The buildings, however, showed no such uniformity.  On the outskirts of this brilliantly tan mystery the blocks seemed to contain nothing save odd heaps of dingy, sun-baked mud.  On the extreme north, however, lay five blocks grouped together, whose buildings, like those in the middle of the city, were rather tall, square-cut and of the same dusty, cream-white hue.

“Down-town” were several structures especially prominent for their height.  They towered to such an extent, in fact, that their upper windows were easily made out.  Apparently they were hundreds of stories high!

Here and there on the streets could be seen small spots, colored a darker buff than the rest of that dazzling landscape.  But not one of the spots was moving.

“We’ll go down further,” said the engineer tentatively, in a low tone.  There was no comment.  He gradually reduced the repelling current, so that the sky-car resumed its descent.

They sank down until they were on a level with the top of one of those extraordinary sky-scrapers.  The roof seemed perfectly flat, except for a large, round, black opening in its center.  No one was in sight.

When opposite the upper row of windows, at a distance of perhaps twenty feet, Smith brought the car to a halt, and they peered in.  There were no panes; the windows opened directly into a vast room; but nothing was clearly visible in the blackness save the outlines of the opening in the opposite walls.

They went down further, keeping well to the middle of the space above the street.  At every other yard they kept a sharp lookout for the inhabitants; but so far as they could see, their approach was entirely unobserved.

When within fifty yards of the surface, all four men made a search for cross-wires below.  They saw none; there were no poles, even.  Neither, to their astonishment, was there such a thing as a sidewalk.  The street stretched, unbroken by curbing, from wall to wall and from corner to corner.

As the cube settled slowly to the ground, the adventurers left the deadlight to use the windows.  For a moment the view was obscured by a swirl of dust, raised by the spurt of the current; then this cloud vanished, settling to the ground with astounding suddenness, as though jerked down by some invisible hand.

Directly ahead of them, distant perhaps a hundred yards, lay a yellowish-brown mass of unusual octagonal shape.  One end contained a small oval opening, but the men from the Earth looked in vain for any creature to emerge from it.

The doctor silently set to work with his apparatus.  From an air-tight double-doored compartment he obtained a sample of the ether outside the car; and with the aid of previously arranged chemicals, quickly learned the truth.

There was no air.  Not only was there no oxygen, the element upon which all known life depends, but there was no nitrogen, no carbon dioxide; not the slightest trace of water vapor or of the other less known elements which can be found in small amounts in our own atmosphere.  Clearly, as the doctor said, whatever air the astronomers had observed must exist on the circumference of the planet only, and not in this sun-blasted, north-central spot.