132 
PROFESSOR W. G. ADAMS ON A COMPARISON OF SIMULTANEOUS 
Zi-ka-wei, and Melbourne, as well as those from the Toronto Observatory in Canada, 
were compai'ed and discussed in the paper read before the British Association at 
York in 1881, and printed among the Beports of that year. 
During the last ten years the number of self-recording magnetic observatories 
where Kew instruments are used has increased to 8 in Europe, 3 in Asia, 1 at the 
Mauritius, 1 at Melbourne, and 2 in the United States of America, whilst there are 
self-recording instruments of a different type at Greenwich, Paris, Utrecht, and 
Toronto. 
For the bifilar magnetometer or horizontal force instrument a scale value not 
greatly differing from '0005 millimetre-milligrammes of horizontal force for 1 mm. of 
scale has been adopted at Kew, St. Petersburg, Vienna, Wilhelmshaven, Lisbon, 
San Fernando, Colaba, and Batavia, and the satisfactory nature of the results now 
obtained at these observatories clearly shows that it would greatly advance the study 
of terrestrial magnetism if the same scale value were adopted at other observatories. 
Nearly the same scale value is adopted for the balance or vertical force magnetometer 
at Kew, St. Petersburg, Lisbon, Batavia, and Mauritius, but at nearly all other 
observatories the instruments are considerably less sensitive, and their indications in 
ordinary magnetic disturbances are too small to be of much service for comparison or 
measurement of the amount of change of the magnetic forces or of the magnetic 
potential of the earth. 
Thus in the disturbances on August 11th, 1880, at the beginning of the magnetic 
storm at 10.20 a.m.,G.M.T., the disturbance of horizontal force is measured at Colaba 
(Bombay) by a rise of 3 mm., at Toronto by 7 mm., at Zi-ka-wei by 19 mm., and at 
Vienna by 13 mm. ; but, as appears from Table I., the values of these deflections are 
very nearly the same, viz., a change of about '005 millimetre-milligramme. With 
the scale value recommended, the measurements for this disturbance would have been 
from 10 to 12 mm. 
>' Table I. 
Scale value for 
Deflection at 
Value in 
Deflection at 
Value in 
1 mm. 
10.20 A.M. 
mm.-mgr. 
11.30 A.M. 
mm.-mgr. 
mm. 
mm. 
Kew. 
•00127 
- 7 
•00889 
Vienna. 
•00051 
+13 
•0066 
-25 
•01275 
St. Petersburg .... 
•00029 
+ 24 
•00696 
-55 
•01595 
Colaba. 
■00167 
+ 3 
•00501 
- 5 
•00835 
Melbourne. 
•00066 
+ 6 
•00396 
-12 
•00792 
Zi-ka-wei. 
•00025 
+ 19 
•00475 
-43 
•0108 
Toronto . 
•00072 
+ 7 
•00504 
+ 13 
•00936 
Thus the sudden changes of force at stations widely separated over the Earth’s 
