Dr. E. Nauwmann— Magnetism and Earth Structure. 548 
follow that there is no secular variation in Japan, but the result can 
be explained by the well-known fact that the amount of secular 
change is different in different places, and may be positive in some, 
whilst it is negative in others. 
From a comparison of Sekino’s and Knott’s maps, however, it 
may be seen that the isogonic lines have shifted very little indeed, 
and we agree with Dr. Knott when he says: “‘ Within the period 
beginning with 1883 and ending 1887, there is practically no 
change, or if there is, it is a very small change indeed. It looks 
almost as if we were just passing through a time of maximum 
declination.” This result is extremely interesting if we take into 
account what Fritsche has said about the line of zero declination 
when it passes through Mongolia. The value of the secular varia- 
tion for stations lying near this part of the line is very small, and 
we may therefore conclude that the wnole magnetic island of western 
declination covering a considerable part of Hastern Asia is at present 
in a nearly stationary condition. 
When speaking of the diurnal means obtained from Sekino’s 
survey, I quoted part of the explanation accompanying his magnetic 
map, and I now direct attention to another passage of that explana- 
tion, which runs as follows: “Il y a un nombre d'irrégularités 
considérables qui sont particuliéres & des régions isolées. Ce ne’st 
pas le caractére spécial de ces irrégularités qui est représenté dans 
ja carte par les lignes en forme de cercle, mais simplement le lieu 
et Vextension.” There is no doubt but that a complete magnetic 
survey of Japan will prove the existence of many small magnetic 
islands, which can at present only be vaguely indicated. 
Dr. Knott’s map does not show the Sado sinuation of the south- 
west curve as I have shown it, but represents the irregularity by 
an isolated line round the island. He prefers this form because— 
as he says—it is well known that the isogonic lines at or near 
islands often present irregularities of quite a local character. I 
am inclined to believe that this is not well known. I, at least, 
was not aware of it, except as regards oceanic islands which may 
—as Rijckevorsel says—appear to have some power of coercion 
on the isogonic lines. Oceanic islands, however, are indications of 
great submarine mountain ranges, which are generally accompanied 
by deep fissures in the earth’s crust, but no power of coercion can 
be expected from an island which is merely a detached piece of the 
main land. 
The problems presented by the magnetic islands of Japan can 
only be solved by a very detailed survey, and it is interesting to 
enquire what such a survey means. (Quite recently a very detailed 
magnetic survey of part of the kingdom of Wiirtemberg has been 
carried out by Hammer, and he estimates that about 90 stations 
would be necessary for the whole kingdom. ‘Taking this as a basis, 
Japan would need 1800 stations, Germany 2500, and Great Britain 
1500 stations. 
These numbers induce me to again refer to the distribution of 
stations in the first Japanese survey, objected to by Dr. Knott. In 
