148 Disturbance of the Earth's Magnetism. 
netism of the earth decreases, recovering its former strength by de- 
grees, often not till the end of twenty-four hours.”* ‘This state- 
ment, founded on observations made in Norway, is a precise de- 
scription of the phenomenon observed in Albany; and should it be 
found a general, or even a frequent occurrence, that a great increase 
of intensity precedes the appearance of the aurora, it would perhaps 
reconcile many apparent discrepancies in the different accounts of 
magnetic influence of the meteor. 
Prof. Hansteen also remarks, in the same paper, that “the polar 
lights seem to be the effect of an uncommonly high magnetic in- 
tensity, which lets itself off, as it were, by the aurora, and thus sinks 
under its common strength.” Nothing, however, can with certainty 
be deduced from these observations, in reference to this supposition 
since the magnetic intensity at any place, as exhibited by the vibra- 
tions of the horizontal needle, may change while the absolute force 
or intensity of the whole earth remains the same. If we present by 
F the whole force in the direction of the dipping needle, by 6 the 
dip in degrees, and by H the horizontal force, we shall have, by 
a well known law, 
a cosin 6° 
Tn this ccs it is evident that F may remain eae although H 
is caused to vary by a change in the value of cosin 6. The fact, 
therefore, of a variation in the absolute intensity, can only be deter- 
mined by combining the observations of the vibrations of the hori- 
zontal needle with simultaneous observations on the dipping needle. ° 
suppose I" constant during the change of horizontal inten- 
sity as observed at Albany, we may, by means of the above formula, 
calculate the change in declination or dip required to produce the 
observed diiccnnce? in the horizontal intensity. Assum ming 6 = 75°, 
(the dip at Pobany nearly,) and H = to the horizontal intensity ob- 
served at 6 o’clock, we can readily find the value of F; and since 
this value is supposed sega by substituting it in the expression 
cosin d = 5 
i 
* I find the same ob tion has alsob de by Humboldt; and also a similar 
= by S oohsameage : who remarks, that the variation of the needle increases when 
aurora alis is ; Sppitaching: Journal Royal Institution. Young’s Natural 
Philosophy, vol, 2, p. 442. : . 
