Dr. E. Naumann—Wagnetism and Earth Structure. 487 
These remarkable words were spoken nearly half a century ago. 
Since then natural sciences have undergone enormous development, 
many methods of investigation have been completely changed, and the 
aim of concatenating facts has greatly influenced the methods of obser- 
vation. Systems which were then descriptive have been replaced by 
others in which the various facts are arranged, so as to exhibit their 
relation to causes, and by reducing the number of isolated facts, differ- 
ent fields of investigation have been brought into closer relationship. 
The change which has taken place is particularly striking in the 
case of Mineralogy. Modern mineralogists devote considerable at- 
tention to Physical Crystallography, a study which borrows its 
methods from a sister science, and the results attained by the 
combination are very satisfactory. 
By the aid of light we are enabled to discern the structure of 
crystallized bodies, ‘and to trace the marvellous connection between 
‘composition, structure, and form. The motions of light, although 
the best, are not the only means of investigating internal structure, 
for those of heat, electricity, and magnetism, also depend on the 
shape, size, and arrangement of the smallest particles and their 
interstices. Any interposition, any fissure, causes changes of wave 
motion, for this motion can only be propagated regularly in homo- 
geneous media, and its character, velocity, and direction of propa- 
gation are altered wherever any variation from homogeneity exists. 
Changes of this kind occur even at the planes of composition of 
twin crystals, where no alteration of substance, but a mere change 
of structure, takes place. In the same way, the electric currents 
passing through the earth’s crust must be diverted, or accelerated, 
retarded, or modified, wherever the uniformity of composition is 
interrupted. Such interruptions will be caused by deep fissures, 
once open at the surface and leading towards the earth’s interior. 
These clefts cannot generally be found by direct observation, but 
require the aid of geological research. 
At the present day, probably the majority of physicists consider 
the globe to be a body of very uniform composition or structure, and 
though it is generally admitted that there is a change of structure 
radially, the irregularities which divide one and the same of the pre- 
sumed concentric layers into unequal parts, are generally neglected, 
although geology has proved the predominance of such irregularities 
in the earth’s crust. 
Modern geology no longer accounts for the differences between 
observed and calculated polar altitudes by assuming constant errors, 
the compensation of which is effected on the method of Least 
Squares. We know that the earth is irregular in form, and that 
its shape cannot be expressed by any mathematical equation. The 
so-called geoid is not geometric in form, and there is in fact an 
infinite number of geoids. These discoveries have greatly limited 
the application of the method of Least Squares. Irregularities exist 
not only in the form of the earth, but also in the distribution of 
matter, and precautions are just as necessary in physiographical 
researches as in geodetic investigations, if a mathematical compensa- 
