Page:The Solar System - Six Lectures - Lowell.djvu/83



(3) The polar caps melt in their summer and accumulate in their winter, thus showing themselves to be seasonal in character.

(4) As they melt, they are bordered by a blue belt, which retreats with them. This negatives carbonic acid as the substance composing them, and leaves to our knowledge only water as a possible explanation.

(5) Their extensive melting shows their quantity to be inconsiderable, and points to a dearth of water.

(6) Comparison with previous observations shows the melting to occur in the same consecutive places year after year. The melting is thus a thing which can be locally counted on.

(7) The greatest local melting is just south of the largest dark (blue-green) regions, the bays in the polar sea in these longitudes being the largest.

(8) The dark regions are subject to a wave of seasonal changes;

(9) which follows upon the melting of the cap. They darken in early summer and fade out in their autumn.

(10) The dark regions are not seas: first, because in Professor W. H. Pickering's experiments their light showed no trace of polarization, while that of the polar sea did;