216 SCIENCE PROGRESS 



The polar patch vanishes in the increasing heat of spring. 

 The blue strip which surrounds it is fluid, for it has been found 

 to polarise light and is exactly the colour of water. The blue 

 strip clings to the dwindling cap, just as pools form around 

 melting snow. When all the white is gone, a dark smudge as 

 of wet ground is seen in its place. By this time, the blue stuff 

 has also disappeared — has either flowed away or evaporated. 



A recent discovery speaks accurately as to the temperature 

 prevailing round the pole in the later part of the summer, for 

 there appears at this time in the subpolar regions of the planet, 

 on the sunrise edge of the disc, a whitish patch which has 

 the unique property of being fixed. It is on the surface of 

 the planet but does not partake of its rotation. It is there- 

 fore a state through which the surface passes at this particular 

 hour of the morning. There can be no doubt that it is hoar- 

 frost. It may seem surprising that this should be visible but the 

 appearance is so striking as to show obviously and unmistakably 

 on many photographs of the planet. 



When I first saw the patch I was so struck with its appearance 

 that I sought for evidence as to whether it had been seen prior 

 to the announcement of the discovery of its nature. I have 

 found many notes of white patches being observed in the 

 appropriate position the meaning of which was not divined at 

 the time. I need scarcely say that this discovery was made 

 2X Flagstaff, where also the majority of the data used in this 

 paper were obtained. 



Lowell has shown that this morning hoar-frost appears exactly 

 where it should do in the coldest part of the autumn hemisphere, 

 which is obviously not the pole but the place where the 

 increasing nights have become long enough to cause the land 

 to lose more heat than it receives daily from the sun. 



The presence on Mars of water in all its three states being 

 indicated, it is natural to inquire what happens to it when it 

 leaves the pole. Most of the greenish areas lie in a belt 

 about the south temperate zone. When the snow at the 

 South Pole begins to melt, this zone of green proceeds to 

 darken, the wave of colour beginning in the southernmost part 

 and gradually spreading northwards. That this change may 

 be due to vegetation is evident. All circumstances are pro- 

 pitious. There is sufficient heat. Water is present to nourish 

 it. And all we know of the Martian atmosphere points to 



