110 L. A. Bauer— Variation of Terrestrial Magnetism. 
ical points distant 20° in longitude and situated on the paral- 
lels of latitude 60° N., 40° N., 20° N., equator, 20°S., 40° S. 
and 60° S. The data (declination and inclination) were scaled 
from isogonic and isoclinic charts. In this way was added to 
the list of secular variation stations already discussed, 7X18= 
126, symmetrically distributed. 
In the case of the declination, material has been gathered 
covering a period of 3$ centuries. Twenty-six charts frora 
1540 to 1890 were scaled, giving about 3000 declinations. In 
some cases the original charts as found in the European libra- 
ries were utilized. The curves expressing graphically the 
secular variation in declination for the 126 hypothetical points 
have all been laid down on a map in their true geographical 
positions. The outcome is a most interesting one and has 
fully repaid the requisite labor. Owing to lack of time, its 
publication and discussion must at present be deferred. 
Inclination data are not so numerous. Little use, if any, can 
be made of Hansteen’s isoclinic charts for the years 1600 and 
1700. The earliest fairly reliable chart that can be utilized is 
the one by Hansteen for 1780. Besides this, the following 
were scaled: Erman and Petersen’s (1829), Sabine’s (1840-45), 
Evans’s and Creak’s (1860, 1870, 1874 and 1880) and Neu- 
mayer’s (1885). 
I then drew up tables of declination and inclination for the 
seven dates, 1780, 18380, 1842, 1858, 1872 and 1885. With 
these data I can construct the secular variation curves for the 
time interval 1780 to 1885 at 126 points uniformly distributed 
over the earth on the principle already laid down.* Iam plot- 
ting them on a large map on Mercator’s projection and on 
twice the scale formerly employed, i. e., I draw the curves 
described by a freely suspended magnetic needle whose total 
length is 80° instead of 40°. On this same map, I intend to 
give the curves derived from actually observed data. The 
_ pursuit of a secular wave around the earth, or the discernment 
of laws, will thus be materially facilitated. This work is fairly 
under way. I would like to suggest that those who are engaged 
in similar investigations adopt the same scale if possible. 
To be sure the material obtained from. isogonic and isoclinie 
charts cannot be relied upon as though it had been actually 
observed. Still it should be remembered that these charts 
were based on observations. While no doubt the constructor 
of the chart had to proceed arbitrarily at times on account of 
defective or deficient data, nevertheless, the data obtained from 
them possess a value of no mean degree, provided they are 
* In “ Beitraege zur Kenntniss des Wesens der Saecular- Variation des Erdmag- 
netismus,”’ Mayer and Miller, Berlin, 1895. See article referred to in ‘‘ Physical 
Review.” 
