Livestigatiovs in the U. S. Signal Office. 51 



one, and must serve to allay the unreasonable fears of the in- 

 habitants of the so-called " tornado districts." It appears that 

 there is no part of the United States in which annually more 

 than one square mile of devastation or severe destruction can be 

 expected for each 185,000 square miles, although cases of limited 

 destruction may occur annually for about every 5,000 square miles 

 of area. In no state may destructive tornadoes be expected, on 

 an average, more than once in two years ; and the area over 

 which total destruction can be expected is, as shown by the fore- 

 going figures, exceedingly small, even in localities most liable to 

 these violent storms. The annual death casualties from torna- 

 does have averaged, in the last 18 years, 102 annually ; but it is 

 l^elieved that the death rate from lightning is greater than that 

 from tornadoes, since during March to August, 1890, the names 

 of 110 are on record who have lost their lives by lightning, al- 

 though the data are incomplete, especially as regards the south- 

 ern states. These statistics cannot be passed by lightly, however, 

 and it is doubtful if in the main they are much in error. By 

 them it appears from five years' record that the average annual 

 death rate liy lightning in the United States is 3.8 per million of 

 inhabitants, or 0.2 above the average. In Sweeden, for sixty 

 years, the average has been 3.0 ; in France, for forty-nine years, 

 3.1 ; in Baden, for seventeen years, 3.8 ; and in Prussia, for fifteen 

 years, 4.4 per million. 



Other figures, given by a life-insurance agent in St. Louis, 

 which the author claims to have compiled with great care, place 

 the average annual rate of death from lightning in the United 

 States at 206, being more than double the deaths from tornadoes. 

 It must be understood that these figures are not vouched for, and 

 must 1)e very cautiously received, as originating with companies 

 interested jiecuniarily in the statistics. 



On the whole, therefore, it may be safely assumed that tiniia- 

 does are not so destructive to life as thunder-storms. 



Professor Thomas Russell has formulated a method for predic- 

 tion of cold waves. They always occur after " lows " and before 

 " highs," and different cold waves vary in extent from three 

 " units " to sixty. A " unit " of temperature-fall is taken as a 

 fall of twent}' degrees over an area of 50,000 square miles. 



The temperature-fall curves in the United States are approxi- 

 mately elliptical in shape. Perfect ellipses represent actual 

 temperature falls with an error not exceeding six degrees in 



