One of the most grotesque and least comfortable of my experiences with the magicians of the air occurred near the forks of the Platte.  There had been a tremendous thunder-storm, lasting all night.  In the morning my party set forward over the soaken prairie under a cloudless sky intensely blue.  I was riding in advance, absorbed in thought, when I was suddenly roused to a sense of material things by exclamations of astonishment and apprehension from the men behind.  Looking forward, I beheld a truly terrifying spectacle.  Immediately in front, at a distance, apparently, of not more than a quarter-mile, was a long line of the most formidable looking monsters that the imagination ever conceived.  They were taller than trees.  In them the elements of nature seemed so fantastically and discordantly confused and blended, compounded, too, with architectural and mechanical details, that they partook of the triple character of animals, houses and machines.  Legs they had, that an army of elephants could have marched among; bodies that ships might have sailed beneath; heads about which eagles might have delighted to soar, and ears—­they were singularly well gifted with ears.  But wheels also they were endowed with, and vast sides of blank wall; the wheels as large as the ring of a circus, the walls white and high as cliffs of chalk along an English coast.  Among them, on them, beneath, in and a part of them, were figures and fragments of figures of gigantic men.  All were inextricably interblended and superposed—­a man’s head and shoulders blazoned on the side of an animal; a wheel with legs for spokes rolling along the creature’s back; a vast section of wall, having no contact with the earth, but (with a tail hanging from its rear, like a note of admiration) moving along the line, obscuring here an anatomical horror and disclosing there a mechanical nightmare.  In short, this appalling procession, which was crossing our road with astonishing rapidity, seemed made up of unassigned and unassorted units, out of which some imaginative god might be about to create a world of giants, ready supplied with some of the appliances of a high civilization.  Yet the whole apparition had so shadowy and spectral a look that the terror it inspired was itself vague and indefinite, like the terror of a dream.  It affected our horses as well as ourselves; they extended their necks and threw forward their ears.  For some moments we sat in our saddles surveying the hideous and extravagant spectacle without a word, and our tongues were loosened only when it began rapidly to diminish and recede, and at last was resolved into a train of mules and wagons, barely visible on the horizon.  They were miles away and outlined against the blue sky.