Between the Platte river, and the head-waters of the Colorado and Sabine rivers, there is an extensive desert tract, which has been called the _Great American Desert_, stretching from the Ozark Mountains to the Chippewan. Over this desert the members of Long’s expedition travelled nearly a thousand miles. The intense reflection of light and heat, from this tract, added much to the fatigue and suffering of their journey. ‘We often met with extensive districts covered entirely with loose and fine sand, blown from the adjacent hills. In the low plains along the river where the soil is permanent, it is highly impregnated with saline substances, and too sterile to produce any thing except a few stinted carices and rushes. As we approached the mountains, we felt or fancied a very manifest change in the character of the weather, and the temperature of the air. Mornings and evenings were usually calm, and the heat more oppressive than in the middle of the day. Early in the forenoon, a light and refreshing breeze often sprung up, blowing from the west or south-west, which again subsided on the approach of night. This phenomenon was so often observed, that we were induced to attribute it to the operation of the same local cause, which in the neighborhood of the sea produces a diurnal change in the winds, which blow alternately to and from the shore. The Rocky Mountains may be considered as forming the shore of that sea of sand, which is traversed by the Platte, and extends northward to the Missouri above the great bend. The rarefaction of the air over this great plain, by the reverberation of the sun’s rays during the day, causes an ascending current, which is supplied by the rushing down of the condensed air from the mountains. * * * * For several days the sky had been clear, and in the morning we had observed an unusual degree of transparency in every part of the atmosphere. As the day advanced, and the heat of the sun began to be felt, such quantities of vapor were seen to ascend from every part of the plain, that all objects at a little distance appeared magnified, and variously distorted. An undulating and tremulous motion in ascending lines was manifest over every part of the surface. Commencing soon after sunrise it continued to increase in quantity until the afternoon, when it diminished gradually, keeping an even pace with the intensity of the sun’s heat. The density of the vapor was often such as to produce the perfect image of a pool of water in every valley upon which we could look down at an angle of about ten degrees. This aspect was several times seen so perfect and beautiful as to deceive almost every one of our party. A herd of bisons, at the distance of a mile, seemed to be standing in a pool of water, and what appeared to us the reflected image was as distinctly seen as the animal itself.[4] Illusions of this kind are common in the African and Asiatic deserts, as we learn from travellers and from the language of poets.’