452 TEE POPULAR SCIENCE MONTHLY 



ical velocities can develop near the center of a great tornado on account 

 of the retarding effects of friction where the wind moves over a rough 

 region like a city, yet it does show where the enormous power resides 

 that is always observed in these conditions. It might develop, there- 

 fore, a pressure of 5,000 or 6,000 pounds per square foot. This is, of 

 course, very much more than would be necessary to make all the de- 

 struction that has been noted. 



Hurricanes such as are observed in the neighborhood of the West 

 Indies, and the typhoon, which is the name of a hurricane in the 

 neighborhood of the Philippine Islands and China Sea, are truncated 

 dumbbell-shaped vortices built on exactly the same principles as the 

 St. Louis tornado, only they are very much larger in their dimensions. 



Fig. 4. Half Section of a Hurricane Vortex. 



The tornado generally ends at a level something like 1,200 meters above 

 the ground, and it is usually much less than half a mile in diameter. 

 The hurricane, however, is probably 12,000 meters thick, and it extends 

 several hundred miles in diameter. This makes the hurricane a very 

 thin mass of air of broad extent, as compared with the word tornado, 

 which is a relatively high mass of air and narrow in extent. We can 

 construct the velocities in the hurricane from our meteorological data, 

 and show that the winds blow at a certain angle, which conforms to the 

 section that cuts off or truncates the vortex at a certain plane. These 

 angles should be more fully explained. On the lowest plane the wind 

 flows radially and directly towards the axis; on the uppermost plane it 

 flows radially and directly away from the axis ; at a middle section, half 

 way between these two planes, it flows in circles tangentially around the 

 axis. In passing from the lower plane to the upper plane the wind 

 gradually makes a larger angle with the radius; first 10°, then 20°, 

 then 30°, and so on up to 90° at the middle plane half way up the tube; 

 then 100°, 110°, and so on up to 180°, which represents the wind 

 flowing radially away from the axis. If now a vortex is truncated at 

 a certain plane, all the winds on that plane will make a given angle 

 with the radius. If a truncated plane is one third the distance up 

 the axis then the wind will make an angle of 60° with the radius; that 

 is, it will blow in at an angle of 30° from a circle. This is about the 

 angle of the wind which observers have recorded in the case of hurri- 

 canes, and hence it is proper to infer that the truncated section should 

 be drawn at about the distance indicated from the lower plane. 



The ocean cyclone is a mass of air still larger than the hurricane, 



