SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY 1, 1886. 233 
Calculation of the Isomagnetic Lines. 
(1.) The Isogonal Lines. 
The isogonals, isoclinals, isodynamics and lines of equal Horizontal Foice are 
drawn through points, at which the values of the magnetic elements to which they 
refer are equal. A general term which shall include them all would be useful, and we 
venture to suggest isomagnetic as the most obvious and convenient. In nothing do 
different magnetic surveys differ more widely than in the methods employed of 
drawing these lines. Some observers have calculated them by least squares; others 
give maps on which they are exhibited, but say nothing about the principles in 
accordance with which they have been drawn. But, inasmuch as the main object and 
result of a survey is the delineation of these isomagnetics, it seems to us that it is 
most important that they should be drawn with all the accuracy that the observations 
will allow. Before proceeding to discuss the method of doing this, it will be well to 
consider the exact meaning to be attached to the operation, and to attempt to give 
'greater precision to the language in which the various curves, and the physical 
phenomena which they represent, are described. 
If we suppose that a series of magnetic curves are drawn, in which all distortions 
due to local magnetism are neglected, except those which are on a scale comparable with 
the dimensions of the earth itself, they would be the terrestrial isomagnetic lines; on 
the other hand, lines w'hich showed every disturbance, however large or small, would 
be the true isomagnetic lines. 
The object of a survey is to determine as nearly as possible the forms of the true 
lines, and to deduce from them the directions of the terrestrial lines in the district 
under investigation. Between these two extremes various grades of accuracy of 
detail intervene, and the terrestrial lines may be regarded as affected with distoi’tions 
of different orders due to disturbances of various magnitudes. It is of course 
inapossible to frame definitions which shall accurately distinguish between them, but 
it is nevertheless convenient to recognise three classes, into which they may be 
divided with respect to any particular survey. 
We may regard a disturbance as being of the third, second, or first class according 
as its range is less than tlie average distance between the stations, greater than this 
distance, but small compared with the dimensions of the entire area under survey, or 
such as to involve the whole or a considerable fraction of that area. 
The term local may be reserved for disturbances of the third class, which affect 
only a single station or its immediate neighbourliood, and are represented by minor 
bends or small loops in the isomagnetic curves. 
Those of the second class may be called regional disturbances. They are repre¬ 
sented by considerable distortions of the curves, but do not seriously interfere wuth 
the determination of the general direction or average distance apart of the terrestrial 
MDCCCXC.—A. 2 H 
