154 METEOROLOGY. 



The central line of minimum pressure is generally of 

 great length, from North to South, sometimes reaching 

 beyond the Gulf of Mexico and the Northern Lakes, while 

 the East and West diameter is comparatively small. 



When a storm commences in the United States — as it does 

 sometimes, — the line of the minimum pressure does not 

 come from the ''Far West," but commences with the storm 

 and travels with it towards the East. 



In great storms, the wind, for several hundred miles 

 on both sides of the line of minimum pressure, blows 

 towards that line directly or obliquely; and at the line of 

 minimum pressure there is generally a lull of wind, and 

 sometimes a calm. On the approach of the line of mini- 

 mum pressure, the wind is Easterly, and immediately after 

 it has passed it changes to the West, and the barometer 

 begins to rise. 



The force of the wind is generally in proportion to the 

 suddenness and greatness of the depression of the barom- 

 eter. At the time of great and sudden depression, there 

 is usually much rain or snow, and generally in sudden 

 great rains or snows, there is a great depression of the 

 barometer near the centre of the storm. 



During the high barometer, on the night preceding a 

 storm, it is frequently clear and calm, with a low temper- 

 ature, accompanied with frost, and during the day preceding 

 a storm, it is generally clear and mild in temperature. 



After the passage of the centre of great storms, it is 

 generally very windy, and the temperature falls suddenly, 

 so that when the centre of a storm is in the middle of the 

 United States, the lowest temperature of the month will 

 occur in the West on the same day of the highest temper- 

 ature in the East. 



Having shown the characteristics of these storms, I will 

 proceed to illustrate the practical use of the barometer 

 and thermometer for indicating their approach. 



