TERRESTRIAL MAGNETISM.—MURPHY. 301 
that the Aurora Borealis was a magnetic phenomenon, has ac- 
quired empirical certainty from Faraday’s brilliant discovery of 
the evolution of light by magnetic forces.” He then follows 
with a beautiful description of the Aurora. 
It is now nearly half a century since Baron Von Humboldt 
reviewed the opinions of Ersted, Arago, Ermen, Ross, Brewster, 
Faraday and Sabine on terrestrial magnetism and, although 
great advances have been since made (in the science of electri- 
city which has many similar manifestations and in dynamical 
geology which may assist in studying the earth as a magnet), 
and presented for our guidance and instruction, still these opin- 
ions are held in as high esteem to-day as the day the first 
volume of the Cosmos was presented to an admiring public. 
In 1873-1874 Rowland made an extensive series of experi- 
ments; the results are said to form one of the most important 
contributions yet made to our knowledge of magnetic induction. 
They were published in the Philosophical Magazine of those 
dates, and are quoted by Professor Chrystal in the article on mag- 
netism in the Encyclopedia Britannica. He treated his results 
graphically. The general conclusions to be drawn from his 
experiments are as follows : 
“1, The magnetic properties of iron, nickel and cobalt at or- 
dinary temperatures differ in degree, but not in quality. 
“2. As the magnetizing force increases from 0 upwards, the 
permeability of iron, nickel and cobalt increases until it reaches 
a@ maximum, and after that diminishes down to a very small 
value. The maximum value is reached when the metal has 
attained a magnetization of from .24 to .388 of the maximum. 
“3. Thecurve showing the relation between the susceptibility 
and the intensity is of such a form that a diameter can be 
drawn bisecting chords parallel to the axis. 
“4. Ifa metal is permanently magnetized its permeability is 
less for low magnetizing forces, but is unaltered for high magnet- 
izing forces. This applies to the permanent state finally attained 
after several reversals of the magnetizing force; but if we 
strongly magnetize a bar in one direction and apply a weak 
