782 The Outline of Science 



days there has been a spell of fine quiet weather, with cloudless 

 sky and a steady high barometer, all over the country. Then one 

 morning the observers at the stations in the West of Ireland see 

 a few slight traces of "mare's tail" cirrus cloud low down on the 

 western horizon. Gradually these clouds, which are five miles 

 high, move up from westward till they are overhead. Behind them 

 the lines of cirrus become denser and eventually close up into 

 a complete thin cloud-sheet, in which there may be seen a "halo" 

 round the sun. Meanwhile a wind has begun to blow from the 

 southward, and on examining their barometers the observers see 

 that a fall has now commenced, which is slow at first, but which, 

 as time goes on, becomes faster. The sky is now covered with a 

 sheet of heavy leaden-grey cloud, in which a "watery" sun can 

 be seen dimly shining. This cloud is called "alto-stratus," it is 

 only one-half as high as the cirrus seen before it, and this means 

 that water vapour is condensing out of the air in increasing 

 quantity. Time passes, the barometer is now falling rapidly, 

 while the southerly wind is blowing with increasing force, and still 

 lower cloud of the "nimbus" type has formed and from it rain has 

 begun to fall. The wind continues to blow harder till it reaches a 

 gale, or even storm force, that is, from forty to seventy miles an 

 hour, and rain falls more heavily. But presently the fall of the 

 barometer begins to slacken, and then ceases, while the wind 

 becomes lighter and rapidly changes from south to south-west, 

 and then swings rather suddenly into the west or north-west. At 

 this moment the barometer begins to rise, and either then or 

 shortly afterwards the rain ceases, and in the west the sky com- 

 mences to clear. The north-west wind is now blowing almost as 

 strongly as did the southerly one, while the barometer is rising fast 

 and the sky is clearing rapidly. Some squalls and showers may 

 occur, but soon all is fine weather once more. It is, however, 

 noticeable that the north-westerly wind is considerably cooler 

 than the southerly one that preceded it. Why should this be the 

 case? The theory suggested by the famous Norwegian meteor- 



