106 Prof. Challis on a Theory of Magnetic Force. 
law should eventually be found to be true of all the magnetic 
poles, the theory will account for it in this manner, The 
north magnetic streams may be supposed to descend into the 
earth from heights at which the gyratory motion of the ether is 
sensibly less than at the earth’s surface, and the south magnetic 
poles to issue from the earth to heights at which the same 
circumstance prevails. Thestreams being supposed to circulate 
from south to north in courses distant from the earth’s surface, 
that diminution of the velocity of gyration will cause them to 
lag behind the earth in rotatory motion, and thus to draw the 
magnetic poles westward. 
I have now completed an outline of a general physical theory, 
of which the leading idea is that all quantitative physical laws 
may be mathematically deduced from a few fundamental facts, 
distinct conceptions of which may be formed from sensation and 
experience. The hypothetical facts on which the theory rests 
are, that all substances consist of minute spherical atoms, of 
different, but constant, magnitudes, and of the same intrinsic 
inertia, and that the dynamical relations and movements of 
different substances are determined by the motions and pres- 
sures of a uniform elastic medium pervading all space not 
occupied by atoms, and varying in pressure in proportion to 
variations of its density. Iam well aware that many of the 
explanations I have given of physical phenomena on these 
hypotheses are expressed in general terms, and are too little 
supported by exact analytical or numerical calculation, Some 
of the explanations, requiring a knowledge of the interior con- 
stitution of bodies, could not be conducted in an exact manner 
in the present state ‘of science. Still so comprehensive a theory, 
resting on so few hypotheses, could hardly fail of meeting with 
contradictions in the attempt to explain facts, unless the hypo- 
theses were true. The number and the variety of the explana- 
tions of physical phenomena which have been drawn from them 
without the support of symbolical calculation, seem almost of 
themselves to justify the conclusion that the ultimate atoms of 
bodies are really such as they have been assumed to be, and 
that the physical forces are modes of action of a single elastic 
medium. 
But no doubt the general theory and the explanations 
derived from it are not fully established till they have borne 
the test of numerical verification. And here I take occasion to 
add that every complete physical theory is necessarily mathe- 
matical. Observation and experiment are essential for furnish- 
ing facts both for the foundation and for the verification of a 
theory, and may also discover laws and relations of facts; but 
