.]

Secondly, there is the modifying influence of this new phenomenon on other
phenomena already known, such, for instance, as that of "aberration." We
saw in the third chapter how Bradley discovered this effect of the
velocity of light, and how the measure of it is obtained by comparing the
velocity of light with that of the earth. This comparison can be effected
in a variety of ways, and we should expect all the results to agree within
certain limits; but this agreement was not obtained, and Chandler has been
able to show one reason why, and to remove some of the more troublesome
differences. It is impossible to give here an idea of the far-reaching
consequences which such work as this may have; so long as there are
differences of this kind we cannot trust any part of the chain of
evidence, and there is in prospect the enormous labour of examining each
separate link until the error is found. The velocity of light, for
instance, may be measured by a terrestrial experiment; was there anything
wrong in the apparatus? The velocity of the earth in its journey round the
sun depends directly upon the distance of the sun: have we measured this
distance wrongly, and if so what was the error in the observations made?
These are some of the questions which may arise so long as the values for
the _Constant of Aberration_ are still conflicting; but it requires
considerable knowledge of astronomy to appreciate them fully.

[Sidenote: Latitude Variation Tide.]

[Sidenote: Earthquakes.]

Another example will, perhaps, be of more general interest. If the axis of
the earth is executing small oscillations of this kind, there should be an
effect upon the tides; the liquid ocean should feel the wobble of the
earth's axis in some way; and an examination of tidal registers showed
that there was in fact a distinct effect. It may cause some amusement when
I say that the rise and fall are only a few inches in any case; but they
are unmistakable evidences that the earth is not spinning smoothly, but
has this kind of unbalanced vibration, which I have compared to the
vibrations felt by passengers on an imperfectly engineered twin-screw
steamer. A more sensational effect is that apparently earthquakes are more
numerous at the time when the vibration is greatest. We remarked that the
vibration waxes and wanes, much as that of the steamer waxes and wanes if
the twin-screws are not running quite together. Now the passengers on the
steamer would be