64 On Atmospheric Magnetism. 



produced, just as the lines which originate in a magnet are 

 held by it, though not in the same degree ; and that any 

 disturbance from above affecting them will cause a greater 

 change in their place and direction in the atmosphere and 

 space above, than in the earth beneath. 



The system of lines of magnetic force around a magnet or 

 the earth is related by a lateral tension of the whole, analo- 

 gous in some degree to the lateral tension of lines of static 

 electrical force ; both the one and the other being easily 

 made manifest by experiment. The disturbance of the ten- 

 sion in one part is accompanied instantly by a disturbance 

 of the tension in every other part ; for as the sum of the 

 external powers of a system, unaltered at its origin, is definite 

 and cannot be changed ; so any alteration either of intensity 

 or direction amongst the lines of force at one place, must be 

 accompanied by a corresponding change at every other. So 

 if a mass of soft iron on the east side of a magnet causes a 

 concentration of the lines of force from the magnet on that 

 side, a corresponding expansion or opening out of the lines 

 on the west side must be and is at the same time produced ; 

 or if the sun, on rising in the east, renders all the oxygen 

 of the air on that side of the globe less magnetic and less 

 able therefore to favour the transition of the lines of terres- 

 trial force there, a greater number of them will be deter- 

 mined through the western region ; and even though the 

 the lines of force may be doubted by some as having a sepa- 

 rate existence such as that above assumed, still no error as 

 to the effects on magnetic needles would in that case be in- 

 troduced, for they by experiment would be and are the same. 



The power of a magnetic body as iron or oxygen to favour 

 the transmission of lines of force through it more than other 

 bodies not magnetic, may be expressed by the term conduc- 

 tion. Different bodies, as iron, nickel, oxygen, conduct in 

 various degrees, and not only that, but the same body as 

 iron or oxygen conducts in different degrees at different 

 temperatures. When space traversed by uniform lines of 

 magnetic force is occupied by a uniform body as air, the dis- 

 position of the lines is not altered ; but if a better conduct- 

 ing substance than the air is introduced, so as to occupy 

 part of the space, the lines are concentrated in it, and drawn 



