If the lands of Mars are framed as are those of our own earth, there should be ridges of mountains constituting what we may term the backbones of the continent. As yet such have not been discerned, which may be due to the fact that they have not been carefully looked for. The only peculiar physical features which have as yet been discerned on the lands of Mars are certain long, straight, rather narrow crevicelike openings, which have received the name of "canals." These features are very indistinct, and are just on the limit of visibility. As yet they have been carefully observed by but few students, so that their features are not yet well recorded; as far as we know them, these fissures have no likeness in the existing conditions of our earth. It is difficult to understand how they are formed or preserved on a surface which is evidently subjected to rainfalls. It will require much more efficient telescopes than we now have before it will be possible to begin any satisfactory study on the geography of this marvellous planet. We can not hope as yet to obtain any indications as to the details of its structure; we can not see closely enough to determine whether rivers exist, or whether there is a coating which we may interpret as vegetation, changing its hues in the different seasons of the year. An advance in our instruments of research during the coming century, if made with the same speed as during the last, will perhaps enable us to interpret the nature of this neighbour, and thereby to extend the conception of planetary histories which we derive from our own earth. [Illustration: Fig. 5.--Comparative Sizes of the Planets (Chambers).]