TEMPEEATUBB IN EESPECT TO ALTITUDE. 



661 



maturation, Wheat and Eye require nearly the same length of time and 

 the same amount of heat. Boussingault's law has been somewhat 

 modijSed by Alphonse De Candolle, who has pointed out many sources 

 of error to bp avoided. It is difficult to fix the time which is to be 

 taken into account ; the temperature of the soil requires to be attended 

 to ; low temperatures, and especially all below 32°, or perhaps 35°, which 

 do not excite the phenomena of vegetable life, should be left out of the 

 calculation ; and the thermometric measurements should be made by 

 observations on the plants themselves, and not merely on the air. By 

 attention to these points, he thinks that useful and accurate conclusions 

 may ultimately be arrived at relative to the temperature required for 

 the performance of vegetable functions. 



Temperature, in its hypsometrical (jj4">s, altitude, and /mt^ov, 

 measure) relation, or as regards its altitudinal range, requires to be 

 considered. In ascending into the atmosphere, a decrease of tem- 

 perature is observed, which varies in its amount at difierent stages of 

 ascent. The following table shows the temperature at different 

 heights in the equatorial and temperate zones — 



Height 



hi feet. 







3,197 



6,394 



9,591 

 12,789 

 15,985 



Taking an average, it may be said that there is a fall of 1° in the 

 thermometer for every 340 feet of ascent. Prof. Forbes states that 

 543 feet of ascent give a difference of 1° of the thermometer in the 

 boiling point of water. The elevation at which constant frost takes 

 place is called the snow-line or line of perpetual congelation. Its 

 limit does not exactly correspond with the height at which the tem- 

 perature is equal to 32° F. The following table, by Buchan, gives the 

 height of the snow-line above the sea (in feet) at different latitudes ; — 



The decrease of temperature on ascending mountams regulates, in a 



