LINES OF MAGNETIC FORCE — AS DELINEATED BY IRON FILINGS. 159 
forms of the lines of force associated with this pole could be determined over the 
place of the nickel hemisphere, under different circumstances, or even when it was 
removed. When the nickel was away, the forms of the lines of force were as in 
fig. 23 ; when the nickel was there, they were as in fig. 24. The application of a spirit- 
lamp to the nickel when in its place, raised its temperature to such a degree (above 
600° Fahr.) that it lost its ordinary magnetic condition ; and then the forms of the 
lines of force, as shown by filings, were the same as if the nickel was away. Re- 
moving the lamp, I was able to obtain the disposition of filings on successive pieces 
of paper, and as many as four results, like fig. 23, could be procured before the tem- 
perature had sunk so much as to cause the production of lines of force corresponding 
to fig. 24. 
3241. These are exactly the same results with nickel as those I have assumed for 
the oxygen of the atmosphere. The change in the forms of the lines about the cool- 
ing nickel in this experiment are the same changes as those I have figured in the 
type globe of cooling air (2865. 2874.). Both nickel and oxygen are paramagnetic 
bodies, and change in the same direction by heating and cooling ; and as the period of 
change with oxygen extends through degrees above and below common temperature 
(2861.), so inflections of the lines of force passing through the atmosphere, corre- 
spondent to those of the heating and cooling nickel, must take place to some extent. 
It is seen in the nickel results, that lines of force entirely outside of it do not for 
that reason continue an undeviating course, but are curved to and fro in conse- 
quence of the disposition of other lines within the niekel ; a result, which, without 
reference to either one view or another of the physical action of the magnetic force, 
must be as true in the oxygen case as in the niekel case, because of the definite 
character of the magnetic force, whether represented by centres of action or by lines 
of power. 
3242. Whether the amount of the deflection in the case of the atmosphere corre- 
sponds with the facts registered by observers, is a question which cannot be answered, 
I suppose, until we know the effect of very low temperatures upon the magnetic 
force of the atmosphere. In the nickel experiment the deflection is in places 30° or 
40°; in nature the effect to be accounted for is not more than 13 or 14 minutes. 
Royal Institution, 
December 20, 1851. 
