RESULTS OF METEOROLOGICAL OBSERVATIONS. 227 



and Lon. 100° W. to Lat. 60° N. and Lon. 130° W. Ultimately this great 

 body of atmosphere, moving from the southeast or south at the eastern base 

 of the Rocky mountains, unites and mingles with the general current from 

 the west, and together the whole passes eastward to the Atlantic, to supply 

 the place of the atmosphere borne from the east into the Gulf of Mexico 

 and over parts of the United States. In the southern part the current is 

 from east to west, and in the northern from west to east, over the belt of 

 land designated. 



The wind from the ocean is commonly warmer than that coming from over 

 land, and the former contains more vapor than the latter. Let equal volumes 

 of two such currents intermingle, and cloud at once is formed : if both be 

 nearly saturated with water, either rain or snow will be formed ; and if the 

 change be extensive, one or the other will fall. When the vapor changes to 

 water, a large quantity of heat is evolved, about one thousand degrees of 

 heat from each pound of vapor. This heat or caloric expands the air which 

 held the vapor, and it consequently rises through the colder air above, while 

 the colder air falls, and thus increases the condensation of vapor into rain 

 or snow. The storm, let it have commenced from any eastward point, meeting 

 with the colder air and the condensation becoming more rapid, will have its 

 direction changed from westward to eastward, and will be swept with the 

 western current towards the Atlantic so long as the elements of storm con- 

 tinue their action. The barometer shows the atmosphere to be lightest at 

 that point or line towards which the winds must blow on each side, so as to 

 bring together the elements for the most powerful action, and produce the 

 greatest changes of temperature. 



The magnitude of the storm will evidently be greater or less according 

 to the extent and power of the actions above indicated. In the great storm 

 of January 1810, so extensive over the northern part of our country, the 

 fall of the thermometer was from 60° at noon to 10 or 12° below zero at 

 nine in the evening, and 20° or more below zero the next morning ; while 

 the snow fell abundantly, and the wind was a tornado. So also in the storm 

 at the beginning of the year 1864, when rain came from the eastward on 

 the last day of December, both west and east of the Mississippi, the cold 

 at St. Louis, and wide over that region, was 19 to 30° below zero, January 

 1 ; a change far greater at the west, than in the State of New-York and 

 farther east. The snow and wind was like that of January 1810. In such 

 cases, the higher and colder atmosphere seems to be precipitated upon the 

 earth for the reason and in the way already mentioned. The great and 

 sudden fall of temperature in many storms seems to require this transfer 

 of atmosphere from a high and cold region to the earth's surface. 



All considerable thunder-showers are a fair illustration of the statements 

 above made. In our country their course is from west to east : they begin 

 with a wind from the south or a little east of south, blowing the vapor and 

 clouds northwards towards the great mass of thunder-heads which is capped 

 by a close cirrose cloud, and is moving eastwards by the general upper 

 current acting on the forming mass of the storm. The wind from the south 



