542 =Dr. E. Naumann—WNagnetism and Earth Structure. 
list of the most important works on the subject, and I consider it 
necessary that every scientific investigator should have a profound 
knowledge of the literature of his subject ; for when so qualified he 
need not discuss works which he has not studied carefully. 
Amongst the works which I have had occasion to peruse during 
my studies in terrestrial magnetism, a memoir by C. A. Schott on 
the secular variation of magnetic declination in the United States 
and at some foreign stations, is of special interest. Any one who 
has devoted any attention to the subject of secular change, knows 
that it is one of the most complicated branches of terrestrial physics, 
and at first sight it seems very curious that by far the greater 
number of copies of the above memoir were sold to lawyers. The 
demand in law circles for the work which treated of a purely 
scientific subject was so great, that several new editions had to be 
issued; and the reason of this is to be found in the difficulties 
experienced in following the old boundary lines of landed property, 
which had been originally laid out by the magnetic compass. This 
case is a good example of how a purely scientific problem may 
suddenly and unexpectedly become one of great practical importance. 
Schott’s investigations tend to show that the secular variation is 
periodic, but unfortunately the observations do not extend over a 
complete cycle. He compares the secular change with the motion 
of a pendulum. In the United States south of the 49th parallel, a 
complete cycle requires 24 to 34 centuries, and during this interval 
the needle describes arcs varying from 3° to 7°; in Paris the ampli- 
tude is 38° and the period about 42 centuries. 
Taken as a whole, the secular variation is perfectly systematic, 
and subject to remarkable laws which will well repay careful study, 
and being a phase in the life-history of our planet, the secular 
change may prove an excellent means of comprehending the internal 
condition of the globe. 
In Japan the magnetic elements have undergone considerable 
changes since the beginning of the present century, as may be seen 
by comparing recent surveys with that of Ino Tadagoski; a Japanese 
astronomer, who, about that period, made a geodetic survey of his 
native country. This observer had heard and read in foreign books 
of the variation of the compass, but, nevertheless, he denied its 
existence, and even went so far as to attribute the declination 
observed by Europeans to errors in their compasses; he also main- 
tained that the fact of his own needles always pointing due north, 
was owing to the superiority of his instruments, which were of his 
own construction. Thus he was led to believe that no variation of 
the compass existed. The average amount of secular change during 
the interval 1800 to 1880 is between 3 and 4 minutes of are per 
annum. Dr. Knott maintains that at the present time there is no 
secular change in Japan, and if this be correct, it is an important 
and unexpected result. Let us see how far it may be relied upon. 
As stated above, Dr. Knott uses a method which is a contradictio 
in adjecto, for if an average of differences be taken, and this average 
proves to be zero, as in the case considered, it does not necessarily 
