CONCEENED IN PEODUCING THE QEEATEE MAGNETIC DISTUEBANCES. 635 
changes in the main body of the disturbing force. Our results regarding the peaks and 
hollows may therefore, perhaps, be viewed as applicable to the main body of the force ; 
but we must bear in mind that while we may thus be enabled to determine the resultant 
line of action of this force, we do not yet learn whether it is of a positive or negative 
nature, whether it may be represented by a north or by a south pole. This will be 
rendered evident when we reflect that a small depression may be due either to the 
weakening of an elevating force, or to the strengthening of one of an opposite character. 
It thus appears that the north or south nature of the whole distubing force must be deter- 
mined by reference to the general appearance of the curve. Indeed it will be afterwards 
shown that we have grounds for supposing two antagonistic forces to be in operation at 
once. To fix our thoughts, however, let us suppose for the following Table the disturbing 
force to be one which increases the earth’s horizontal intensity at Kew, and let us con- 
sider its action on the north pole of the needle. 
Table VI. — Eepresenting the Astronomical Azimuth and Dip of the various disturbing 
forces. 
Date of disturbance. 
Direction in which the force 
tends to make the north pole 
of a needle point. 
Dip which the force would 
give to the north pole 
of a needle. 
1858. March 14 — 15 
North 53 46-5 West 
15 36-5 
April 9—10 
64 28-5 
13 0*5 
June 23 
47 13 
15 53 
October 28 
54 5*5 
14 43-5 
December 4 — 5 
54 5-5 
14 43-5 
1859. February 26 — 27 
56 27-5 
14 23*5 
June 8 — 9 
56 10-5 West 
13 53-5 
September 1 — 4 
3 27*5 East 
16 46 
September 1 — 4 
40 30*5 West 
15 11 
October 17 — 18 
64 21-5 
12 52-5 
December 14 
59 13-5 
14 1-5 
1860. April 10—13 
65 54 
12 55 
July 5 
67 54 
12 17 
August 10 — 11 
North 40 1*5 West 
16 59-5 
32. It has been already mentioned that we do not ascertain by our method whether 
the whole disturbing force is positive or negative. The following considerations, however, 
may serve to elucidate this subject. 
We have seen in art. 18 that a disturbance cannot be represented by the action of a 
single force, and we have also seen (art. 29) that the various forces which compose a 
disturbing group have a very small range as regards their mode of action upon the three 
elements, this range being especially small when the two force-elements are compared 
together. 
We might therefore hope to find, on inspecting the general appearance of the curves, 
that the two force-elements are simultaneously raised or depressed with reference to 
their normals nearly in the proportion denoted by the nature of the disturbing force 
whose character we have been analysing. 
