At three o'clock P. M. the planet Venus was distinctly visible. Its distance from the sun at 3h. 45m. was east 36° 15[']. There were a few broken cumulostratose clouds from the south-west, otherwise the sky was clear, and near the zenith, where the star was seen, of a deep and beautiful azure. Our actual elevation, at this time, must have been considerable, and might be supposed to effect, in some degree, the transparency of our atmosphere.
Several magpies were seen about the islands in the river, where it is probable they rear their young.
On the 30th we left the encampment at our accustomed early hour, and at eight o'clock were cheered by a distant view of the Rocky Mountains. For some time we were unable to decide whether what we saw were mountains, or banks of cumulous clouds skirting the horizon, and glittering in the reflected rays of the sun. It was only by watching the bright parts, and observing that their form and position remained unaltered, that we were able to satisfy ourselves they were indeed mountains. They were visible from the lowest parts of the plain, and {174} their summits were, when first discovered, several degrees above our horizon. They became visible by detaching themselves from the sky beyond, and not by emerging from beneath the sensible horizon, so that we might have seen them from a greater distance had it not been for the want of transparency in the atmosphere. Our first views of the mountains were indistinct, on account of some smokiness of the atmosphere, but from our encampment, at noon, we had a very distinct and satisfactory prospect of them. A small part only of the intervening plain was visible, the convexity of the surface intercepting the view from the base of the mountains, and that portion of the plain adjacent to it.
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.
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 by a thin vapour which is spread over the ground in hot weather in India, but this appearance was entirely different, and on looking along the ground no vapour whatever could be perceived. The ground was quite level and smooth, and the weather very hot. It is only found in level, smooth, and dry places; the position of the sun, and the degree of heat, are not material, for it was afterwards seen in Damaun, when the weather was not hotter than in England." On the frontier of Caubul, Elphinstone saw what he calls a most magnificent mirage, {176} which looked like an extensive lake, or a very wide river. The water seemed clear and beautiful, and the figures of two gentlemen, who rode along it, were reflected as distinctly as in real water. "It is common in our own country," says the London Monthly Review, "for ground-mists to assume the appearance of water, to make a meadow seem inundated, and to change a valley into a lake; but these mists never reflect the surrounding trees and hills. Hence, the mirage must consist of a peculiar gas, of which the particles are combined by a stronger attraction of cohesion than the vapours of real water; the liquor silicum of the alchymists is described as exhibiting, in some circumstances, this glassy surface, yet as being equally evanescent."[125] It is afterwards suggested, in the same paper, that the gas which occasioned these extraordinary reflections, may probably be the substance of the pernicious wind called Simoom. The explanation here offered will not probably be thought satisfactory. It seems to belong to the epoch of great and brilliant discoveries in pneumatic chymistry, when "a peculiar gas" was thought the agent of every phenomenon.
The images of pools of water, which we saw in the deserts of the Platte, appeared to us similar to those mentioned by Elphinstone, likewise to those observed by Nieburgh[126] in Arabia, where inverted images were seen.
To the more common effects of light passing through a medium charged with vapours, we had become familiar. We had, for many days, seen the low bluffs of the valley of the Platte suspended over the verge of our apparent horizon, as distant capes are suspended over the sea; but in viewing these perfect images of lakes, we could scarce believe they were occasioned by refraction, to which the phenomena of mirage have usually been attributed.[127] {177} The circumstance that these pools could only be seen when we looked down at a considerable angle upon some valley; the perfect manner in which the image of the sky was returned from the surface, and the inverted position of the objects seen, induced us to inquire whether the effect might not be produced by reflection from the lower stratum of watery vapour.[128] These appearances are sufficient to justify the conclusion that the quantity of evaporation is much greater here than in less elevated districts of country, where such things are not seen.
Distant View of the Rocky Mountains