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IV. Comparison of Simultaneous Magnetic Disturbances at Several Observatories. 
By Professor W. Grylls Adams, D.Sc., F.R,S. 
Received June 11,—Read June 18, 1891. 
[Plates 8, 9.] 
In my former papers on Magnetic Disturbances and Earth Currents, which were 
read at the meetings of the British Association at Swansea in 1880 and at York in 
1881, a comparison was made of the declination and the horizontal force traces 
given by the self-recording instruments at five European stations, also at one station 
in India, one in China, and one in Australia. An attempt was made to determine the 
relative amounts of the simultaneous changes at the several observatories by com¬ 
paring them with one another by means of the scale values of the instruments 
employed, which were all of the pattern of the self-recording instruments at the Kew 
Observatory. It was found on comparison that there were great differences in the scale 
values of the instruments of the same kind at the different observatories, and in some 
cases there "was great uncertainty as to the scale values, because no determination of 
them had recently been made. Hence great difficulty was found in arriving at the 
true meaning of the records which were taken regularly at the different observatories. 
The comparison was sufficient to show the great importance of adopting the same 
scale values for the like instruments at all observatories. In my paper a recom¬ 
mendation was made that for horizontal force records a scale value of '0005 millimetre- 
milligramme for a difference of scale reading of 1 mm. should be adopted as being the 
most convenient. The same scale value was recommended by Dr. Wild, of the St. 
Petersburg Observatory, and for the vertical force magnetometer the same scale 
value might conveniently be adopted. With this scale value the instruments would 
be sufficiently sensitive to give for a considerable magnetic disturbance changes 
which are capable of being measured, but yet would not be so sensitive as to send 
the spot of light off the photographic paper, even in a violent magnetic storm. 
A violent magnetic storm was experienced in August, 1880, and the records from 
these observatories, viz., Kew, Stonyhurst, Lisbon, Vienna, St. Petersburg, Bombay, 
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