PRESIDENT’S ADDRESS. 169 
results have as yet been deduced, and we are thus culpable of 
holding back a great mass of valuable evidence which would 
undoubtedly throw additional light on almost every problem of 
terrestrial magnetism. 
Hitherto we have dealt with terrestrial magnetism in its general 
aspect or “in its totality.” 
There remains the local aspect to be considered, namely, that 
in which the magnetic operations are principally intended to 
determine the magnetic conditions of a limited region in minute 
detail, and constituting what are generally called “magnetic 
surveys.” 
Every magnetic survey of a country is not only of special 
service to that country in regard to the requirements of the land 
surveyor, the mining engineer, and the geologist, but forms a 
distinct and much-valued contribution to magnetic science. In 
these undertakings the field-work is carried on “by one or a number 
of observers, who move from place to place with their portable 
instruments, observing at selected stations the three magnetic 
elements, namely, declination, horizontal intensity, and dip, also 
noting and investigating disturbances in their relation to geolc- 
gical features. 
The selection and number of stations in a magnetic survey 
depend upon the nature of the country and on the more or less 
elaborate character which the survey is intended to possess. 
It may be remarked that the more numerous the stations the 
more complete will ke the survey, provided their distribution be 
judicious ; indeed, there can be very little fear of erring in the 
direction of making a magnetic survey too minute in detail. 
From the field observations results are deduced to represent 
the value of the magnetic elements for a common epoch, which is 
called the epoch of the survey. For this purpose it is necessary 
to know the variations and perturbations of the magnetic ele- 
ments on the days of observation ; and this necessary information 
is supplied by the permanent magnetic observatory, which is 
taken as the base station of the survey, and where the registra- 
tion of these variations is obtained photographically or by frequent 
observations throughout the twenty-four hours of the day. It is 
very desirable that the base station should not be too far removed 
from the locality of the survey. 
The next step is to plot on the map the isogonic, isoclinic, and 
isodynamic lines, or lines of equal declination, inclination, and 
force respectively. 
After the lapse of time the direction of these lines becomes 
affected by the secular variation, and the magnetic map requires 
rectification ; but the secular variation can ‘only be determined 
by a new ma gnetic survey of the same regions after an interval 
of several years. 
