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Magneto-Electricity, and Electro-magnetical Machines. 127 
running in mercury grooves in such a manner that the needle, by 
its own motion, produced the necessary reversals, when, upon the 
application of a vigorous calorimotor, it performed one hundred 
and fifty revolutions per minute, in the plane of the magnetical 
meridian, thus exhibiting terrestrial magnetism in a very agreea- 
ble manner. 
The north end of the earth shown to be virtually a Magnetical 
south pole.—While the needle was revolving by terrestrial mag- 
netism, I brought the south pole of a feeble artificial magnet to 
the lower point of the dip, so that the pole of the needle passed 
near to it. The motion was immediately accelerated. On pre- 
senting a north pole at the same point the motion was retarded, 
stopped, or reversed, according to the strength or proximity of 
that pole. 
The south polarity of the north end of the earth still more stri- 
kingly exhibited.—I constructed a semicircular steel magnet, the 
inside diameter of which just permitted the dipping-needle to re- 
volve within it, and attached it in such a manner, that the south 
pole of it was at the lower point of the dip, and bending round to 
the south had its north pole at the upper end of the dipping axis. 
The battery being applied, the revolutions were exceedingly rapid, 
and in the SAME DIRECTION as by terrestrial magnetism. On re- 
versing the semicircular magnet and bringing the north pole at 
the lower point, the motion was reversed, and conrrary to that 
pire by terrestrial magnetism. In this form, the instrument 
resembles Messrs. Davenport and Cooke’s model, as exhibited last 
spring in New York, one of their semicircular magnets being re- 
moved and the instrument being turned down on one side, so as 
to bring the diameter of the other into the dipping axis. 
Magneto-Electricity produced by Terrestrial Magnetism.— 
Removing the semicircular magnet and the battery, connecting 
the poles of my thermoscopic galvanometer with the mercury 
grooves, and giving the needle a smart whirl by hand, say one 
hundred and fifty revolutions per minute, I obtained a sufficient 
quantity of electricity to deflect the galvanometer needle 40° by 
impulse, and and that too against a torsion wire six inches long, 
weighing one third of a grain. And here I ought to remark, that 
the mechanism used for the reversal of electrical currents, and 
consequently of the polarity, when the instrument is used as a 
self-revolving machine, was precisely what was required in pro- 
