many of the canals, and which in some parts of the surface are very
numerous, are said to be more difficult of detection than even the
lines, being often blurred or rendered completely invisible by slight
irregularities in our own atmosphere, while the canals themselves
continue visible. About 180 of these have now been found, and the more
prominent of them are estimated to vary from 75 to 100 miles in
diameter. There are however many much smaller, down to minute and barely
visible black points. Yet they all seem a little larger than the canals
which enter them. Where the canals are double, the spots (or 'oases' as
Mr. Lowell terms them) lie between the two parallel canals.

No one can read this book without admiration for the extreme
perseverance in long continued and successful observation, the results
of which are here recorded; and I myself accept unreservedly the
substantial accuracy of the whole series. It must however always be
remembered that the growth of knowledge of the detailed markings has
been very gradual, and that much of it has only been seen under very
rare and exceptional conditions. It is therefore quite possible that, if
at some future time a further considerable advance in instrumental power
should be made, or a still more favourable locality be found, the new
discoveries might so modify present appearances as to render a
satisfactory explanation of them more easy than it is at present.

But though I wish to do the fullest justice to Mr. Lowell's technical
skill and long years of persevering work, which have brought to light
the most complex and remarkable appearances that any of the heavenly
bodies present to us, I am obliged absolutely to part company with him
as regards the startling theory of artificial production which he thinks
alone adequate to explain them. So much is this the case, that the very
phenomena, which to him seem to demonstrate the intervention of