296 Account qf' the recent Magneilcal Discoveries 
begins to oscillate ; and when the elongation is 20°, then the ob- 
servation commences. 
The times are marked by a chronometer at the instant of the 
beginning of the 1st, 10th, 20th, &c. oscillation, (or rather at the 
termination of the preceding one), continuing thus till 360 oscil- 
lations are completed, of which the last one vibrates within an arc 
of 2° only, large enough, however, to be distinctly observed. I 
now take the difference of the times marked at the first commence- 
ment and the beginning of the 300th vibration; also the diffe- 
rence between the 10th and 31 0th ; likewise the difference be- 
tween the 20th and 320th ; and so on to the difference of the 
times marked at the beginning of the 60th and 360th. This 
gives seven different determinations of the duration of 30Q vi- 
brations each. If the arches through which the vibrations take 
place were not shortened by the opposition of the air, these dura- 
tions would equal; but now the last duration is about shorter 
than the first. I take therefore a mean of ail the seven. After 
this I mark the time at every 6th duration ; and thence I get a 
mean of 11 durations of 300 vibrations. The following ex^ 
ample will explain the wliole. 
' OcTOBKR 1. 1820. 
Order of the Vibra- 
tions. 
Instant of the 
Ceasing of the 
Vibrations. 
Order of the Vi- 
brations. 
Instant of the 
Ceasing of the 
Vibrations. 
Duration 
of 300 Vi- 
brations. 
Ceasing of Othvibr. 
o' 35,0 — 35,0 
Ceasing of 300th vibi*. 
14 5,3= 845,8 
810,8 
6 
51,2 51,2 
306 
22,0= 862.0 
810,8 
12 
1 7,6 = 67,6 
312 
38,2 rr. 878,2 
810,6 
18 
23,8 =1 83,8 
318 
54,4= 894,4 
810,6 
24- 
40.0 - 100,0 
324 
15 10,4= 910,4 
810,4 
30 
56,4 = 116,4 
330 
26,8 = 926,8 
810,2 
S,6 
2 12,6 132,6 
336 
42,8= 942,8 
810,2 
42 
28,8 148,8 
342 
59,2 = 959,2 
810,4 
48 
45,2 165,2 
348 
16 15,2 = 975,2 
810,0 
54 
3 01,4 = 181,4 
354 
31,6= 991,6 
810,2 
60 
17,6 197,6 
360 
47,7 = 1007,7 
816,1 
Mean, 
1 810,41^ 
Since the intensities are reciprocally as the squares of the du- 
rations of equal numbers of oscillations, I may assume any one 
of these durations as unity, and express the other ones in parts 
® Mr Hansteen is not wrong j q" upon the mean of 11 durations. 
