Lovering and Bond on Magnetic Observations at Cambridge. 57 
Aurora and the irregular perturbations of the magnetic meridian. 
It must not be inferred that other causes do not exist, in codpera- 
tion with the auroral phenomena, to derange violently the earth’s 
magnetism. According to Ampere’s theory of currents a large 
fund of such derangements must be deposited under the crust of 
the earth. The equilibrium although permanently stable must be 
subject to constant fluctuations. Theory supplies the reason and 
observation asserts the fact. Many of the small daily derangements 
have no apparent relation to the Aurora; and in regard to the mag- 
nificent strides of the Magnetometer it cannot be told which is 
cause and which is effect. If further search shall prove that an 
Aurora never fails to attend a great disturbance we may conclude 
that the Aurora itself is seldom displayed in the daytime. For the 
remarkable changes of declination almost always begin during the 
night and seldom continue into the next day. If, however, an un- 
seasonable Aurora should occasionally arise we may be able to 
perceive indications of its presence from the magnetic perturbations, 
although its light were eclipsed by the brightness of the sun. 
So far we have attended to relative only and not to absolute dec- 
linations. The former are sufficient when the object is to find the 
times of maxima and minima, the daily range and the diurnal curve. 
But it sometimes becomes necessary to know the absolute declina- 
tion, so that the process will now be described of referring any 
reading of the scale to its absolute value. It is clear that if the ab- 
solute value of one reading can be ascertained that of all the rest 
is known at once. It is convenient to have the absolute declination 
always referred to the same number of the scale; we will suppose 
this number to be, therefore, 100. To find then the absolute de- 
clination corresponding to 100 of the scale we proceed thus: the 
Variation-transit with which the Gauss Magnetometer is observed 
8 
