Natural Phihsophy — Magnetism. W7 
cate fibre of silk, whose torsion was insensible. The mica from 
Zinwald oscillated twelve times in 55 ", and the Siberian mica 
only seven times in the same period. Hence, the magnetic 
forces were as (12)^ to (7)®, or nearly as 6.8 to 20. If we sup- 
pose, therefore, that the quantities of iron which they respec- 
tively contain are proportional to these forces, the Siberian 
mica must contain 6.8 per cent, of the oxide of iron, and the 
Bohemian mica contained 20 per cent. It is very remarkable 
that this result accords exactly with M. Vauquelin’s analysis,, 
which was sent to M. Biot after his own experiments were 
finished. M. Biot does not say that he took the precaution of 
making the micas oscillate at the same temperature, as it has been 
ascertained that the magnetic qualities of minerals that contain 
oxide of iron, and also those of mica, are developed more pow- 
erfully by heat A full account of these interesting experiments 
will be found in the Memoirs of the Academy of Sciences for 
1816. 
21. Variation of the Needle at Stockholm and at Tronyem err 
Drontheim^ in Norway. — The following results respecting the 
declination of the magnetic needle at Stockholm, have been 
published in the Memoirs of the Swedish Academy of Sciences, 
Variation. 
1763, 18th May 
1811, June 
1817, March 
1817, April 
ir 50' 0" West. 
15 51 40. 
15 35 51 
15 34 
51 I 
17) 
15° 35" 4" Mean, 
The following observations on the declination were made at 
Drontheim in Norway, in north latitude 63° 26' 16", and east 
longitude. 
1769, 
15° 
25^ 
1777, 
17° 
45^ 
1770, 
15 
SO 
1778, 
17 
50 
1771, 
15 
40 
1779, 
18 
0 
1772, 
16 
6 
1780, 
18 
0 
1773,, 
16 
40 
1781, 
18 
29 
1774, 
16 
46 
1782, 
18 
SO 
1775, 
16 
58 
178S„ 
18 
32 * 
1776, 
17 
SO 
22. tHiurnal Variation of the Needle. — We’ are glad to observe 
that the very interesting and hitherto inexplicable phenomena 
of the daily changes in the position of the magnetic needle, are 
* See Dr Clarke’s Travels ^ Part iii.. Sect. 1. p. 751. 
