132 
The whole of the discussions of these eminent men involved 
this supposition of the equality or near equality of the periods 
of rotation and revolution whilst the moon’s figure was forming, 
as the bases of their solutions, and having only the theory of 
gravitation as the foundation of their methods, they had no 
option left to them. 
Since about the year 1820, we have gradually become ac- 
quainted with other powerful forces in nature. (Ersted’s dis- 
covery that a magnetic needle sets itself at right angles to a 
circuit over or under which it is placed when a galvanic current 
passes along that circuit, was quickly followed by others still 
more surprising, and the science of electro-magnetism in its 
various branches, has in less than 50 years from its birth grown 
to be a most extensive as well as a most important science. 
The branch which we call magneto-electricity arose afterwards, 
and it is to the forces of magneto-electricity that I shall refer 
for explanation of the lunar phenomena before related. 
I must be allowed to diverge from the simple course of 
procedure to remind the meeting that bodies may be now 
arranged in two classes which we call magnetic and diamag- 
netic bodies. The class of magnetic bodies comprehends those 
which are attracted towards the poles of powerful magnets, and 
the class of diamagnetic bodies comprehends all the others, 
whether neutral or repelled from the poles of powerful magnets. 
The first class comprehends many metals, as iron, nickel, 
cobalt, manganese, chromium, Wc. crown-glass, potassium, 
sodium, many salts and oxides of the magnetic metals, as 
well as magnetic minerals, and oxygen gas. The second class 
contains bismuth, phosphorus, antimony, heavy glass, or si- 
licated borate of lead, zinc, &c., flint-glass, mercury, lead, silver, 
copper, gold, &c., nitrogen gas. 
Hot air is diamagnetic with respect to cold air, and hence 
the peculiar action of powerful magnets on flames. 
The repulsion of the most diamagnetic substances, as bis- 
