Snow could be seen on every part of them which was visible above our horizon. The thermometer immersed in the water of the river fell from 80°, the temperature of the atmosphere, to 75°. Observations had been made daily to ascertain the temperature of the water of the Platte.[123] Notwithstanding there were only about five degrees of difference between the temperature of the air and that of the water, it was remarked by several of the party, that a sensation of extreme cold was felt on passing from the one to the other. [123] The results of several observations are as follows: Temperature of the water. Temperature of the air. June 27, 68° 83° 28, 70 79 29, 74 82 30, 75 80 July 1, 71 60 At eleven, A. M. Before sunrise the mercury fell usually as low as 60°.—JAMES. It is possible, that at the elevation we had now attained, the rapidity of evaporation, on account of the diminished pressure of the atmosphere, might be something greater than we had been accustomed to. 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 vapour were seen to ascend from every part of the plain, that all objects, at a little distance, appeared magnified, and variously distorted. An undulating or tremulous motion in ascending lines was manifest over every part of the {175} 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 vapour 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 effect 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. Illusions of this kind are common in the African and Asiatic deserts, as we learn from travellers, and from the language of poets. They are called by the Persians _sirraub_, "water of the desert;" and in the Sanscrit language, _Mriga trichna_, "the desire or thirst of the antelope." Elphinstone relates,[124] that at Moujgur, in the kingdom of Caubul, towards evening many persons were astonished at the appearance of a long lake enclosing several little islands. Notwithstanding the well known nature of the country, which was a sandy desert, many were positive that it was a lake, and one of the surveyors took the bearings of it. "I had imagined," says he, "this phenomenon to be occasioned