188 SCIENCE IX SHORT CHAPTERS. 



In like manner, a steady, regular rise, going on for some 

 days in the midst of wet weather, may be regarded as a hope- 

 ful indication of coming continuous fine weather the more 

 gradual and steady the lise, the longer is the fine weather likely 

 to last. 



The least reliable of all the barometric changes is a sudden 

 rise. In winter it may be followed by hard and sudden frost, 

 in summer by sultry weather and thunder-storms. All that 

 may be safely said of such sudden rise is, that it indicates a 

 change of some sort. 



The barometer is usually high with N.E. winds, and low 

 with S. W. winds. The preceding explanations show the reason 

 of this. In a given place the extreme range of variation is 

 from 2 to 2-J inches. 



It has been proposed that the following rules should be 

 engraved on barometer-plates instead of the usual words : 



1st. Generally, the rising of the mercury indicates the 

 approach of fair weather ; the falling of it shows the approach 

 of foul weather. 



2d. In sultry weather, the fall of the barometer indicates 

 coming thunder. In winter, the rise of the mercury indicates 

 frost. In frost, its fall indicates thaw, and its rise indicates 

 snow. 



3d. Whatever change in the weather suddenly follows a 

 change in the barometer, may be expected to last but a short 

 time. 



4th. If fair weather continues for several days during which 

 the mercury continually falls, a long succession of foul 

 weather will probably ensue ; and again, if foul weather con- 

 tinues for several days while the mercury continually rises, a 

 long succession of fair weather will probably follow. 



5th. A fluctuating and unsettled state of the mercurial 

 column indicates changeable weather. 



As the barometer is subject to slight diurnal variations, 

 irrespective of those atmospheric changes which affect the 

 weather, it is desirable in making comparative observations to 

 do so at fixed hours of the day. Nine or ten in the morning 

 and same hour in the evening are good times for observations 

 that are to be recorded. These are about the hours of daily 

 maxima or highest readings due to regular diurnal variation. 



The true reading of the barometer is the height at which it 

 would stand if placed at the level of the sea at high tide ; but, 

 jus barometers arc always placed more or less above this level, 



