362 De la Rive on the Aurora Borealis. 
insulating layer in all was a centimeter thick, giving 4 centime- 
ers for the thickness of the bar thus covered. Within the globe, 
a ring of copper surrounded the bar and its insulating bed, at 
the part most distant from the tubulure. ‘This ring was arranged 
to be put in communication with a source of electricity exterior 
to that of the bar by means of a metallic wire insulated with 
care, which passed through the tubulure and ended without in a 
ook. A stopcock attached to the other tubulure of the globe, 
was arranged for obtaining a vacuum. When the air within is 
sufficiently rarified, the hook is connected with the conductor of 
an electric machine, and the outer extremity of the bar of iron 
with the soil; by this means the electricity forms within the globea 
luminous sheaf, more or less irregular, which passes from the ring, 
and terminates at the inner extremity of the soft iron. But im- 
mediately on placing the outer extremity of the soft iron on the 
pole of an electro-magnet, the electric light takes a wholly dif- 
ferent aspect. Instead of proceeding indifferently from different 
points of the upper surface of the cylinder of iron, it proceeds 
from all points in the circumference of this surface, so as to form 
around it a continuous luminous ring. ‘This is not all: this ring 
has a movement of rotation around the magnetized cylinder, 
sometimes in one direction and sometimes in the other, according 
to the direction of the electric current, and the nature of the 
magnetisation. Finally, jets of brilliant light are seen to proceed 
from this luminous circumference, which are distinct from the 
rest of the mass of light. When the magnetization ceases, the 
Juminous phenomena return to the condition familiar in the ex~ 
periment, known under the name of the Electric Egg. 
There is some advantage in using for the experiment here de- 
scribed Armstrong’s hydro-electric machine, in which the boiler 
is Made to communicate with the hook which is united by a mé 
tallic connection to the ring of copper within the globe, whilst 
the conductor which receives the vapor is put in connection wit 
the bar of soft iron. Thus we have in the globe an electric cut 
rent of ‘great intensity which may be changed in direction, by 
inverting the connections. 
3. Agreement of the theory with the facts. 
on the aurora borealis published in the History of the Voyage © 
Captain Franklin. Lieutenant Hood and Dr. Richardson were 
55 miles miles apart for the purpose of making simultaneous ob- 
Servations, in order to ascertain the parallax of the phenomenon 
aud consequently its height. The results from three trials place 
