OF ALL THE PHENOMENA OF TERRESTRIAL MAGNETISM. 
105 
eighth of an inch deep and broad : and lastly, a groove of the same breadth, 
but of double the depth, was cut like a meridian, from pole to pole, half round. 
These grooves were for the purpose of laying in the wire, which was effected 
thus. The middle of a copper wire, nearly ninety feet long and one-tenth of 
an inch in diameter, was applied to the equatorial groove, so as to meet in the 
transverse meridian ; it was then turned down that groove, one end towards 
one pole, the other towards the other pole, as far as the first parallel ; it was 
then made to pass round this parallel, returned again along the meridian to 
the next parallel, then passed round this again, and so on till the wire was 
thus led in continuation from pole to pole. 
The length of wire still remaining at each pole was bound with varnished 
silk, to prevent contact, and then returned from each pole along the meridian 
groove to the equator : at this point, each wire being fastened down with 
small staples, the two wires for the remaining five feet were bound together 
to near their common extremity, where they opened, to form two points for 
connecting the poles of a powerful galvanic battery. When this connection 
was made, the wire became of course an electric conductor, and the whole 
surface of the globe was put into a state of transient magnetic induction ; and 
consequently, agreeably to the laws of action above described, a neutralized 
needle freely suspended above such a globe, would arrange itself in a plane 
passing from pole to pole through the centre, and take different angles of in- 
clination according to its situation between the equator and either pole. 
In order to render the experiment more strongly representative of the actual 
state of the earth, the globe in the state above described was covered by the 
gores of a common globe, which were laid on so as to bring the poles of this 
wire arrangement into the situation of the earth’s magnetic poles, according 
to the best observations we have for this determination. I therefore placed 
them according to the mean results of Sir Edward Parry and Captain Foster 
in latitude 72° N. and 7 2° S., and on the meridian corresponding with longi- 
tude 76° W., by which means the magnetic and true equators cut one another 
in about 14° E. and 166° W. longitude. 
The globe being thus completed*, a delicate needle must be suspended above 
* This globe was constructed in 1824, and exhibited by Dr. Birkbeck, March 26th of that year, 
at the London Institution. 
MDCCCXXXI. 
P 
