128 SCIENTIFIC AGRICULTURE. 



as man has ever ascended, and probably beyond this point to 

 the very limit of the atmosphere. The interior of the earth is 

 supposed to be yet in a fluid state from the effects of heat ; 

 the solid outside crust constituting only y^ part of its whole 

 diameter: at 50 to 40 feet below the surface, invariable tem- 

 perature prevails; that is, there is always an equilibrium, so 

 that the mercury in a thermometer would remain stationary 

 at this depth, whatever might be the temperature above in 

 the open air. This point would be at the surface if the tem- 

 perature of the air was always the same. The increase of 

 cold upwards from the earth is at the rate of 1 F. for every 

 100 yards. The snow line, or line of perpetual congelation, 

 varies less in proportion to latitude than altitude : thus it will 

 be seen by the table below, that this line is much lower at the 

 equator than in higher latitudes in proportion. 



Table of Snow Lines from Mutter. 



Coast of Norway, 2,340 feet above sea level. 



Iceland, 3,042 " " 



Alps, 8,801 " " " " 



MtEtna, 9,441 



Himmalayas, 14,625 " x 



Mexico, 14,625 " " " " 



Quito, 15,600 " " " " 



There are three reasons given by Dr. Brande, why the cold 

 increases as we ascend, viz: 1. The absorption of the sun's 

 rays in the denser strata of the atmosphere near the surface 

 of the earth. 2. Radiation of caloric from the earth. 3. The 

 ascending current of air. 



Configuration of the land varies the climate of a country : 

 a plain is hotter than an uneven surface, all other conditions 

 being equal. The sand on the desert plains of Africa some- 

 times attains a temperature of 122 F. The side of a moun- 

 tain or hill, which faces the sun, is warmer than the opposite 



