After having lasted for some
months, the markings fade out gradually and disappear until another
season equally favorable for their formation. Thus it happens that in
certain other seasons (especially near the southern solstice of the
planet) few are seen, or even none at all. In different oppositions the
gemination of the same canal may present different appearances as to
width, intensity, and arrangement of the two stripes; also in some cases
the direction of the lines may vary, although by the smallest quantity,
but still deviating by a small amount from the canal with which they are
directly associated. From this important fact it is immediately
understood that the gemination cannot be a fixed formation upon the
surface of Mars and of a geographical character like the canals. The
second of our maps will give an approximate idea of the appearance which
these singular formations present. It contains all the geminations
observed since 1882 up to the present time. In examining it it is
necessary to bear in mind that not all of these appearances were
simultaneous, and consequently that the map does not represent the
condition of Mars at any given period; it is only a sort of
topographical register of the observations made of this phenomenon at
different times.[D]
The observation of the gemination is one of the greatest difficulty, and
can only be made by an eye well practiced in such work, added to a
telescope of accurate construction and of great power.
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