comprehension the practical difference is perhaps not great.
Scientifically speaking, Helmholtz and Thomson are as well
entitled to reason upon the assumption of a perfectly
frictionless fluid as geometers in general are entitled to assume
perfect lines without breadth and perfect surfaces without
thickness. Perfect lines and surfaces do not exist within the
region of our experience; yet the conclusions of geometry are
none the less true ideally, though in any particular concrete
instance they are only approximately realized. Just so with the
conception of a frictionless fluid. So far as experience goes,
such a thing has no more real existence than a line without
breadth; and hence an atomic theory based upon such an assumption
may be as true ideally as any of the theorems of Euclid, but it
can give only an approximatively true account of the actual
universe. These considerations do not at all affect the
scientific value of the theory; but they will modify the tenour
of such transcendental inferences as may be drawn from it
regarding, the probable origin and destiny of the universe.

The conclusions reached in the first part of this paper, while we
were dealing only with gross visible matter, may have seemed bold
enough; but they are far surpassed by the inference which our
authors draw from the vortex theory as they interpret it. Our
authors exhibit various reasons, more or less sound, for
attributing to the primordial fluid some slight amount of
friction; and in support of this view they adduce Le Sage's
explanation of gravitation as a differential result of pressure,
and Struve's theory of the partial absorption of light-rays by
the ether,--questions with which our present purpose does not
require us to meddle. Apart from such questions it is every way