Though strange rock structures abound in all this region, it is in the specially arid portions that they are most common. The strata being unprotected by vegetation, the wearing away is more rapid, and follows more eccentric lines. The higher and drier a locality, there--provided there is some rainfall--will be found the most extraordinary rain carvings. The lack of abundant rain prevents the growth of vegetation and the altitude permits the rain torrents to carry loads of sand, and the more sand and velocity the greater the scouring. In some of these intermittent stream courses the sand and bowlders scoop out deep holes like huge pots--a variety, in fact, of the hole known in geology as "pothole" (Fig. 13). These are very deep and sometimes provide a thirsty traveler with a draught of clear water that has lingered from the last shower. In some places these "pockets" or "tanks" supply the only water to be had, and it is a glad sight when one sees a pocket before him. Each formation has its own peculiarities of erosion, or as Dutton aptly puts it, "its own school of natural architecture." Given, then, a particular formation exposed to the atmosphere, it can be foretold just what its natural architectural forms will be, whether domes, minarets, pinnacles, arches, towers, or what. Architectural forms are not confined to the United States, nor to the American continent. The Giant's Causeway in Ireland is a familiar example of what they have in those parts, while under the Arabian sky the conditions resemble those in our arid Southwest, and there we find many fantastic rain carvings. Among these is the Sphinx of El Guerrah, carved by the rain-sculptor doubtless expressly to furnish answers to our "whys and wherefores" concerning this "wondrous architecture of the world." [Illustration: FIG. 13.--POTHOLES IN INTERMITTENT STREAM COURSE.] * * * * * Among the applications proposed for the newly liquefied gases is their use as aids in balloon navigation. An aëronaut having a few bottles of liquefied hydrogen or illuminating gas has the means of increasing the inflation of his balloon when necessary, and need not fear to waste a little gas when he wishes to diminish its levity. Prof. L. Errera, of Brussels, has suggested an apparatus which he calls the _ceinture de natation_, or natation belt, by which these objects can be conveniently accomplished. THE HERDS OF THE YELLOW ANT. BY JAMES WEIR, JR., M. D. As far as my observations go, and they extend through a period of twenty years, the aphides which live on grapevines, and which are the peculiar cows of the common yellow ant (_Lasius flavus_) of our gardens, show many curious phases in their life history. Especially is this true of the manner in which they perpetuate their species.