"One thing always puzzled me when examining maps of the Moon," observed Ardan, "and I can't say that I see it yet as clearly as if I had thought over the matter. It is this. I could understand, when looking through a lens at an object, why we get only its reversed image--a simple law of optics explains _that_. Therefore, in a map of the Moon, as the bottom means the north and the top the south, why does not the right mean the west and the left the east? I suppose I could have made this out by a little thought, but thinking, that is reflection, not being my forte, it is the last thing I ever care to do. Barbican, throw me a word or two on the subject." "I can see what troubles you," answered Barbican, "but I can also see that one moment's reflection would have put an end to your perplexity. On ordinary maps of the Earth's surface when the north is the top, the right hand must be the east, the left hand the west, and so on. That is simply because we look _down_ from _above_. And such a map seen through a lens will appear reversed in all respects. But in looking at the Moon, that is _up_ from _down_, we change our position so far that our right hand points west and our left east. Consequently, in our reversed map, though the north becomes south, the right remains east, and--" "Enough said! I see it at a glance! Thank you, Barbican. Why did not they make you a professor of astronomy? Your hint will save me a world of trouble."[C] Aided by the _Mappa Selenographica_, the travellers could easily recognize the different portions of the Moon over which they were now moving. An occasional glance at our reduction of this map, given as a frontispiece, will enable the gentle reader to follow the travellers on the line in which they moved and to understand the remarks and observations in which they occasionally indulged. "Where are we now?" asked Ardan. "Over the northern shores of the _Mare Nubium_," replied Barbican. "But we are still too far off to see with any certainty what they are like. What is the _Mare_ itself? A sea, according to the early astronomers? a plain of solid sand, according to later authority? or an immense forest, according to De la Rue of London, so far the Moon's most successful photographer? This gentleman's authority, Ardan, would have given you decided support in your famous dispute with the Captain at the meeting near Tampa, for he says very decidedly that the Moon has an atmosphere, very low to be sure but very dense. This, however, we must find out for ourselves; and in the meantime let us affirm nothing until we have good grounds for positive assertion."