portion of such a cap would be reduced to a much smaller size, than we see it to-day.  Now, this is exactly what happens to Mars.  The long year, nearly double our own, permits the ice to accumulate during the polar night of ten or twelve months, so as to descend in the form of a continuous layer as far as parallel 70 degrees, or even farther.  But in the day which follows, of twelve or ten months, the sun has time to melt all, or nearly all, of the snow of recent formation, reducing it to such a small area that it seems to us no more than a very white point.  And perhaps this snow is entirely destroyed; but of this there is at present no satisfactory observation.

Other white spots of a transitory character and of a less regular arrangement are formed in the southern hemisphere upon the islands near the pole, and also in the opposite hemisphere whitish regions appear at times surrounding the north pole and reaching to 50 degrees and 55 degrees of latitude.  They are, perhaps, transitory snows, similar to those which are observed in our latitudes.  But also in the torrid zone of Mars are seen some very small white spots more or less persistent; among others one was seen by me in three consecutive oppositions (1877-1882) at the point indicated upon our chart by longitude 268 degrees and latitude 16 degrees north.  Perhaps we may be permitted to imagine in this place the existence of a mountain capable of supporting extensive ice fields.  The existence of such a mountain has also been suggested by some recent observers upon other grounds.

As has been stated, the polar snows of Mars prove in an incontrovertible manner that this planet, like the earth, is surrounded by an atmosphere capable of transporting vapor, from one place to another.  These snows are, in fact, precipitations of vapor, condensed by the cold, and carried with it successively.  How carried with it if not by atmospheric movement?  The existence of an atmosphere charged with vapor has been confirmed also by spectroscopic observations, principally those of Vogel, according to which this atmosphere must be of a composition differing little from our own, and above all, very rich in aqueous vapor.  This is a fact of the highest importance because from it we can rightly affirm with much probability that to water and to no other liquid is due the seas of Mars and its polar snows.  When this conclusion is assured beyond all doubt another one may be derived from it of not less importance—­that the temperature of the Arean climate notwithstanding the greater distance of that planet from the sun, is of the same order as the temperature of the terrestrial one.  Because, if it were true, as has been supposed by some investigators, that the temperature of Mars was on the average very low (from 50 degrees to 60 degrees below zero), it would not be possible for water vapor to be an important element in the atmosphere of that planet nor could Water be an important factor in its physical changes, but would give place to carbonic acid, or to some other liquid whose freezing point was much lower.