And then came another surprise.  As the machines neared the surface, a familiar odor floated in through the open windows of the air-craft; and the four found themselves looking at each other for signs of irrationality.  A moment, and they saw that they were not mistaken.

For, although that kaleidoscopic expanse of buildings showed not the slightest break, yet they were now located on the sea.  The houses were packed as closely together as anywhere; apparently all were floating, yet not ten square yards of open sea could be seen in any one spot.

Van Emmon almost forgot his resentment in his growing wonder.  “That gets me, Myrin!  Those houses seem to be merely floating, yet I see no motion whatever!  Why are there no waves?”

The doctor snorted.  “Shame on you, Van!  Don’t let our friends think that you’re an absolute ignoramus.”  He added:  “Venus has no moon, and no wind, at least under the roof.  Therefore, no waves.”

Smith put in:  “That being the case, there is no chance to start a wave-motor industry here.  Neither,” as he thought further, “neither for water-power.  Having no rain in your mountains, Estra, where do you get your power?”

But it was Myrin who answered.  “I suppose you are all familiar with radium?  It is nothing more or less than condensed sunlight, which in turn is simply electromagnetic waves; although it may take your scientists a good many centuries to reach that conclusion.

“Well, every particle of the material which composes this planet, contains radioactivity of some sort; and we long ago discovered a way to release it and use it.  One pound of solid granite yields enough energy to—­well, a great deal of power.”

They had now been flying for two hours, and still no end to that thickly-housed, ever different appearance of the ground.  Also, although they saw a great many birds, they noted no animals.  Finally, Billie could hold in no longer.

“Are we to understand,” she demanded of Estra, “that the whole of this planet is as densely populated as we see it?”

“Just that,” replied the Venusian.  “Why not?  The roof makes our climate uniform from pole to pole, while our buildings are such that, whether on land or on sea, they are equally livable.”

“But—­Estra!” expostulated the girl.  “Venus is nearly as big as the earth.  And it looks to be as thickly populated as—­as Rhode Island!  Why, you must have a colossal population; let me see.”  And she scribbled away in her memorandum book.

But both Smith and the doctor had already worked it out.  They looked up, blinking dazedly.

“Over three hundred billion,” murmured the doctor, as though dizzy.

The Venusian checked Smith’s correction with, “You dropped one cipher, doctor.  There are three and a half trillion of us!”

“Good lord!” whispered Van Emmon, all his antagonism gone for the moment.  And again the explorers were silent for a long time.