, 8, 9, 11, May 4, 14, 28, 31, 
June 1, 2, 7, 24), various days having been omitted in consequence of a doubt on the 
uniformity of the clock-movement of the registering-barrel, which afterwards proved to 
be unfounded. The results of the comparison were exhibited in curves, engraved copies 
of which are given in the volume of publication. I expressed my opinion that it was 
impossible to doubt the general causal connexion of the Galvanic Currents with the 
Magnetic Disturbances, but that some points yet remained to be cleared up. 
As soon as circumstances permitted, I undertook the examination of the whole of the 
Earth-currents recorded during the establishment of the Croydon and Dartford Wires 
(namely from 1865 April 1 to 1867 December 31), as far as they should appear to bear 
upon this and similar questions. For this purpose the days of observation were divided 
by Mr. Glaisher into three groups. Group No. 1 contained days of considerable mag- 
netic disturbance (or days of considerable galvanic disturbance, which are always the 
same), including, besides the seventeen days above-mentioned, the thirty-six days of the 
following list:— 1865, April 15, 16, 17, 18, 19, May 14, 17, July 7, 15, August 14, 19, 
26, September 8, 16, 28, October 4, 6, 10, 12, 14, November 1; 1866, August 11, 23, 
September 8, 9, 12, 13, 17, 18, 25, October 6, 7, 10, 30, November 26 ; 1867, February 8 ; 
making in all fifty-three days of considerable magnetic disturbance. Group No. 2 con- 
sisted of days of moderate magnetic disturbance, and of these no further notice was taken. 
Group No. 3 contained the days of tranquil magnetism, and the discussion of these will 
form the principal part of the present Memoir. 
The comparisons of the additional thirty-six disturbed days were made in every respect 
by the same process as those of the seventeen days in the former paper ; the operations 
of every kind were directed, as before, by Mr. Glaisher ; and the results are exhibited 
2 f 2 
