264 
MR. A. W. RtiCKER AND DR. T. E. THORPE ON A MAGNETIC 
the ordinary decrease of the Horizontal Force with latitude. If the centre were 
sufficiently powerful, the decrease might be converted into an increase ; and since 
immediately over the centre the value must be normal, a miinmum and maximum 
must follow in order. Another minimum and maximum may occur on the further 
side. The effect of a centre of attraction on the lines of equal Horizontal Force is 
thus the same as that of a centre of repulsion on the isogonals. In the case of the 
Dip, as in that of the Declination, there can only he two critical points on opposite 
sides of the point defined above. 
The Vertical Force will increase with latitude at more than the normal rate to the 
south of a centre of attraction, and at less than the normal rate to the north of it. 
The lines of equal Vertical Force will thus be drawn southwards in the neighbourhood 
of sucli a centre. 
If the Vertical Force attains a maximum value it will be at a point above the 
centre, and it must be followed by a minimum—at which, however, the Vertical 
Force will be greater than its calculated value. 
It is evident from this discussion that there are a number of signs of a centre 
of attraction which may not all coexist, and which will be complicated in actual 
practice by irregularities in the distribution of the attracting masses, but which 
may nevertheless be of considerable practical help in a survey of local magnetic 
disturbances, {Cf. Lamont : ‘ Erdmagnetismus in Nord-Deutschland,’ 1859, p. 21.) 
We have dwelt on tliem, not because they present any difficulty, or even because 
they are altogether novel. Mr. Bennett Brough, A.Pt.S.M., has given a full account 
of the methods employed by the Swedish mining engineers in exploring for iron ore. 
(“ Use of the Magnetic Needle,” &c, ‘ Journal of the Iron and Steel Institute,’ No. 1, 
1887, pp. 289-303.) They are accustomed to map out the neighbourhood of a mass 
of ironstone with a magnet, in order to determine its exact position. We believe, 
however, that the systematic use of the forms of the isomagnetic curves has been 
largely overlooked in the case of surveys comparable with that described in this paper. 
In Plates V. to VIII., the values of the Declinations, Horizontal Forces, Dips, and 
Vertical Forces determined at the various stations are entered and true magnetic curves 
are drawn. These curves have not been chosen for equal differences of the values of 
the element to which they refer, but those have been selected which exhibit the most 
marked peculiarities. In this way attention is best drawn to disturbed stations and 
districts which can then be studied in accordance with the plan above suggested. 
Turning next to the investigation of the disturbances or differences between the 
observed and calculated values of the elements, we note that apart from the assurance 
they afford that the peculiarities studied are common to a district and do not depend 
only on a single station, they often supply additional information to that which can 
be gained from the true isomagnetics. 
Thus, in the ctise of the Horizontal Force, a maximum followed by a minimum may 
be either to the north or the south of the centre of attraction. In the former case. 
