The mass of the northern snow cap of Mars is, on the other hand, centered almost exactly upon its pole.  It is located in a region of yellow color, which we are accustomed to consider as representing the continent of the planet.  From this arises a singular phenomenon which has no analogy upon the earth.  At the melting of the snows accumulated at that pole during the long night of ten months and more the liquid mass produced in that operation is diffused around the circumference of the snowy region, converting a large zone of surrounding land into a temporary sea and filling all the lower regions.  This produces a gigantic inundation, which has led some observers to suppose the existence of another ocean in those parts, but which does not really exist in that place, at least as a permanent sea.  We see then (the last opportunity was in 1884) the white spot of the snow surrounded by a dark zone, which follows its perimeter in its progressive diminution, upon a circumference ever more and more narrow.  The outer part of this zone branches out into dark lines, which occupy all the surrounding region, and seem to be distributary canals by which the liquid mass may return to its natural position.  This produces in these regions very extensive lakes, such as that designated upon the map by the name of Lacus Hyperboreus; the neighboring interior sea called Mare Acidalium becomes more black and more conspicuous.  And it is to be remembered as a very probable thing that the flowing of this melted snow is the cause which determines principally the hydrographic state of the planet and the variations that are periodically observed in its aspect.  Something similar would be seen upon the earth if one of our poles came to be located suddenly in the center of Asia or of Africa.  As things stand at present, we may find a miniature image of these conditions in the flooding that is observed in our streams at the melting of the Alpine snows.

Travellers in the arctic regions have frequent occasion to observe how the state of the polar ice at the beginning of the summer, and even at the beginning of July, is always very unfavorable to their progress.  The best season for exploration is in the month of August, and September is the month in which the trouble from ice is the least.  Thus in September our Alps are usually more practicable than at any other season.  And the reason for it is clear—­the melting of the snow requires time; a high temperature is not sufficient; it is necessary that it should continue, and its effect will be so much the greater, as it is the more prolonged.  Thus, if we could slow down the course of our season so that each month should last sixty days instead of thirty, in the summer, in such a lengthened condition, the melting of the ice would progress much further, and perhaps it would not be an exaggeration to say that the polar cap at the end of the warm season would be entirely destroyed.  But one cannot doubt, in such a case, that the fixed