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.