THE BAROMETER. 



observations continued for several years, has been found to be 

 29-7*7 inches. 



18. The periodical variations of the barometric column are 

 extremely complicated, though very minute. In winter, it is 

 found that the column attains a maximum height at nine in the 

 morning ; it falls from this hour until three in the afternoon ; it 

 then begins to rise, and attains another maximum at nine in the 

 evening. In summer, the hour of the first maximum is eight 

 in the morning, and that of the minimum four in the afternoon ; 

 that of the second maximum being eleven at night. In spring 

 and autumn, this maximum and minimum take place at inter- 

 mediate hours. 



19. The accidental variations of the barometer, or, to speak 

 more properly, those which are not periodic, and which are much 

 greater in magnitude, have been generally supposed to be prog- 

 nostics of change in the weather, and hence the barometer is 

 sometimes called a weather-glass. Rules have been attempted to 

 be established by which from the absolute height of the mercurial 

 column the coming state of the weather may be predicted ; and 

 we accordingly find the words, Rain, Fair, Changeable, &c., 

 engraved upon the scale attached to common domestic barometers, 

 as if, when the mercury stands at the heights marked respectively 

 by these words, the weather is always subject to the vicissitudes 

 expressed by them. 



It requires but little reflection on what has been stated to show 

 the fallacy of such indications. The absolute height of the mer- 

 curial column varies with the position of the instrument. A 

 barometer in Fleet Street will be higher at the same moment than 

 one on the top of St. Paul's, and consequently two such barometers 

 would indicate different coming changes of the weather, though 

 absolutely situate in the same place. Two barometers, one of 

 which is placed at the level of the Thames, and the other at 

 the top of Hampstead Hill, will differ by half an inch, and, con- 

 sequently, would indicate, according to the usual scales, different 

 coming changes. 



20. It is evident, therefore, that the absolute height of the 

 barometer cannot in itself be an indication of anything but the 

 weight of the atmosphere in the place where the instrument 

 stands, and the words engraved on barometric plates, which have 

 been just referred to, are altogether unworthy of serious attention. 



Nevertheless, at a given place the column varies between certain 

 limits, usually from 2 to 2| inches, and when the mercury is at 

 its highest limit, the prevailing character of the weather is fair, 

 when at its lowest it is rainy and stormy, while at the intermediate 

 altitude it is variable. 

 186 



