MARS AS A WORLD. 163 



cloud, though here, again, the thickest clouds ever seen 

 on Mars fall very far short of the dense cloud masses with 

 which our globe is so frequently shrouded. There can 

 be no doubt that portions of the permanent features on 

 the planet are temporarily obscured by the intervention 

 of clouds. Perhaps it would be more correct to regard 

 these objects in the atmosphere of Mars as rather in the 

 nature of mists or fogs than of clouds in the ordinary 

 sense of the word. 



Still greater caution must be used in the attempt to 

 interpret the varied hues of the different permanent 

 features of the face of the planet. The general appear- 

 ance of the surface of the disc, as represented in Mr. 

 Green's drawings, is of " a warm yellow, heightening into 

 orange " in some of the regions. W^ith these are strongly 

 contrasted the dark portions which Mr. Green well de- 

 scribes as being of " a greenish grey, partly due to con- 

 trast with the orange." 



It is customary to speak of these latter portions as lakes, 

 or seas, or oceans, according to their size and surround- 

 ings ; while the yellow and warmer portions generally are 

 spoken of as islands or as continents. There can be no 

 inconvenience in using this language so long as we clearly 

 remember that the distinction between land and water on 

 the planet is purely of a conjectural character. 



Those oppositions of Mars in which the northern hemi- 

 sphere of the planet is turned towards the earth, are, as 

 already explained, of a disadvantageous character for a 

 minute scrutiny of the planet's surface. It is then at an 

 exceptionally great distance from the earth. Mr. Knobel 

 has well observed that, as the planet is then near itr 



