[126] Karsten Niebuhr (1733-1815) was a Danish scientist and traveller, who visited Egypt, Arabia, India, Palestine, and Syria during the years 1761-67. The first volume of his travels, _Beschreibung von Arabien_ (Copenhagen, 1772), was published by the Danish government; the second and third volumes appeared in 1774 and 1778, bearing the title _Reisebeschreibung von Arabien und anderen umliegenden Ländern_; while a fourth was brought out in 1837, by Niebuhr's daughter.—ED. 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. [Illustration: Distant View of the Rocky Mountains] [127] See Humb. Pers. Nar. vol. ii. p. 196. vol. iii. pp. 358, 542.—JAMES. [128] Rays of light, falling with any degree of obliquity upon the particles of that portion of watery vapour which lies near the surface of the earth, may be reflected, and pass off at an equal corresponding angle; so that if the eye be raised a few feet above the reflecting surface, an image of the corresponding arc of the sky is produced, as in the case of a sheet of water where the image, seen by the reflected light, is not that of the water, but the sky. Hence any object, which obstructs the rays of light in their passage from the parts of the atmosphere beyond the reflecting surface to that surface, is returned to the eye in a darkened image as from water.—JAMES. Towards evening the air became more clear, and our view of the mountains was more satisfactory, though as yet we could only distinguish their grand outline, imprinted in bold indentations upon the luminous margin of the sky. We soon remarked a particular part of the range divided into three conic summits, each apparently of nearly equal altitude. This we concluded to be the point designated by Pike as the highest peak. Its bearing was taken a short time before we halted for the evening, and found to be south, 73° west.[129] As we were about to encamp, some of the party went in pursuit of a herd of bisons, one of which they killed, and returned to camp a little before sunset.