THE REASON WHY. 157 



" Can any understand the spreadings of the clouds, or the iioise of his 

 tabernacle f " JOB xxxvi. 



sphere of its influence are drawn, it is a miniature of the roaring wlrirlpjol 

 or the wilder maelstrom of the Norwegian seas. 



Nature rehearses all her parts in mild whispers ; and for every picture that 

 she paints, she places a first study upon the canvas. Man need not go into the 

 heart of her terrors to understand their laws. Many an unknown Humboldt, 

 sitting by the river's side, may rejoice in the " aspects of nature," and share the 

 bliss of knowledge with the great philosopher. 



CHAPTER XXXII. 



691. What is a barometer ? 



A barometer is an instrument which indicates the pressure of 

 the atmosphere, and which takes its name from two Greek words 

 signifying measurer of weight. 



692. Why does a barometer indicate the pressure of the 

 atmosphere ? 



Because it consists of a tube containing quicksilver, closed at one 

 end and open at the other, so that the pressure of the air upon the 

 open end balances the weight of the column of mercury (quick- 

 silver), and when the pressure of the air upon the open surface of 

 the mercury increases or decreases, the mercury rises or falls in 

 response thereto. 



693. Why is a barometer called also a " weather-glass ?" 



Because changes in the weather are generally preceded by altera- 

 tions in the atmospheric pressure. .But we cannot perceive those 

 changes as they gradually occur ; the alteration in the height of the 

 column of mercury, therefore, enables us to know that atmospheric 

 changes are taking place, and, by observation, we are enabled to 

 determine certain rules by which the state of the weather may be 

 foretold with considerable probability. 



694. Why are barometers constructed with circular dials, 

 and an index to denote changes ? 



Beeauso that is a convsnient mechanical arrangement, by which 



