28 CHANGES IN THE ATMOSPHERE. 



column of mercury presses upwards upon perfectly dry 

 air confined in an enlargement (c) of the bent tube. 



When the weight of the atmosphere increases, the 

 mercury is pressed doivnwards in the long arm and rises 

 in the short arm of the tube, the dry confined air, from 

 its elasticity, yielding to the pressure. The length of the 

 column of mercury is marked upon a graduated scale 

 placed on one side. A movable scale, (E F) called a 

 vernier, is attached, so contrived as to measure the 

 height of the column to hundredths of an inch. 



88. Careful observation of the winds, and of the barom 

 eter, with a knowledge how to observe, will often enable 

 a person to foresee rain for some hours, or a day, or pos 

 sibly longer, before it comes ; but no person can yet 

 predict, with any certainty, whether the succeeding month 

 will be dry or rainy. 



It is only of late that careful and continued observa 

 tions have been carried on, upon a large scale, to discover 

 the laws of storms. It is found that nearly all storms, 

 in the Atlantic States, come from the west, and travel 

 pretty rapidly from west to east. Hereafter we may 

 know, certainly, the approach of a storm many hours 

 before it reaches us. Prof. Henry, at the Smithsonian 

 Institution in Washington, having telegraphic communi 

 cation with many parts of the country, is usually able to 

 predict the approach of a rain-storm twelve hours before 

 it comes. 



89. Of what are commonly considered the Signs of Rain, 

 none are entirely reliable. When the sun sets clear, with 

 a westerly wind, and the clouds float high and in round, 

 compact, well-defined masses, we may expect the next 

 day to be fair. But when the sun sets in a deep mass of 



