490 Expedition to the 



of the water of the Platte.* Notwithstanding there were 

 only about five degrees of difference between the tempera- 

 ture 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, appear- 

 ed magnified, and variously distorted. An undulating or 

 tremulous motion in ascending lines was manitest over every 

 part of the surface. Commencing soon after sunrise it con- 

 tinued to increase in quantity until the afternoon, when it 

 diminished gradually, keeping an even pace with the intensi- 

 ty 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 per- 

 fect and beautiful as to deceive almost every one of our par- 

 ty. 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 



* The results of several observations are as follows: 



Temperature of the water. Temperature of the air. 



June 27, 63° 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°. 



