212 Influence of Terrestrial Magnetism on Iron. 



tion of the compass before and after this section or axis 

 has been made to turn to a known angle. Other formulae 

 enable us even to avoid this operation, and to be content with 

 merely observing the direction of the compass before and after 

 the addition of a mass of iron, always placed in the same 

 manner, and so as easily to be brought near to the compass 

 to change its direction. 



Influence of Temperature on Magnetism. 



Among the actions that facilitate magnetisation by terres- 

 trial magnetism, one of the most efficacious consists in heat- 

 ing the magnetic body to redness, and allowing it to cool 

 under the influence of this magnetism. The cooling that fol- 

 lows a much lower elevation of temperature is even sufficient. 

 MM. Moser and Riess have proved that, to this kind of ef- 

 fect, we must refer the phenomena of magnetisation that have 

 been supposed to be produced by rays of light, and especially 

 by the violet rays. They have proved that, as these effects 

 only take place when the needles which experience them 

 are in a position perpendicular to the magnetic meridian, they 

 can be attributed to terrestrial magnetism alone, the action 

 of which is facilitated by the elevation of temperature brought 

 about by the solar rays, or rather by the cooling that follows 

 it. Heat, in fact, far from increasing, notably diminishes, on 

 the contrary, the intensity of the magnetic virtue. A mag- 

 netised steel bar, when brought to a red-white heat, totally 

 loses its magnetism ; should it have become magnetic during 

 cooling, it is due to the action of the earth. A soft iron bar 

 is no longer magnetic, that i3 to say, is no longer attracted 

 by the magnet, when it is simply brought to a red heat. Nickel 

 ceases to be magnetic at the temperature of boiling oil. With 

 regard to cobalt, its magnetic force does not seem gradually 

 to diminish, as is the case with other substances, in propor- 

 tion as its temperature increases ; but it suddenly ceases at 

 an extremely high temperature, and it then appears again 

 just as rapidly when the metal is made to descend from this 

 high temperature. 



The remarkable influence that is exercised upon magnetism 



