24 THE CLIMATOLOGY OF THE UNITED STATES. 



which the percentage of verification has been very large 

 considering the great difficulty of the subject, and very 

 likely in this short period many lives have been saved. 

 Many people in the east consider that the large destruc- 

 tion of property in the west by tornadoes is partly due 

 to the light construction of the wooden houses there ; but 

 it should be borne in mind that brick and stone buildings 

 succumb to these blasts almost as quickly as those of 

 wood. 



When a tornado strikes a building it generally tears it in 

 pieces, carrying the debris aloft within the funnel-shaped 

 cloud and throwing it out from the top to either side as it 

 advances, leaving the wreckage of a homestead scattered 

 along in a northeasterly line for distances, varying from 

 a few yards to one or two miles. 



It sometimes, however, happens that a house will ex- 

 plode by the expansion of the air within, as the rarefied 

 air of the funnel passes over it, and the four walls will be 

 thrown out in as many directions. This may occur fre- 

 quently without being observed, as the parts may be car- 

 ried away by the in-blowing currents and thus all trace 

 of this action may be obliterated. Tornadoes have been 

 found to travel in connection with some cyclonic disturb- 

 ance to the north and their courses are generally parallel 

 with the course of the main storm, though generally from 

 200 to 500 or 600 miles away. The theory which is now 

 generally accepted is, that when a body of cool air flows 

 southward and meets a mass of warmer air, it sometimes 

 flows over instead of under the warmer air and in seeking 

 a condition of stable equilibrium the warm air forces an 

 opening through the stratum of cool air above, an inter- 

 change of positions thus taking place. Taking account 

 of the temperatures, amount of moisture in the air and 

 barometric pressures at the time, the Signal Service has 



