And now another difficulty arose. The deadly gas used by the Huns, having a density greater than air, has a tendency to fill the hollows and leave the high ground comparatively clear. On Malcolm's front the ground rose gradually to a height of about twenty feet. While it might afford protection from the noxious vapour, the ridge was certainly open to rifle-fire. Nor could Carr understand why, in a temporarily-deserted expanse, there should be such a persistent hail of machine-gun fire.