How does Neal Stephenson use imagery in Cryptonomicon?

Imagery:

"The sun will be down soon; it is already down in Berlin, as in most of the hellish empire that Hitler has built from Calais to the Volga. Time for the radio operators to begin their work. Radio does not, in general, go around corners. This can be a real pain when you are conquering the world, which is inconveniently round, placing all of your most active military units over the horizon. But if you use shortwave, then you can bounce the information off the ionosphere. This works a good deal better when the sun is not in the sky, sluicing the atmosphere with wideband noise. So radio telegraphers, and the people who eavesdrop on them (what the Brits call the Y Service) are alike, nocturnal beings."

"The generator starts instantly, with a nearly inaudible purr."