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THE POPULAR SCIENCE MONTHLY. 



from twelve to twenty-four hours, or longer. Sometimes these clouds, 

 before reaching the zenith, will recede and disappear beneath the 

 southern horizon. This indicates a backward oscillation of the south- 

 ern current, caused by the greater resistance of the polar current. 

 But in such case the stratus clouds will reappear next day, or sooner, 

 and uniting and, becoming denser, they will advance over the zenith, 

 and cover the whole heavens, discharging rain, snow, or sleet, accord- 

 ing to the thermal conditions present. 



Fig. 4. Cumclo-Sthatus Clouds. 



Thus, by observing the clouds, a northeast or winter storm may 

 always be predicted from one to three days beforehand, while the 

 barometer shows no change until the stratus clouds from the south 

 have reached and passed over the zenith, when it begins to fall ; but 

 the thermometer indicates no change. 



At this stage of the storm the wind from the north rises and blows 

 more violently, while the clouds move northward against the wind, 

 and the rain or snow, driven by the prevailing wind, comes down 

 obliquely from the north. After some time the direction of the wind 

 changes, and there is a calm. The air is warmer, the thermometer 

 rises suddenly, the barometer has reached its lowest point, and the 

 rain or snow falls vertically. This calm continues for a longer or 

 shorter time, and the wind gradually changes until it comes from 

 nearly or quite the opposite quarter from which it came at the begin- 

 ning of the storm, and blows more powerfully than before. The 



