54 
NATURE 
[JANUARY 13, 1923 

on their way—living semi-pelagically—to the east- 
ward, until the metamorphosis is completed, and the 
small eel young have acquired a dark covering of pig- 
ment. In Northern Europe they move—by way of the 
Channel and round the north of Scotland—through into 
the North Sea and farther, via the Danish waters, to the 
western parts of the Baltic, where they have been found 
so far east as E. of Bornholm, at stages where the meta- 
morphosis was not yet quite completed (Dr. A. C. 
Johansen, with the Thor), Fig. 1. In the northern parts 
of the Baltic, elvers are not known, or indeed any eels 
less than 20-30 cm., though the eel occurs right in to the 
innermost waters of that sea. The eels found in Finland 
are large females, and on the east coast of Sweden no 
males have been found north of lat. 57° 08’ (off Oland) ; 
see Fig. x. This peculiar fact evidently answers to 
what we know from the great rivers, where the female 
eels generally move farther up into the higher reaches 
than the males. 
The eel fry enter the Mediterranean at an early 
stage, as unmetamorphosed larve, most often not 
even having attained their full larval size, between one 
and a half and two years old. As unmetamorphosed 
larve they are found throughout the western basin; 
west of Italy, and at times, perhaps, still farther to the 
eastward. Even in the most easterly parts of the Medi- 
terranean, an ascent of elvers takes place, these being 
transparent, and thus not having fully completed their 
metamorphosis. I have in this connexion received 
some information, with samples, from Mr. Geoffrey W. 
Paget, Director of Fisheries Investigations in Cairo. At 
a pumping station near Alexandria, where fresh water 
is pumped in large quantities inté a channel leading 
direct to the sea, Mr. Paget found, on February 24, 1920, 
“that elvers were present in prodigious quantities, 
being unable to proceed further on account of the 
station. From this date—February 24—until April 15, 
fishing was practically continuous, and we transported 
Over 5,000,000 elvers to the canal systems inland.” 
Mr. Paget’s observations are highly interesting, show- 
ing as they do that elvers which have not yet completed 
their metamorphosis can occur in such great quantities 
so far east as about 30°E. long. Together with 
observations from northern Europe, they give us a clear 
picture of the remarkable power of migration possessed 
by the eel fry. From the breeding grounds in the 
western Atlantic to the mouth of the Nile is a distance 
approaching go degrees of longitude, or one-fourth of 
the earth’s circumference, and this distance is covered 
by the eel fry in the space of about three years. They 
may reach the Nile and the western Baltic before their 
metamorphosis is yet complete, and the greater part of 
the journey is made while they are still in the leaf-shaped 
larval stage. No other instance is known among fishes 
of a species requiring a quarter of the circumference of 
the globe to complete its life history, and larval migra- 
tions of such extent and duration as those of the eel are 
altogether unique in the animal kingdom. 
Theories of Magnetism. 
By Dr. A. E. Oxtey. 
UCH attention has been devoted in recent years 
to theories of magnetism, and an interesting’ 
survey of the position of the subject is given in the 
report of a committee of the U.S. National Research 
Council issued by the National Academy of Sciences, 
Washington, in August last (vol. 3, part 3). It is 
difficult in a descriptive article of moderate length to 
present judicially the various views which have been 
advanced, but an attempt will here be made to do this, 
using the report referred to as a basis, and supplement- 
ing it with accounts of one or two advances not recorded 
therein. 
Poisson in 1820 published a mathematical theory of 
magnetism which was based on Coulomb’s inverse 
square law. He merely regarded magnetic sub- 
stances as possessing positive and negative magnetic 
fluids which could be separated by the application of 
an external magnetic field resulting in the production 
of the magnetic effects as observed in bar magnets. 
This theory was eventually (1831) shown to be un- 
tenable by Faraday’s discovery of the phenomenon of 
diamagnetism. 
Ampére’s theory (1825), based on Oersted’s dis- 
covery (1820) of the magnetic effects of an electric 
current, may be regarded as the foundation of modern 
magnetic theories, though at that time the laws of 
electromagnetic induction were unknown. This theory 
led Weber (1854) to develop a theory which aimed 
at an explanation of the magnetic effects of bar 
magnets on the assumption that the molecules were 
always equivalent to miniature magnets, whether the 
NO. 2776, VOL. 111 | 
substance were magnetised or not, the action of the 
external field being merely to align the miniature 
magnets along the direction of the applied field. No 
explanation of the phenomenon of hysteresis was given, 
however, until Maxwell (‘‘ Electricity and Magnetism,” 
§ 444) extended Weber’s views and interpreted the 
more complicated hysteresis effects in terms of certain 
quasi-elastic forces. 
The theory of Ewing (1890) enabled us to visualise 
the nature of these hypothetical controlling forces by 
attributing the sluggishness of the response to an 
applied field as due to the interaction between special 
groups of molecules. This gave a rough explanation 
of hysteresis effects in terms of the mutual actions 
between complex groups of molecular magnets, and 
accounted for the shape of the hysteresis loops, the 
coercive force and the retentivity of a ferro-magnetic 
substance like iron. 
At the beginning of the present century, attempts 
were made by Voigt and J. J. Thomson to outline an 
electron theory of magnetism based on the magnetic 
effects of a moving electron, but it was not until the 
theory of paramagnetism and diamagnetism of Lan- 
gevin appeared (1905) that a satisfactory interpreta- 
tion of these phenomena was presented. 
The classical researches of Curie (1895) had shown 
that substances could be divided into three groups as 
regards their magnetic properties under an external 
field. These are, (1) diamagnetic substances, which 
show a minute negative induced moment, practically 
independent of temperature ; (2) paramagnetic sub- 
