104 SCIENCE OF COMMON THINGS. 



Difference between a thermometer and barometer. Peculiarities of climates. 



the correspondence between the heights marked, and 

 the state of the weather. 



653 What is the difference "between a thermometer and a barometer? 



In a thermometer the mercury is sealed up from the 

 air ; and rises or falls as the varying temperature of 

 the air expands or contracts it ; but in a barometer the 

 mercury is left exposed (or open) to the air / and rises 

 or falls as the varying weight ot the air presses upon 

 the open column. 



653 Why is the tube of a barometer left open f 



That the air may press upon it freely ; and, as this 

 pressure varies, the mercury rises or falls in the tube. 



654 Why does the mercury in the barometer rise at the approach of 

 fair weather ? 



Because the air is becoming more dry^ and the drier 

 the air, and the more free it is from vapor, the greater 

 the pressure. 



655 Why does the mercury sink at the approach of foul weather f 



Because the air is laden with vapor or disturbed by 

 wind. 



656 Why does vapor in the air cause the mercury to sink f 



Because air containing vapor is lighter than dry 

 air ; and its pressure on the mercury is therefore less. 



657 Why will there be no rain if the air be very dry f 



Because dry air will absorb moisture, and not part 

 with it in rain. 



CIIAPTEE YIII. 



PECULIARITIES OF CLIMATES. 

 658 What do we mean by the term climate ? 



By climate, we mean the condition of a place in 

 relation to the various phenomena of the atmosphere, 



