68 Prof. Challis on a Theory of Magnetic Force. 
inertia of the ether on the side of A. Thus the motion will be 
maintained so as to be symmetrical with respect to a neutral 
position N, mid-way between A and B, the partial streams con- 
verging towards the parts about B, and diverging in like manner 
from the parts about A. The velocity of the ether will be 
greatest, and its density least, at N, and the velocity will de- 
crease, and the density increase, in both directions from this 
position by the same gradations. Hence the atoms, assumed 
to be of finite dimensions, will be urged on both sides towards 
N, by reason of the excess of pressure on the halves of their sur- 
faces turned from N, and the total moving forces in the opposite 
directions will be equal. Consequently the theory not only 
accounts for the weil-known fact, that the magnetism of a steel 
bar is equal and opposite on the opposite sides of a middle neutral 
line, but explains also why terrestrial magnetism, being supposed, 
for reasons that will be hereafter adduced, to act as the primary 
current of the theory, produces no perceptible motion of 
translation of the bar. These mferences will not be altered when 
the dynamic effect of the motion in the secondary streams 1s taken 
into account, as will be shown in a subsequent part of the theory. 
5. It also follows, conformably with experience, that if a 
magnetized bar be divided into two or more parts by being cut 
transversely, each part becomes a magnet, because it may be 
assumed that the gradation of density from end to end is the 
same, and in the same direction, in each, as when it formed a 
part of the whole bar. 
6. The theory of the action between two magnets requires the 
investigation of the mutual influence of two steady streams, 
having separate origins and interfering courses. The following 
laws, which may be admitted on hydrodynamical principles, will 
suffice for my present purpose. The resultant of two interfering 
steady streams is steady. Where two streams meet, the velocity 
is in general less, and consequently the density greater, than 
that which would be due to either stream flowing separately. 
When two streams unite in the same course, the velocity is 
greater, and the density less, than in either of the component 
streams. When the courses of two streams cross at right angles, 
the gradations of density in the directions of the courses are very 
nearly the same as in the separate streams. 
7. Conceive, now, that two bar-magnets, BN A, BIN'A!, are 
brought near each other, with their axes in the same straight 
line; and, for distinctness, let the axes be in the plane of the 
magnetic ‘meridian, and let A, A!’ be the ends from which the ° 
secondary streams always issue. The two currents under these 
circumstances will interfere with each other, so that the sym- 
metrical distribution of density with reference to the neutral 
