September 14, 1888.] 



SCIENCR 



127 



A MAGNETIC SURVEY OF JAPAN. 



A RECENT number of the Journal of the College of Science of 

 Japan contains the results of a magnetic survey of the islands, 

 carried out by Cargill G. Knott and Aikitsu Tanakadate. The 

 determination of the magnetic elements of Japan is of peculiar in- 

 terest, as the results of former researches led Dr. Naumann to the 

 conclusion that intimate connections exist between magnetic ele- 

 ments and geological structure. This opinion was based on the 

 magnetic survey of Japan, carried out by Messrs. Sekino and Kodari 

 in 1S82-S3. A careful review of the methods and results of this 

 survey was therefore very desirable, and Dr. Knott undertook this 



a fairly good distriliution, and a shunning of local disturbances due 

 to volcanic rocks. The second condition was extremely difficult to 

 fulfil, particularly in the northern parts of Japan, where magnetic 

 rocks abound. 



Eighty-one stations were occupied in the course of the survey. 

 As the results of the observation on declination are of particular 

 interest in connection with Naumann's theory, we reproduce Knott's 

 map of lines of equal magnetic declination. The lines were drawn 

 by hand from consideration of the observations of contiguous sta- 

 tions. From these he has computed parabolic lines by the method 

 of least squares. We have reproduced these also, although they 

 seem to be of little value, considering the great amount of local 



MAP OF JAPAN SHOWING THE LINES OF EQUAL MAGNETIC DECLINATION. (According to K.sott and Navmann. 



task. He found that it would be unsafe to deduce from them any 

 definite conclusions as to the general magnetic characteristics of 

 Japan. His principal reason was the inadequate selection of 

 routes, and the fact that the observations were made in two sets, 



— one in the fall of 1S82, the other in the fall of 1883. — the obser- 

 vations being made usually about 9 A.M. or 3 p.m., but not with 

 absolute regularity. He found that no satisfactory attempt had 

 been made to reduce all observations to one hour. 



It thus appeared that the thing to be desired was a new survey, 



— what might be called a preliminary survey of all Japan, special 

 attention to be paid to the distribution of stations. The work 

 was carried out by two parties, one visiting the northern half of the 

 territor)' to be studied ; the other, the southern half. In selecting 

 the stations, two considerations were principally borne in mind, — 



variation. The lines as constructed by Naumann are shown for 

 comparison on the smaller map. 



The following remarks of Dr. Knott are of interest in reference 

 to Naumann's theory. Based on the broad features of Sekino's 

 chart, Naumann finds in the form of the isogonic line of 50 \V. a 

 close relation to the so-called ' Fossa Magna." Just where this 

 great break in the geological continuity of the countr>- occurs, there 

 a Large sinuosity seemed to show itself on the isogonic line. This 

 great fault, the Fossa Magna, almost stretches right across the central 

 part of Japan in a nearly north-and-south direction. The well- 

 known volcano Fujiyama is included in it, and so. it is generally sup- 

 posed, is the line of volcanic islands stretching south-easterly. The 

 Fossa Magna hardly reaches the northen coast of Japan ; but, if 

 continued northwards, it would be found to run between the penin- 



