16 Improvement in Physical Science (Jan. 
duced in the needles themselves ; their magnetic virtue appears to 
be increased or diminished in consequence of various states of the 
atmosphere, with which we are but imperfectly acquainted. Tem- 
perature is obviously one of the agents in these changes, though not 
the only one. The dryness and moisture of the atmosphere seems 
to be another. It would seem as if the magnetic intensity of the 
needle were increased by a moist atmosphere. It is not unlikely 
that the electrical state of the atmosphere may also have its in- 
fluence. The repetition of these experiments, while the state of 
the hygrometer and electrometer at the same time with the obser- 
vation of the variation is noted down, would probably throw much 
new light on this obscure subject. 
That the diurnal variation is owing to changes in the needle 
itself I conclude from a fact observed by Colonel Beaufoy. He 
employed different needles in his observations, and he found that 
each gave a variation of its own different from the others. Thus, 
for example, his observations for the six months of 1815, be- 
ginning with April, differ in the quantity of declination from those 
of the same six months of 1813 and 1814, and they were made 
with a different needle. ‘To decide this question in a satisfactory 
manner, it would only be necessary to repeat the observations with 
a variety of needles employed immediately after each other. 
5. Cavallo showed long ago, that when iron is acted upon by 
diluted sulphuric acid, its magnetic intensity is increased. He 
put a quantity of iron filings into a watch glass, and placed a mag- 
net at such a distance as scarcely to act upon them. On pouring 
diluted sulphuric acid on the filings, they were powerfully acted 
upon by the magnet. This experiment has been lately confirmed 
and varied by Mr. Rubland. As electricity is evolved in such 
abundance during the action of acids on metals, it was natural to 
expect, considering the striking analogy between clectricity and 
magnetism, that a similar evolution of magnetism would take 
place when acids are made to act on iron. At the same time the 
experiments of Cavallo and Ruhland are not quite free from ambi- 
guity ; for we may conceive the liquid to act chiefly by giving the 
filings greater mobility in consequence of the diminution produced 
in their specific gravity by being plunged in a liquid. 
6. There is a curious paper on the magnetism of the earth by 
Hansten, published in Schweigger’s Journal for 1813 (vol. vil. 
p- 79). He has collected a great many observations on the varia- 
tion. He shows that the earth must have four magnetic poles. In 
the year 1769, one of the north magnetic poles was situated in 
north latitude 70° 17’, and east longitude from Ferro 277° 40°5’. 
The Siberian north magnetic pole in the year 1805 was situated in 
north latitude 85° 21°5’, and Jongitude east from Ferro 133° 49’. 
In the year 1774 one of the south magnetic poles was in south 
latitude 71° 26-5’, and 153° 533’ east longitude from Ferro; 
the second in south latitude 77° 16°75’, and 254° 23’ east longitude 
from Ferro. Hansten’s calculations respecting the periodic times 
