LOCAL CLIMATOLOGY. 71 



any place. Thus, to elevate it 300 feet, would produce the same 

 effect on its climate as the placing it sixty miles farther north. 



But to this rule there are certain exceptions. It makes a great 

 difference whether the ascent be steep or gradual. The position 

 of the surface from which the ascent is made, is also important in 

 its influence. The average temperature in the Great Salt Lake 

 valley in our own continent, though it is four to five tiiousand 

 (4,351) feet above the level of the sea, is a.s warm as the places 

 on the California coast in the same latitude and at the sea level, 

 as Fort Humboldt, for example ; Fort Madison, Iowa ; and Ottawa, 

 Illinois : the average for the year in Great Salt Lake city being 

 about 52; that at Fort Humboldt is given at 52.1 ; Fort Arm- 

 strong, 50.3 ; and Ottawa the same. 



Again : the cereals, as barley, &c., will grow on the north side 

 of the Himalayas at an elevation of some two or three thousand 

 feet higher than on the south side, notwithstanding a difference of 

 some two degrees in latitude. But on the south side the reflected 

 rays of the sun are sent up from a plain which is nearly on a level 

 with the sea ; while on the north side, the reflecting surface is the 

 elevated plateau and table land of Thibet. 



In apparent contradiction to this law, we often find the weather 

 much colder in Ioav narrow valleys, especially in autumn and 

 winter. I have known the thermometer to fall eio^ht de«:rees in 

 a descent of some two hundred feet, and at a distance of not more 

 than half a mile ; and all persons have doubtless observed the 

 fact that frosts frequently cut off»the tender crops much earlier in 

 the low lands, than on the hill-sides and hill-tops even adjoining 

 them. The reason is, doubtless, to be found in the fact that the 

 cold air, being heavier than warm, descends into the valle3^s by the 

 force of its own w^eight. This phenomena, therefore, can occur 

 only when the stratum of air is colder than the earth below it. 



III. Inland distance, or distance from the sea coast, is an impor- 

 tant element in determining temperature. 



Perhaps this fact will make no great difference in the general 

 average for the year. Its influence is chiefly felt in the contrasts 

 between summer and winter, and the heat of the day when com- 

 pared with the coolness of the night. 



The following examples will illustrate this principle : 



Winter. Summer. DiflFeretice. 



f Shetland Isles __ 38.5'^ W 15.5^^ 



Lat. 60 { St. Petersburg l(i.3 60.8 44.5 



1^ Yakutsk _. —38. 63. lOL 



