through several holes. Another small compass belonging to the transit instrument,
to the needle of which no card was attached, was placed in a recess at
the opposite side of the house, in the open air. The variation of the latter
was inspected by Mr. Franklin, and of the former, by me, at several hours
of the day and night. Both were very frequently affected by the Aurora, but
in different degrees ; and they recovered their former positions at different
times. Upon some occasions they were affected in the day, and upon others,
one compass alone was affected. Mr. Franklin remarked, that a certain
position of the Aurora was always attended by a variation of his compass in
the same direction, and that the motions of the needle were so immediate, as
to be often visible during his observation. Mine, on the contrary, however
various the shape and situation of the Aurora, in the course of two or three
hours, declined gradually to the eastward or westward, without ever retrograding
; and I could not perceive that the increase or decrease of the variation
depended upon any particular position of the Aurora, for it never was stationary
for one-fourth of the above-mentioned period. However, all the remarkable
aberrations of both instruments took place during the same nights, and were
found to be at their maximums on the following mornings, and they generally
recovered before 8h. p. m. As these maximums were the last result of the
impulses given by the Aurora, and their duration the longest, they were compared
together, and about half of them found to be in contrary directions from
the magnetic meridian, but not equal in quantity. Considering the Aurora as
magnetic effluvia, the aberrations of both needles should have been always
in the same direction with respect to the magnetic meridian; because all
bodies which are not ferruginous, are equally conductors of magnetism, and
will transmit it in the same straight line—whereas, electricity conveys itself
through the channels which are the best conductors, and may act from different
quarters upon two needles differently situated: and its effect continues, till
the charge received by the needle is carried off by the pivot supporting it,
which, by experiment with an excited electric, appears to require not less
than 10 or 12 hours.
The common cork-ball electrometer not having on any occasion given signs
of a charge, I tried the following experiment, in order to attain further evidence
on the subject. A brass needle was attached to a compass card, and balanced
on a copper pivot in a wooden box. It was about four inches in radius, and a
copper arch of 60° to that radius, was fixed at one end of the box, which was
closed by a wooden slide, and paper pasted over every crevice to exclude the air.
To give it the same advantages for conducting electricity as the compass boxes,
(which are made of brass,) I introduced an iron wire, eight inches in length,
perpendicularly through the lid, in such a manner, that its lower extremity was
in a horizontal plane with the needle; and a pane of glass at that end of the
lid, enabled me to see into the interior of the box. Having previously ascertained
that it contained no magnetism, the instrument was placed, on the 2d
of May, on a covered shelf, at the outside of the house, in a position nearly
east and west; the brass needle being 25' from the conductor, and a small
glass bubble adjusted on the box, in order to prevent its otherwise unperceived
movement. At 12h. p. m., I examined the needle, and found its position
unaltered. No Aurora was then visible, but one was afterwards seen by Mr.
Franklin; and at 8h. a. m., May 3d, the needle and conductor were in contact.
I moved the needle 40' from the conductor, and it was similarly affected at
some period on the nights of May 3d, 5th, 6th, 9th, 10th, and 11th. The
thermometer, during this period, ranged in the day between + 26° and +, 56°;
and in the night, between +, 10° and + 33°, I did not see the Aurora, except
on the nights specified above; and did not perceive any alteration in the
needle till the succeeding mornings.
The night of the 12th furnished a more satisfactory proof of the agency of
the Aurora. At lOh. p. m., the needle was not affected, and no Aurora was
visible. At Oh. 30' a. m., May the 13th, several arches appeared across the
sky from N.W. to S.E., and the needle was attracted to the conductor from
the distance of 1°. The temperature of the air was + 12. I now determined
to convert the instrument into a kind of electrometer, by insulating the needle
and conductor. The pivot which supported the former was fixed upon sealing
wax, and the part of the latter which passed through the lid was covered with
the same substance.
Paper was pasted on the box as before, and it was re-placed at 2h. p. m., on
the 14th, the temperature of the air being 54°. A heavy gale of wind from
N.N.W. with snow, immediately followed, and the .temperature of the air, at
midnight, was reduced to 19°. At 9h. a. m. May 15th, the needle was removed
30° from the conductor, and both were still charged, so that I could not bring
them together till the conductor Was accidentally touched. I believe this
4 F 2