444 



NATURE S TEACHINGS. 



in the illustration. That on the right represents the common 

 Wheel Barometer, which marks the weight of the air by a 

 hand moving in front of a dial. If the hand move towards the 

 right, the weight of the air is increasing ; if to the left, it is 

 decreasing. 



There are certain words, such as Wet, Change, Fair, Dry, 

 &c., on the face of the dial, but they are only conventional, the 

 real test of the weather being the direction in which the hand 

 moves. .For example, if with a west wind the hand moves 

 from Dry towards Fair, rain may be expected ; whereas, if it 

 should move from Wet to Change with an east wind, we may 

 reasonably think that fine weather is coming. 



The whole cause of this revolution of the hand may be found 

 in the weight of the atmosphere. 



It is found that a column of water thirty feet high, or a 

 column of mercury thirty inches high, is exactly equal in 

 weight to a column of air of the same diameter, but some forty 

 odd miles high, so that the two columns precisely balance each 

 other. 



Suppose, then, the water or mercury to be placed in tubes 

 closed at the top and open at the bottom, the water or mercury 

 will exactly balance the air, and will not escape from the tubes. 



SUCKING SUGAR-CANE. SUCKING AN EGG. 



BAROMETERS. 



It necessarily follows that if the air be heavier than .usual, it 

 will force the liquid higher into the tubes, and, if it be lighter 

 than usual, will allow them to fall lower. This is the principle 

 of the Barometer. 



