492 Expedition to the 



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 chemistry, 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 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.* The circumstance that these 

 pools could only be seen when we looked down at a consid- 

 erable angle upon some valley ; the perfect manner in which 

 thf 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.f These appearances 

 are sufficient to justify the conclusion that the quantity of 

 evaporation is much greater here than in less elevated dis- 

 tricts of country, where such things are not. 



* See Humb. Pers. Nar. vol. 2. p. 196. vol. 3. pp. 358. 542. 



f Rays of light, falling with any degree of obliquity upon the particles 

 of thai, poriiofi 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 tiie 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 

 iD 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. 



