30 Lovering and Bond on Magnetic Observations at Cambridge. 
polarity is one of the phases of the electrical fluid. The magnetic 
character of currents and the facility with which they are excited 
by motion and differences of temperature are now well approved 
facts. The earth’s rotation; the sun’s heat; volcanos; and the 
great eastern and western metallic ranges furnish all the materials 
and machinery that are needed for making the planet a grand elec- 
tro-magnet. Aside from the magnetic phenomena, the existence of 
the earth’s currents is thought to have been shown by direct experi- 
ment. If we do not admit them as the prime source of Terrestrial 
Magnetism they offer themselves as sufficient and satisfactory causes 
of the observed fluctuations about the average state. ‘The belief in 
the earth’s currents seems unavoidable ; and if any proportion sub- 
sists between their magnetic energy and those of artificial electro- 
magnets the fund of magnetism which they supply must be ample 
to explain all the chief facts of the earth’s magnetism. But if we 
adopt Gauss’s estimate that the whole magnetic power of the earth 
is equivalent, on the average, to 7,8 steel magnets of 1 pound 
weight magnetized to saturation for every cubic metre, there may 
be some difficulty in conceiving of sufficient iron ore in the earth 
to furnish the depositary of such a magnetic force according to the 
common motion of steel magnets. Besides, there may be a ques- 
tion whether it is philosophical to look round for any such new prin- 
ciple when it is not needed to explain the facts. If currents are 
finally adopted not as the auxiliary but the main and standing cause 
of Terrestrial Magnetism, the unavoidable fluctuations and occasion- 
al overflowings with the daily ebb and flow to which from their na- 
ture and remote source they must be subject afford a liberal expla- 
nation of the regular and irregular perturbations in the declination 
of the meridian. 
In so complex a subject, no attempt could be made at present to 
