L. A. Bauer— Variation of Terrestrial Magnetism. 315 
It will be recalled that we found that the main distribution 
phenomena could be represented by two rectangular magnetic 
systems, a polar and an approximately equatorial, the former 
of about 5 or 6 times the strength of the latter. This conclusion 
_ was reached with the aid of the magnetic observations for 1885. 
Suppose we carry out the same investigation for other dates, 
shall we reach the same result ? 
Let us begin with the year 1780, since for this date we 
possess the earliest fairly reliable isoclinic chart. This chart 
gives us the approximate distribution of the magnetic dip for 
that date. It was constructed by Christopher Hansteen.* 
The first thing to be done is to construct the “isapoclinics” 
according to the method explained in No. II. For this 
purpose, I have scaled the dips for points 20° distant in longi- 
tude and in latitudes +-60°, +40°, +20° and 0°. The data 
for latitude 60° S. could not be safely obtained and hence the 
“isapoclinies” are not extended beyond 40° S. For the 
regions of the two foci (secondary dip poles), I have made a 
few closer scalings. 
Having obtained the data as based upon observation, the 
computed dip I, is again derived with the aid of equation: 
tall == 2 taney, 
@m being the geographical latitude. It will be remembered 
that this is the dip that would prevail at the place where the 
latitude is pif the earth were uniformly magnetized about the 
geographical axis and were at rest. The departure of the 
actual (or observed) dip from this computed dip is: 
ZX I = Tuva iy Tessa == I, al L 
These differences we form for all the scaled values, plot 
them on a map in their true geographical position, and finally 
draw lines through all the places having the same AI, or 
departure, paying due regard to sign. The lines obtained thus 
I have termed lines of equal-departing dip or, briefly, isapo- 
clinics. The result for 1780 is given on the diagram, page 316. 
For the sake of convenience in comparison the same curves 
for 1885 are reproduced from No. II.t+ | 
It will be seen that the main result is the same for both 
dates. The secondary polarization is again nearly equatorial. 
Hence, the final conclusion of No. II is borne out by this 
early date. We shall see later that this is also true for an inter- 
mediate year, 1829. 
* The chart will be found in the Atlas to his ‘‘ Untersuchungen wtber den 
Magnetismus der Erde, Christiania, 1819. A reduced copy is given in Neumayer’s 
“Atlas des Erdmagnetismus,” Gotha, 1891, plate No. 41. 
+ In the reproduction the isapoclinic + 25° has been slightly improved, 
