reeing with this, starts from the fact of aberration, or the
displacement of the apparent from the actual position of the stars in
the direction of the earth's motion. Aberration depends partly on the
velocity of light, partly on the velocity of the Earth; and the latter
being known, the former can be computed. Now, these two deductive
arguments, verifying each other, have also been verified experimentally.
Foucault's experiment to measure the velocity of light is too elaborate
to be described here: a full account of it will be found in the treatise
above cited, Sec. 687.

When the phenomena to be explained are of such a character, so vast in
extent, power or duration, that it is impossible, in the actual
circumstances of the case, to frame experiments in order to verify a
deductive explanation, it may still be possible to reproduce a similar
phenomenon upon a smaller scale. Thus Monge's explanation of mirage by
the great heat of the desert sand, which makes the lowest stratum of air
less dense than those above it, so that rays of light from distant
objects are refracted in descending, until they are actually turned
upwards again to the eye of the beholders, giving him inverted images of
the objects as if they were reflected in water, is manifestly incapable
of being verified by experiment in the natural conditions of the
phenomenon. But by heating the bottom of "a sheet-iron box, with its
ends cut away," the rarefied air at the bottom of the box may sometimes
be made to yield reflections; and this shows at least that the supposed
cause is a possible one (Deschanel, _Optics,_ Sec. 726). Similarly as to
the vastest of all phenomena, the evolution of the stellar system, and
of the solar system as part of it, from an immense cloudlike volume of
matter: H. Spencer, in his Essay on _The Nebular Hypothesis_, says,
amidst a great array of deductive arguments from mechanical principles,
that "this _a priori_ reasoning harmonises with the results of
experiment. Dr. Plateau has shown that when a mass of fluid is, as far
as may be, protected from the action of external forces, it will, if
made to rotate with adequate velocity, form detached rings; and that
these rings will break up into spheroids, which turn on their axes in
the same direction with the central mass." The theory of the evolution
of species of plants and animals by Natural Selection, again, though, of
course, it cannot be verified by direct experiment (since experime