of the Magnetic. Poles of the-Earth. ~ ay 
for us to diye with our bodily eyes into the bowels of the 
earth, the greatest depth to which we ‘have arrived being 
trifling compared with the actual diameter of the globe :—yet 
the interior of the earth is revealed by its effects,on its sur- 
. face. Thus, the experiments.on the deviation of the plummet 
from the vertical line in the vicinity of mountains, show that 
the mean density of the mass of the earth is five times greater 
than that of water; and that consequently this mass is denser 
than most kinds of stone, and is therefore for the most part 
metallic. ‘Thus the periodical annual and diurnal motions of 
the magnetic needle are a mute language, ‘telling us what is 
passing within the earth. Thus the aurora borealis is perhaps 
the result of a contest of powers set in motion by the different 
substances of the earth, substances which by these means may 
one day become known tous. For we may justly conclude 
of the causes from their effects, which is the usual way of extend- 
ing our knowledge of nature. : 
Yet, although this investigation affords a great interest both 
for theory and practice, it is not every person’s business to 
enter into mathematical investigations. I thought, therefore, 
that I might gratify many readers in giving here a popular 
sketch of the results of my investigations on terrestrial mag- 
netism. 
The accompanying charts (Plates I. and II.) represent two 
segments of the surface of the earth from the poles to the 50th 
degree of latitude. The longitudes are calculated from the 
meridian of Greenwich, as most observations have been made 
by British seamen, who calculate from that meridian. The 
arrows on the charts indicate the directions of the magnetic 
needle; the end of them, towards the opposite side of the pole, 
denotes the place of observation; and the angle formed by the 
agen meridian with this end of the arrow is there- 
ore the variation of the needle found in the observation. The 
observations given on the southern chart are all Captain 
Cook’s, and were made between the years 1772 and 1777: the 
observations on thenorthern segment are by Captains Cook and 
Phipps, Admiral Lovenérn, Captain Billings, and others, made 
about the same period. Some of them have the time of the ob- 
servation affixed to them. The most important observations 
made during the last English north-polar expedition (1818— 
1820) are marked with an asterisk. As these observations 
embrace so short a space of time, they may be considered as 
being contemporaneous, and thus show the magnetic condition 
of the earth in the vicinity of the poles during the quarter of a 
century just elapsed. 
The variation in all Europe is now westerly. | If we go from 
east 
