De la Rive on the Aurora Borealis. 359 
netic pole, the greater the number of currents that will act upon 
it: near the equator, it will not be subject to any influence from 
the currents which are formed beyond the region around the nee- 
dle. In winter these differences are less sensible, because the 
inally, according to our theory, the same effects should be 
manifested in the southern hemisphere, only that all is reversed ; 
and this is fully established by the various results of recent ob- 
servers, including those of Colonel Sabine and a large number of 
travellers! 
{ should however acknowledge that there are some anomalies, 
either in the hours or in the direction of diurnal variation, at cer- 
tain places, especially at St. Helena and the Cape of Good Hope, 
anomalies which it is difficult to explain by the theory proposed. 
ut I am convinced that when further examined, they. will be 
found to be due to local and accidental causes, such as the vicin- 
ity of the sea, which influences very notably the diurnal varia- 
tions of temperature, and especially their amplitude and the hours 
of the maximum and minimum of heat. he question whether 
there are not places of no variation, proposed by Arago, is of 
little importance in this connection. The points of the earth’s 
Surface without diurnal variation, will be those where the two 
vary with the sun, the temperature, the winds, and other disturb- 
ing causes. i 
But I do not dwell on this point, as my object is not to treat 
of the diurnal motions of the needle. My end is simply to prove 
from the diurnal variations, the existence of the terrestrial cur- 
Tents. In continuation, we may obtain another proof still more 
direct, although less general, of the presence of these currents, 
by making use of the telegraph wires for collecting them. This 
I have done in England, as has also Mr. Barlow; and M. Baum- 
gartner has performed similar experiments in Germany. In these 
trials, the currents have in all cases been detected by means of 
the galvanometer. M. Baumgartner, having introduced a ve 
Sensitive galvanometer into the circuit formed by the telegraph 
Wire between Vienna and Prague, which has a length of about 61 
miles, obtained the following results when the two extremities of 
Wire were buried in the earth, 
1. The magnetic needle never stood at zero, but was more or 
less deviated. 
