TORNADOES. 149^ 



confined to the land areas, but may and frequently do occur over large and small 

 bodies of water. 



Thunder-storms. — These phenomena are atmospheric disturbances of great 

 variability of extent and power. They are invariably accompanied by such man- 

 ifestations of the presence of electricity as are ordinarily termed thunder and 

 lightning, the former being entirely consequent upon the existence of the latter. 

 Thunder is but the reverberations of the concussion produced by the inconceiva- 

 bly rapid propulsion through the air of that physical element we are pleased to 

 term electricity. Thunder-storms may be a few miles or several hundred in ex- 

 tent, and their length of duration is quite as uncertain, viz : from a {^^^ hours tO' 

 one or more days. There is no regular time of day for their occurrence, al- 

 though they are perhaps more frequent in the afternoon. However, they may 

 occur at any time during the day or night. As to the season of year, summer is 

 the period of greatest prevalency. There is no month of the year entirely free 

 from them. Whether the precipitation be rain or snow the presence of electricity 

 has still been manifested in the usual form. With the former character of con- 

 densation of vapor, the evidence of electricity is most common, while with the 

 latter it is the rare exception. 



As regards geographical distribution, thunder-storms are most frequent be- 

 tween the equator and parallel 40° N. and from thence to parallel 70° N. the 

 average frequency diminishes with considerable rapidity. In the vicinity of par- 

 allel 80° N. it is believed they never occur, although this in the main is mere 

 supposition. There are certain portions of the United States where thunder- 

 storms are unusually frequent as compared with other parts. They seldom occur 

 in the Pacific Coast States, especially California, and are most frequent and violent 

 in the Eastern Rocky Mountain Slope, the Lower Missouri Valley and in the 

 Lake Region. 



Having briefly outlined the characteristics of the various classes of storms we 

 will now proceed to consider more in detail the most important (at least in certain 

 respects) of all atmospheric disturbances. At this stage of our inquiry in regard 

 to the character and classes of storms, I presume it will be admitted, that no two 

 of the several storms defined, at least appear to be alike. There are, however, 

 points of resemblance and in some, these features are stronger than in others. As 

 each is studied more carefully, the essential points of difference will be more 

 clearly contrasted. It is not within the province of this memoir to discuss at 

 length the points of difference or harmony nor enter into an intricate analysis of 

 meteorological phenomena and the multiform operations of atmospheric changes 

 attending the origin, development and complete formation of these disturbances. 

 On the contrary it is my desire to make this publication, in general, a brief but 

 comprehensive resume of the leading features of storms, as known at least in the 

 United States, if not in North America, and in particular, to present rather a 

 minute consideration of the peculiarities of tornadoes, with a view to place at the 

 disposal of the people most interested, the facts and practical results of past and 



