OF FERRUGINOUS BODIES. 225 
43. The first applications of the spirit lamp 
to hard steel, tend to lower its temper, and con- 
sequently to lessen the resistance to magnetic ex- 
citement on that side of the pole to which the lamp 
is applied, without producing a corresponding di- 
minution of resistance on the other side. Hence 
the magnet, under these circumstances, is similarly 
situated to a magnetic pole approached on one 
side by a piece of soft iron (41); the polar force 
will therefore move in that direction, or towards 
the point of heat. But when this resistance be- 
comes so far diminished on every side of the pole, 
by the frequent application of the lamp warming 
and softening the steel, as to be overcome by the 
direct calorific force, the latter force prevails in 
producing the polar displacement; which is inva- 
riably from the point of heat. When the steel is 
soft and thin, the resistance of the metal is easily 
overcome, and the polar transplacement from the 
point of heat, is accomplished even by the first 
applications of the lamp. 
44. One of the peculiarities attending longitu- 
dinal polar transplacement by heat, is, as I have 
shown in my third Memoir,* a diminution of space 
passed over by the disturbed poles on each suc- 
* Annals of Electricity, &c. vol. iv. p. 144. 
