Centripetal force keeps us at least for a while under the lunar
influence, but centrifugal force drives us away irresistibly."
These words were uttered in a tone that killed Ardan's last and fondest
hope.
* * * * *
The portion of the Moon they were now approaching was her northern
hemisphere, found usually in the lower part of lunar maps. The lens of a
telescope, as is well known, gives only the inverted image of the
object; therefore, when an upright image is required, an additional
glass must be used. But as every additional glass is an additional
obstruction to the light, the object glass of a Lunar telescope is
employed without a corrector; light is thereby saved, and in viewing the
Moon, as in viewing a map, it evidently makes very little difference
whether we see her inverted or not. Maps of the Moon therefore, being
drawn from the image formed by the telescope, show the north in the
lower part, and _vice versa_. Of this kind was the _Mappa
Selenographica_, by Beer and Maedler, so often previously alluded to and
now carefully consulted by Barbican. The northern hemisphere, towards
which they were now rapidly approaching, presented a strong contrast
with the southern, by its vast plains and great depressions, checkered
here and there by very remarkable isolated mountains.
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