SCIENCE OF COMMON THINGS. 105 



Mean daily temperature. Temperature varies with the altitude. 



as temperature, moisture, etc. Thus, we speak of a 

 warm or cold climate, a moist or dry climate, etc. 



659 What is meant by the mean daily temperature f 



The mean or average temperature of the day is 

 found by observing the thermometer at fixed intervals 

 of time during the twenty-four hours, and then divid- 

 ing the sum of the temperatures by the number of 

 observations. 



6 6O How is the mean annual temperature of a particular point ascer- 

 tained ? 



By taking the average of all the mean daily tempe- 

 ratures throughout the year. 



661 How does temperature vary with the latitude ? 



The average annual temperature of the atmosphere 

 diminishes from the equator towards either pole. 



663 Give examples of this variation ? 



At the equator, in Brazil, the average annual tem- 

 perature is 84 Fahrenheit's thermometer ; at Calcutta, 

 lat. 22 35' 1ST., the annual temperature is 78 F ; at 

 Savannah, lat. 32 5' N., the annual temperature is 

 65 F. ; at London, lat. 51 31' 1ST., the annual tempe- 

 rature is 50 F. ; at Melville Island, lat, 74 47' 1ST., the 

 mean annual temperature is 1 below zero. 



663 How does the temperature vary with the altitude above the earth's 

 surface ? 



Temperature diminishes with the altitude. As a 

 general rule, a loss of heat occurs to the extent of one 

 degree F. for every 343 feet of elevation. 



664 How does the gradual reduction of temperature as we ascend from 

 the surface of the earth affect the moisture of the air ? 



In every latitude there is a point above the surface 

 of the earth where moisture, once frozen, always re- 

 mains congealed. 



665 Why are tlie tops of very high mountains always covered with 

 snow ? 



Because, at the great elevation of their summit, the 

 temperature of the atmosphere is so low that the con- 

 gealed moisture which falls upon them never melts. 



