PRESIDENT’S ADDRESS. 171 
accurate knowledge of the magnetic meridian must be of a neces- 
sity for its surveyors ; and second, for the great assistance it would 
render to the sea-going magneticians in surveying vessels, who 
require to verify their instruments by connecting their observa- 
tions with as many permanent stations as can be obtained. 
This last attribute of permanent magnetic observatories is 
applicable to all the stations here contemplated. 
These propositions for the expansion of magnetic work in Aus- 
tralasia may appear exorbitant, and out of all proportion with 
the resources of these colonies; but the science of terrestrial mag- 
netism is now in a phase of remarkable activity in many parts of 
the world, and we, by keeping out of its course, simply retard and 
probably impede its progress. 
This is therefore another task which will at least claim our 
earnest attention in the immediate future, and we may rest 
assured that by so doing we shall receive the approval and the 
encouragement of all the authorities in this branch of terrestrial 
physics, whose objects are so interesting, and whose laws are so 
obscure. 
The next subject to which I desire to draw your attention is 
terrestrial gravitation, a force which, like terrestrial magnetism, is 
obscure in its origin, but which is capable of exact measurement 
at any accessible point of the earth’s surface, and its distribution 
can therefore be ascertained. 
If the earth were a regular and homogeneous spheroid, with a 
smooth surface, we might expect that its attractive force would 
vary uniformly, according to a simple law, from the Poles to the 
Equator, and would have the same intensity at all places of equal 
latitude ; so that it wé6uld be sufficient to determine its value at 
one single point, in order to deduce its variation at any other. 
But the ranges of mountains and deep oceans, the great 
differences in the geological conditions of the earth’s crust, and 
probably other irregularities in its figure, disturb this simple dis- 
tribution, and cause variations and perturbations in the force of. 
gravity, which cannot perhaps be represented by any mathematical 
formula. 
Consequently, if we desire to know its accurate value at any 
given place, we must simply determine it by actual observation. 
Hence the necessity of carrying on this class of observations in 
every part of the world, in order to improve as much as possible 
our information on a subject which can hardly be considered 
secondary in importance to any other branch of terrestrial physics. 
This importance may perhaps be better understood in its con- 
nection with geodesy, and the figure and size of the earth. 
It is well known that discrepancies in geodetic determinations 
of latitudes and longitudes, when compared with corresponding 
values from astronomical observations, are frequently considerable, 
