mathematician, so that, if science is ever to become popular, and yet
remain scientific, it must be by a profound study and a copious
application of those principles of the mathematical classification of
quantities which, as we have seen, lie at the root of every truly
scientific illustration.

There are, as I have said, some minds which can go on contemplating
with satisfaction pure quantities presented to the eye by symbols, and
to the mind in a form which none but mathematicians can conceive.

There are others who feel more enjoyment in following geometrical
forms, which they draw on paper, or build up in the empty space before
them.

Others, again, are not content unless they can project their whole
physical energies into the scene which they conjure up. They learn at
what a rate the planets rush through space, and they experience a
delightful feeling of exhilaration. They calculate the forces with
which the heavenly bodies pull at one another, and they feel their own
muscles straining with the effort.

To such men momentum, energy, mass are not mere abstract expressions
of the results of scientific inquiry. They are words of power, which
stir their souls like the memories of childhood.

For the sake of persons of these different types, scientific truth
should be presented in different forms, and should be regarded as
equally scientific whether it appears in the robust form and the vivid
colouring of a physical illustration, or in the tenuity and paleness
of a symbolical expression.