OF THE AURORA BOREALIS ON THE MAGNETIC NEEDLE. 
99 
placed on the comparison of measures thus made by itself at different times. 
I have in fact already discovered, by using- these precautions, such an increase 
of intensity as was to be anticipated during the cold season now come on. My 
observations have been too desultory, owing to numerous other avocations, to 
permit me to look out for a diurnal change of intensity. 
The balance of the watch makes 290 vibrations in a minute, so that the vi- 
brations are not commensurate to the seconds ; recourse therefore must be had 
to reading off the time by approximation on the seconds dial-plate. The time 
is estimated to the nearest quarter of a second by means of a microscope, and 
the watch relieved from the point where it stops for a new observation ; thus 
neutralizing any incorrectness in the reading off, and any inequality in the divi- 
sions of the dial-plate on the principle of the repeating circle. The stop is on 
the balance of the watch, and is therefore instantaneous. 
After trials of various arcs, I have found it most convenient, for insuring that 
the different series of observations shall commence in equal arcs, to make the 
needle vibrate by its own breadth at the extremity, that is, in arcs of nearly 
12°, which is correctly determined when the corners of the needle come alter- 
nately to the cross wires of the micrometer. The watch is relieved the instant 
the centre of the needle comes to the cross wires the third time after the extent 
of the arc has been noted by the corners ; 50 oscillations are then reckoned, 
and the watch stopped for reading off the time. A very small piece of iron is 
employed for moving the needle, which is instantly deposited at some distance 
at right angles to it ; and for night observations my candlestick is of glass. 
I had no leisure for making observations on the intensity for several days 
previous to the 14th of December. The last I made were on the 2nd, 3rd, and 
4th of December, — 4 sets of 50 oscillations each day at 8 p. m. ; average of 
each set, and of the whole, in 225".25 : Therm, from 35° to 42°. 
I shall now copy from my journal the observations of the 14th of December, 
They were set down at the time, at intervals of a quarter of an hour. 
5f p. m. — Aurora seen in various quarters of the sky. 
6 p. m. — Arch of nebulous light in N. about 25° high ; another S. about 30° 
high ; vertex of each about the mag. mer. : S. arch just over a con- 
tinued line of clouds. Many detached clouds in region of N. arch. 
o 2 
