actual surface of the planet, whereas the markings displayed by our
telescopes on some of the other planetary members of our system are mere
effects of atmospheric changes, which, though visible for several years
and showing well defined periods of rotation cannot be accepted as
affording the true periods. The behavior of the red spot on Jupiter may
closely intimate the actual motion of the sphere of that planet, but
markings of such variable, unstable character can hardly exhibit an
exact conformity of motion with the surface upon which they are seen to
be projected. With respect to Mars' case, it is entirely different. No
substantial changes in the most conspicuous features have been detected
since they were first confronted with telescopic power and we do not
anticipate that there will be any material difference in their general
configurations.

"The same markings which were indistinctly revealed to the eyes of
Fontana and Huyghens in 1636 and 1659 will continue to be displayed to
the astronomers of succeeding generations, though with greater fullness
and perspicuity owing to improved means. True, there may possibly be
variations in progress as regards some of the minor features, for it has
been suggested that the visibility of certain spots has varied in a
manner which cannot be satisfactorily accounted for on ordinary
grounds. These may possibly be due to atmospheric effects on the planet
itself, but in many cases the alleged variations have doubtless been
more imaginary than real. The changes in our own climate are so rapid
and striking, and occasion such abnormal appearances in celestial
objects that we are frequently led to infer actual changes where none
have taken place; in fact, observers cannot be too careful to consider
the origin of such differences and to look nearer home for some of the
discordances which may have become apparent in their results."