152 The Realm of Nature CHAP. 



small district of Kansas on the Missouri River, and in the 

 south-west of Illinois, near the Mississippi and Ohio, that 

 more than 50 have been recorded in the last hundred 

 years. 



210. Waterspouts and Cloudbursts. The rapid con- 

 densation of water-vapour in the axis of a tornado, or in 

 the comparatively harmless whirlwinds that sometimes occur 

 in all parts of the world, produces a dark funnel-shaped 

 cloud tapering downward to the Earth. Such a cloud 

 occupying the centre of an ascending eddy of air is called 

 a waterspout. When it strikes the ground the heavy fall 

 of rain on a very small area sometimes produces great 

 destruction. At sea, or in passing over a lake or river, 

 the low pressure of the whirling air of a waterspout often 

 sucks up a column of water and whirls it on for consider- 

 able distances. In this way shoals of fish or swarms of 

 frogs are sometimes raised high in the air, carried for miles 

 inland, and dropped as showers of fish or frogs to the 

 wonder of country people. It often happens that the upward 

 rush in a tornado is strong enough to prevent the condensed 

 water from falling until a great quantity has accumulated ; 

 then it descends not as rain but like a river, and the 

 phenomenon is spoken of as a Cloudburst. On mountain 

 slopes cloudbursts have been known to hollow out deep 

 ravines in a few minutes. Hail as well as rain may be 

 similarly accumulated, and the worst hailstorms occur dur- 

 ing the passage of a tornado. 



211. Weather-charts. The gradual growth of know- 

 ledge about the atmosphere showed that the barometer 

 could be used for predicting changes of weather in certain 

 cases. Most barometers have a series of words from "Set 

 fair" to "Stormy" engraved on the scale, as if high or 

 rising pressure always means calm and fine weather, and 

 low or falling pressure always foretells wind and rain. A 

 few weeks' observation will in most cases convince any 

 one that this is a mistake, and that a single barometer is 

 of little value for forecasting the weather. Fig. 30 shows 

 that it is not the actual height of the barometer at one place, 

 but the difference in the height of many barometers at con- 



