58 NOTICES OF THE MEETINGS [April 11, 
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 atmo- 
sphere 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, analogous 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 tension 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 there- 
fore to favour the transition of the lines of terrestrial force there, a 
greater number of them will be determined through the western 
region ; and even though the lines of force may be doubted by some 
as having a separate existence such as that above assumed, still no 
error as to the effects on magnetic needles would in that case be 
introduced, 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 conduction. Different. 
bodies, as iron, nicke], 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 
disposition of the lines is not altered; but if a better conducting 
substance than the air is introduced, so as to occupy part of the 
space, the lines are concentrated in it, and drawn from other parts as 
shown by P. P. in the figure, or if a worse conducting substance is 
introduced, the lines are opened out as at D. D. In both cases the 
lines of force are inflected, and a small magnetic needle standing in 
them at the inflected part would have its direction changed accordingly. 
Experimental illustrations of these changes in direction are given 
, 
a. 
