"Professor," exclaimed M'Allister, "what is the matter? I cannot see where we are going!" "I can guess what it is," I replied; "we have run into one of those sand-clouds I told you of the other evening, and until we get through, or it passes away, we shall see nothing else. Perhaps we had better go on very slowly." We went on accordingly, but instead of our getting through it, the cloud seemed to become denser and denser. However, we still pressed on, and, after what seemed quite a long time, we emerged into somewhat clearer air, although there was still a thin yellow cloud below us. Our course had been well maintained, for we seemed to be within ten miles of our destination, which we could just make out through the thin dust-cloud. Presently M'Allister called out to me, "Professor, I don't know what is wrong, but the machinery is slowing down so much that I am afraid we shall soon come to a dead stop! I have switched on more power, but it does not seem to make any difference!" "Well, try a little stronger current," I suggested; "but be careful not to overdo it, or we may land upon Mars more suddenly than we shall like." He tried this, but we had not moved more than a hundred yards when he found that farther progress was impossible. So here we were, only a few miles from our destination, yet prevented by an impalpable and unknown obstacle from reaching it! We consulted together, but could find no solution of the mystery of this invisible barrier to our progress. Then John suggested that, as we could not go straight on, we should try a different course. So M'Allister altered our course a few points, and once more put on the speed power, only to be brought to a standstill again after a very short spurt. "My word!" he exclaimed, "I'll not be beaten like this. I've driven an old iron tramp-steamer through scores of miles of thick seaweed out in the tropics, although the machinery was almost worn out and the engines leaking at every joint. Here goes for full speed ahead!" he cried; and, so saying, he switched on full power, quite heedless of my shout of "Do be careful, M'Allister, or we shall all be smashed to pieces!" "She's got to go!" he replied grimly, "smash or no smash! I never was beaten yet when pushing my way through obstacles, and I'm too old a hand to be beaten now!" However, he found he was beaten this time, for although he switched on the utmost power, it refused to give any evidence of its existence, and we had to rely on our neutral power in order to maintain our position in the air; though, as events proved, we could not have fallen. The excitement and tension of the work had thrown M'Allister into a profuse perspiration; and, as he stood moodily mopping his brow with his handkerchief, I heard him muttering and swearing softly to himself. His blood was evidently up, for he made another desperate attempt to get the _Areonal_ to move forward, wrenching his switches with angry jerks, but it all proved labour in vain.