making observations near the limb of the planet.  Lowell concludes it must be a calm and serene atmosphere; probably only one-seventh of our own in density.  The normal height of the barometer in Mars would then be but four and a half inches.  This is a pressure far less than exists on the top of the highest terrestrial mountain.  A mountain here must have an altitude of about ten miles to possess so low a pressure on its summit.  Drops of water big enough to form rain can hardly collect in such a rarefied atmosphere.  Moisture will fall as dew or frost upon the ground.  The days will be hot owing to the unimpeded solar radiation; the nights bitterly cold owing to the free radiation into space.

We may add that in such a climate the frost will descend principally upon the high ground at night time and in the advancing day it will melt.  The freer radiation brings about this phenomenon among our own mountains in clear and calm weather.

With the progressive melting of the snow upon the pole Lowell connected many phenomena upon the planet’s surface of much interest.  The dark spaces appear to grow darker and more greenish.  The canals begin to show themselves and reveal their double nature.  All this suggests that the moisture liberated by the melting of the polar snow with the advancing year, is carrying vitality and springtime over the surface of the planet.  But how is the water conveyed?

Lowell believes principally by the canals.  These are

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constructed triangulating the surface of the planet in all directions.  What we see, according to Lowell, is not the canal itself, but the broad band of vegetation which springs up on the arrival of the water.  This band is perhaps thirty or forty miles wide, but perhaps much less, for Lowell reports that the better the conditions of observation the finer the lines appeared, so that they may be as narrow, possibly, as fifteen miles.  It is to be remarked that a just visible dot on the surface of Mars must possess a diameter of 30 miles.  But a chain of much smaller dots will be visible, just as we can see such fine objects as spiders’ webs.  The widening of the canals is then accounted for, according to Lowell, by the growth of a band of vegetation, similar to that which springs into existence when the floods of the Nile irrigate the plains of Egypt.

If no other explanation of the lines is forthcoming than that they are the work of intelligence, all this must be remembered.  If all other theories fail us, much must be granted Lowell.  We must not reason like fishes—­as Lowell puts it—­and deny that intelligent beings can thrive in an atmospheric pressure of four and half inches of mercury.  Zurbriggen has recently got to the top of Aconcagua, a height of 24,000 feet.  On the summit of such a mountain the barometer must stand at about ten inches.  Why should not beings be developed by evolution with a lung capacity capable of living at two and a half times this altitude.  Those steadily