AGRICULTURAL TEXT-BOOK. 41 



try may become incapable of sustaining human life, as in the 

 marshes surrounding Rome, Italy ; and the waters may again 

 encroach and cover land that was previously retrieved from them, 

 as in the Aragua Valley, Venezuela, So. America. (Boussin- 

 gault.) 



129. The cultivation of perennial plants is limited by the 

 lowest temperature of winter ; that of annual plants by the mean 

 temperature of summer. Thus a country so cold as to be inca 

 pable of sustaining trees, may yet have a sufficiently long and 

 hot summer to mature grains and vegetables ; and annual plants 

 may prosper in a latitude so for North that perennial ones, indi 

 genous to the same climate, would perish from cold. 



The heat of the atmosphere is measured by an instrument called a 

 thermometer, consisting of quicksilver or alcohol, enclosed in a vacuum 

 tube, with a hollow bulb at one end. This is placed upon a scale, 

 graduated by actual experiment ; and the quicksilver rises and fal s in 

 the tube, according to the amount of heat. Three different modes of 

 graduation are employed in different countries ; Fahrenheit s in America 

 and Great Britain ; Centigrade in France ; and Reaumur s in France and 

 other parts of Europe. 



212 degrees, ( c ) of Fahrenheit equal 100 Centigrade, and 80 

 Reaumur. 



32 degrees of Fahrenheit equal Centigrade, and Reaumur. 



There are various forms of the Thermometer, but all on the same 

 principle. 



By the term mean temperature, is meant the temperature obtained by 

 adding together the temperatures of the days or months required, and 

 dividing by the number of days or months. Mean annual temperature 

 is obtained by adding together the temperatures of all the months of the 

 year, and dividing the sum by the number of the months ; so that the 

 mean annual temperature expresses* the height at which the thermometer 

 would stand at any place, if we could suppose it perfectly stationary 

 throughout the year. 



1 30. For the cultivation of annual plants, it is only necessary 

 to know the mean temperature of the climate during the num 

 ber of months requisite for growth and the maturation of the 

 seed. 



