any sensational theories calculated to attract the popular imagination.

[Footnote 1: Mercury also seems to have a scanty atmosphere, but as its
mass is only one-thirtieth that of the earth it can retain only the
heavier gases, and its atmosphere may be dust-laden, as is that of Mars,
according to Mr. Lowell. Its dusky markings, as seen by Schiaparelli,
seem to be permanent, and they are also for considerable periods
unchangeable in position, indicating that the planet keeps the same face
towards the sun as does Venus. This was confirmed by Mr. Lowell in 1896.
Its distance from us and unfavourable position for observation must
prevent us from obtaining any detailed knowledge of its actual surface,
though its low reflective power indicates that the surface may be really
visible.]

But in the case of Mars the progress of discovery has had a very
different result. The most obvious peculiarity of this planet--its polar
snow-caps--were seen about 250 years ago, but they were first proved to
increase and decrease alternately, in the summer and winter of each
hemisphere, by Sir William Herschell in the latter part of the
eighteenth century. This fact gave the impulse to that idea of
similarity in the conditions of Mars and the earth, which the
recognition of many large dusky patches and streaks as water, and the
more ruddy and brighter portions as land, further increased. Added to
this, a day only about half an hour longer than our own, and a
succession of seasons of the same character as ours but of nearly double
the length owing to its much longer year, seemed to leave little wanting
to render this planet a true earth on a smaller scale. It was therefore
very natural to suppose that it must be inhabited, and that we should
some day obtain evidence of the fact.