CHAPTER XVI.

THE FIRE PLANET. I think I should explain now a little about the physical conformation of Mercury--the "lay of the land," so to speak--in order that the events I am about to describe may be more readily understood. It has already been made clear by Bob Trevor, I believe, that Mercury revolves on its axis only once during the time of its revolution around the sun. Thus, just as a similar condition always makes our moon present very nearly the same face to us, so Mercury presents always the same portion of its surfac…