391 



The minimum of intensity here oc- 

 cui-rcd at 10'' 32'5 the maximum 

 later in the afternoon, after the 

 last observation at 1^ 41' p.'"m. 

 This day was a maguetical term, 

 and the results of the vibration 

 of the needle were in good har- 

 mony with the corresponding ob- 

 servations on the bifilar magne- 

 tometer. 



Aurora Borealis. 



[ 



In Gottingen I made 96 observations between 8 a. m. and 44 p. m., from 

 Aug. 27 to Sept. 10. In Dresden only 4, between Aug. 15 and 20 ; two a. m. 

 and two p. m. When it is possible I generally in every place 

 make at least two observations, one between 10 and 11 a.m., and 

 the other between 4 and G p. m., in order to eliminate the daily 

 variation. 



Since 1819, 1 have very often observed the dip here in Chris- 

 tiania, with three different instruments, 1«/, A 5-inch instrument 

 by Dollond, divided to 20', with two needles ; the one cylindi-i- 

 cal, with conical ends, and in the middle a cube, which is per- 

 forated in two directions, so that the axis can be inserted from 

 four different sides, and turned round ad libitum ; the other flat 

 lancet-formed, in which the axis can be put in from two sides, and ^ 

 turned. 2d, A C-inch instrument by Ertet, in Munich, divided to 

 10', with a flat needle and three axes, which can be turned round 

 ad libitum. 3d, An 8-inch instrument by Gamboi, in Paris, with 

 three needles and fixed axes. By the instrument 1st and 2d, 

 after every complete observation, the limb east and west, and 

 the needle iu the four usual situations, the dip read of four 



o 



o 



times in every position, after having been lifted from the agate 

 planes (in all thirty-two readings), the axis was turned 90°, and a 

 second obser\-ation made. This was continued four times, till 

 the axis arrived in its former position. In all three instru- 

 ments the needle very often was brought out of equilibrium, by 

 the application of a little tube, with a screw perpendicular to 

 its axis, upon the axis of the needle. This is useful, in order to 

 diminish the influence of the form of the pivots, especially on 

 needles with fixed axis ; and the influence of magnctical particles in the 

 circle ; shortly, to diminish constant errors. After this proemium, I shall 

 communicate my observations and their results. 



(ED 



