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it in fact; your thoughts wander amid the very atoms of
your steel, and you conclude that each atom is a magnet,
and that the force exerted by the strip of steel is the mere
summation or resultant of the forces of its ultimate par-

Here, then, is an exhibition of power which we can call
forth or cause to disappear at pleasure. We magnetize our
strip of steel by drawing it along the pole of a magnet;
we can demagnetize it, or reverse its magnetism, by prop-
erly drawing it along the same pole in the opposite direc-
tion. What, then, is the real nature of this wondrous
change ? What is it that takes place among the atoms of
the steel when the substance is magnetized? The question
leads us beyond the region of sense, and into that of imagi-
nation. This faculty, indeed, is the divining-rod of‘the man
of science. Not, however, an imagination which catches
its creations from the air, but one informed and inspired by
facts, capable of seizing firmly on a physical image as a
principle, of discerning its consequences, and of devising
means whereby these forecasts of thought may be brought
to an experimental test. If such a principle be adequate to
account for all the phenomena, if from an assumed cause
the observed facts necessarily follow, we call the assump-
tion a theory, and, once possessing it, we can not only re-
vive at pleasure facts already known, but we can predict
others which we have never seen. Thus, then, in the prose-
cution of physical science, our powers of observation, mem-
ory, imagination, and inference, are all drawn upon. We
observe facts and store them up; imagination broods upon
these memories, and by the aid of reason tries to discern
their interdependence. The theoretic principle flashes, or
slowly dawns upon the mind, and then the deductive fac-
ulty interposes to carry out the principle to its logical con-
sequences. A perfect theory gives dominion over natural
facts; and even an assumption which can only partially

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