Lovering and Bond on Magnetic Observations at Cambridge. 21 
among the heavenly bodies are so few and clear that the character 
of the disturbance will generally indicate something in regard to the 
cause which produces it. But the Elements of the Earth’s Magnet- 
ism are exposed to abrupt and violent fluctuations, which, so far as the 
circumstances are known, acknowledge no periods and, although 
perhaps capable of being explained by many conceivable causes 
which are in constant operation and therefore at the disposal of the 
philosopher, they cannot be distinctly brought home to any single 
one and are at present regarded as inexplicable. These magnetic 
hurricanes, as they have been fancifully called, are often exhibited 
during Auroral appearances, but many of them, so far as has been 
observed, are not coincident in time with this or any other class of 
natural phenomena. Now, every observed position of the needle for 
a given moment is beset with all these regular and irregular varia- 
tions ; which must be carefully eliminated by multiplying the number 
and shifting the exposure of the observations before we can be as- 
sured what is the exact and absolute value of the element for that 
time. And when we are seeking the amount of any particular order 
of perturbations, we must proceed as in the astronomical case by 
selecting as far as may be times for observation when the disturb- 
ance in question is a maximum and all others are of minimum value. 
The practice of the observer will supply many artifices of this sort 
for eluding or grappling with difficulties which appear at first sight 
insurmountable. It is obvious, that when the object is to ascertain 
the steady and periodical variations of the meridian we should ex- 
clude from the comparison those days whose serenity is affected 
by what Humboldt has denominated magnetic storms; just as we 
should pass over days of violent winds and tempests in deducing 
the gradual rise and fall of temperature during the 24 hours. No 
attempt should be made to frame an hypothesis or even to hazard 
