304 THE POPULAR SCIENCE MONTHLY. 



The cumulo-stratus cloud, which is the precursor of this kind of 

 storm, can usually be observed only from one to eight hours, and, in 

 some cases of the most violent kind, only about twelve hours before it 

 will burst upon a place. Although these storms are the most danger- 

 ous and destructive not unfrequently ending in tornadoes and hurri- 

 canes the barometer is of no practical service in predicting it. This 

 is explained by the fact that in such storms the plane of meeting of 

 the two currents moves southward with its lower extremity, or region 

 of lowest barometer, in front, while the plane itself is more or less 

 inclined northward. Hence the barometer shows no change until 

 this region of lowest barometer moves over it, when it suddenly falls g 

 but it is then already in the most dangerous part of the storm, and its 

 warning, therefore, comes too late ; while the clouds, if properly ob- 

 served, always give warning in time to provide against the dangers 

 of such a storm. 



Tornadoes. This class of storms includes hailstorms, water- 

 spouts, hurricanes, and all storms in which rotary and lateral motions 

 are more or less combined. They are the most violent and destruc- 

 tive of all storms, as well as the most complicated and difficult to 

 understand and explain. They are the offspring of progressive polar 

 or summer storms, and in the temperate zone occur only during 

 summer. 



When in the development of a summer storm, as above described, 

 the two conflicting currents attain a state of equal power or resistance, 

 and thus balance each other, which is indicated when the dense cumu- 

 lus clouds over the plane of conflict become stationary, then the storm 

 is at its crisis. The air within the region of conflict is compressed and 

 very sultry, and this condition is always felt before a toimado by per- 

 sons within its area. If, now, during this critical stage of the storm, 

 no topographic or other disturbance of its tension take place in its 

 plane of meeting, a return oscillation of the polar current northward 

 will set in, and the storm will gradually clear away. But if, in this 

 crisis of the storm and during this high state of compression and re- 

 sistance, either current becomes stronger, and forces back the other 

 over some hill or valley, or if some other obstruction or configuration 

 of the surface of the earth breaks the tension or disturbs the resistance 

 between the two currents at any point, so that the polar current will 

 sink as in a valley, then the tropical current will suddenly rush into 

 this depression and generate a succession of violent whirling and zig- 

 zag motions along the diagonal of the two currents within the plane 

 of conflict, as the waters of a dam would rush through a sudden break 

 or depression in an embankment. This conclusion respecting the 

 origin of tornadoes Prof. Blasius reached after his careful study of 

 the West Cambridge tornado of 1851, and it was subsequently con- 

 firmed by the facts and phenomena connected with the tornado of 

 Iowa and Illinois, in May, 1873, as obtained from the report of the 



