METEOROLOGY. 



19 



From these data a good estimate may be formed of the temperatures of the mountain passes 

 between each set of posts of observation, by allowing 3° decrease of temperature for each 

 1,000 feet elevation, as estimated by Professor Heniy, of the Smithsonian Institution. From 

 this calculation the following table is constructed: 



In the latter case the interval between the two posts, which exceeds 6, 000 feet in altitude, 

 is over two hundred miles, all of which would have a winter temperature ranging between 

 15°. 1 and 23°. 5. 



On the other hand, only 165 miles on the northern route exceed 3,000 feet in altitude, having 

 a winter temperature between 17°. and 26°. 0, and only six miles exceed 5,000 feet. 



The two coldest days in January, 1854, furnish a good evidence of the general correctness of 

 the rule followed, as here shown: 



SNOW AS AN OBSTRUCTION TO THE RAILROAD LINE. 



Details of the information in regard to snows in the Cascade mountains are given in Volume I, 

 and more recent observations do not show any probability of their being under-estimated. The 

 great mildness of the winters must affect the temperature of the passes sufiSciently to prevent 

 the constant accumulation of snow for more than one winter month, while rains frequently take 

 their place. Compared with Steilacoom, where the mean winter temperature is 39°. 5, that of 

 Snoqualme Pass should be at least 29°, while for much of the time it would rise above the 

 freezing point and cause thaws or rain. It has already been shown that the diflerence of 3° 

 for each 1,000 feet was too great, as determined by actual observation of the coldest days of 

 January, 1854, at Fort Benton and Cadotte's Pass; and the great rarity of cold northeast winds, 

 as we approach the Pacific coast, must make the difference there still less than in the Rocky 



