1816.) during the Year 1815. 13 
than the declination. Hence it would be of considerable import- 
ance to the theory of magnetism to be in possession of aset of good 
observations of the dip in different latitudes, and might lead to 
important deductions respecting the position and depth of the mag- 
netic poles of the earth. On that account the following observa- 
tions by Humboldt, giving the magnetic dip in different parts of . 
the North Atlantic, for the year 1799, are of considerable value. 
4 (Number of, 
N. iati- | W- Lonei- Magnetical oscillations 
tude from . 3 5 
tude. Geenoniali dip. in ten mi- 
; nutes. 
38° 52 18° 42 68°18 242 {Good observation. 
3T 26 18 52 67°81 242 |Almost perfect calm. 
34 30 19 05 65°10 234 =|Perfect calm. 
31 46 19 24 64-71 237  |Doubtful, especially the intensity. 
28 28 20 53 62°41 238  |Good. 
24 53 23 18 60°34 239° =| Very good. 
= 29 28... 2 58°18 237 Good. 
19 54 ef rs 57°27 236 |Geod. 
4 15 50 23 50°67 239 |Good. 
13 20 5a so 45°60 234 |Dip good, intensity doubtful. 
aL iy fame Bi 42°34 237 Good. 
10 46 63 14 42°25 229 Good. 
3. The variation of the compass, or the alteration which takes 
place in its declination, or in the point towards which it is directed 
in different longitudes, was first observed by Columbus. That the 
declination varies in the same place was first discovered in England, 
though the name of the discoverer is not accurately known. Wallis 
ascribes it to Gellibrand, who, according to him, made the dis- 
covery in 1645. According to Bond, the discovery was by Mr. 
John Mair. In the year 1657 there was no variation of the com- 
pass in London, or in other words, the needle pointed due north. 
In 1580 it pointed 11° 15’ east. In 1692 the variation was 6° 
west. Ever since the year 1657 the declination has been advancing 
west, and in the year 1814 it was 24° 22’ 22”, according to a 
mean of the very accurate observations of Colonel Beaufoy, which 
1 consider as superior in precision to any that were ever made 
before him. At first the declination varied at a considerable rate ; 
thus, during the first 15 years after 1657, the declination had ad- 
vanced west two degrees and a half, which gives a variation amount- 
ing to ten minutes for each year. But of late years this declination 
has been progressively diminishing, and according to the observa- 
tions of Colonel Beaufoy, the increase from 1813 to 1814 was only 
31”; or 40”, if we confine ourselves to the state of the needle at 
noon. 
Dr. Halley was the first person who endeavoured to form a 
theory capable of explaining this variation in the declination. He 
supposed that the earth contained an immense magnet within it, 
poised upon its axis, and having four poles, two weaker than the 
