138 
PROFESSOR W. G. ADAMS ON A COMPARISON OF SIMULTANEOUS 
great magnetic disturbances, but there is a great resemblance between the traces 
given by the three Utrecht instruments. This resemblance is due to the fact that the 
horizontal and vertical force changes are given by means of deflecting magnets and 
iron induction bars according to Lamont’s plan ; hence, the horizontal and vertical 
force needles are affected by changes in declination, and the vertical force trace is also 
affected by changes in horizontal force. 
Thus the change in horizontal force in metric units 
SX = -000376 (pj -p), 
where p x and p are the changes in ordinates of horizontal force and declination 
expressed in millimetres. 
If SZ be the increase of vertical force, and p 2 the change in the ordinate of the 
vertical force in millimetres, then 
SZ = -00326 (p 2 - p) + -00172 (p 1 - p). 
Thus, at 10*32 p.m. on June 24, p = — 6"4 mm., 
p x — -j- 15"5 mm. and p 2 = — 13"6 mm. 
Hence 
SX = -000376 (15-5 + 6'4) = -0082 metric unit, 
and 
SZ = -00326 (— 13-6 + 6-4) + -00172(15-5 -f 6’4) = ’0142 metric unit. 
Plate 9 contains only traces of horizontal force for June 25, 1885, from mid-day 
until 8 p.m. From 2.10 p.m. there is a large and continuous well-marked period of 
disturbance, beginning with an increase of force varying from -006 to -01 metric unit, 
followed by a decrease of very neaidy the same amount at '3 P.M. 
Another similar period of disturbance, well-marked at Melbourne, Batavia, and 
Bombay, as well as throughout Europe, occurs between 5.30 p.m. and 6.30 p.m. on 
the same day. 
The values of these changes in metric units are given in Table IV. 
