632 
ME. B. STEWAET ON THE NATHEE OE THE EOECES 
zontal component of the earth’s magnetism bears always a nearly invariable proportion 
to its effect upon the vertical component of the same. In this case it is clear that two 
or more disturbing forces superimposed would, as far as regards the horizontal and vertical- 
force changes, behave almost in the same manner as a pure elementary disturbance. This 
. also explains why it is almost impossible to find a peak in the one component of the force 
corresponding to a hollow in the other. 
24. This curious fact may likewise be stated in the following language : — 
Whatever he the nature of the disturbing force at work, its resolved jjortion, ivhich acts in 
the gtlane of the magnetic meridian at Kew, is always in nearly the same line. 
25. It is very easy to find the direction of this line. 
A disturbance of the horizontal force, as it appears in the curve, is, we have seen, very 
nearly double that of the vertical force. Now an inch in the ordinate of the horizontal- 
force curve denotes a change amounting to nearly ‘010 of the whole force, while for the 
vertical-force curve it represents a change of '0025 of the whole ; and increasing ordinates 
denote decreasing force for both elements. Also the absolute value of the horizontal 
force is 3’8, while that of the vertical force is 9’6. 
Hence if we represent by 38 the value of the horizontal component of the disturbing 
force in the plane of the magnetic meridian, that of the vertical component will be denoted 
by 12, and the dip of the whole force acting in this plane will be I7°‘5 nearly. 
26. From what has been already mentioned (art. 22), it may be inferred that it is 
difficult to obtain similar corresponding changes for all the elements together. The 
following Table exhibits those instances in which this has been accomplished ; but the 
results can only be regarded as very rough approximations. 
In some cases, where the vertical-force disturbance was very small, it was not attempted 
to measure it. 
Table V. — Similar corresponding changes for all the elements. 
Date. 
Greenwich Mean 
Time. 
Character and size of change (vertical-force change = unity 
in each instance). 
Declination. 
Horizontal force. 
Vertical force. 
1858. 
March 14 
h m 
2 19 P.M. 
rise after =2-1 
rise after =1*9 
rise after =1*0 
15 
9 53 A.M. 
rise after =1‘8 
rise after =2*0 
rise after =1*0 
15 
9 56 
fall after =1*7 
fall after =1*8 
fall after =1*0 
15 
9 35 
rise after =2*1 
rise after =2*0 
rise after 
April 9 
11 14i 
rise after =3*1 
rise after =2*0 
rise after 
10 
4 21 
rise after =3*0 
rise after =2*0 
rise after 
10 
4 23 
fall after =2*7 
fall after =2*1 
fall after =1*0 ; 
10 
6 321 
fall before =3*0 
fall before =1*9 
fall before =1*0 
June 23 
3 25 
fall after =1*4 
fall after =2*0 
fall after i 
23 
3 29 
rise after =1*5 
rise after =2*0 
rise after 
23 
3 33 
fall after =1*6 
fall after =2*0 
fall after 
23...... 
3 37 
rise after =1*6 
rise after =2*0 
indistinct I 
1 
