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LXIX. On the Barometer, By Richard Walker, Esq, 

 To Mr. Tilloch. 



Sir, An consequence of its having been intimated to me, 

 that a short rationale, or general view of the various changes 

 in the weather, and the indications of the barometer, as 

 connected with them, might not be unacceptable, 1 beg 

 leave to transmit the following, which maybe considered as 

 an 1 appendage to the paper " On the Application of the Ba- 

 rometer for indicating the Weather, &c." you did me the 

 favour to insert in your last Number. 



Water exists in the atmosphere in two different stales, 

 viz. 1st, in a state of chemical combination; that is, so 

 completely incorporated with the air, as to form with it 

 one homogeneous transparent fluid; — and, 2dly, in a state 

 of mechanical combination; which is, when the minute 

 particles of water are merely suspended in the air, forming 

 that state of the atmosphere, which is denominated cloudy 

 or misty. 



The dense state of the air being fittest for the chemical 

 combination above mentioned; clear, dry weather, generally 

 speaking, accompanies the higher degrees of the mercury 

 in the barometer, whilst, a rare state of the air being less 

 capable of receiving the water into chemical combination, 

 it is then merely suspended in a state of mechanical com- 

 lination, forming, clouds, mists, &c. 



Hence it follows, that, when the mercury stands at or 

 near fair, clear dry weather is indicated generally ; and 

 when at or near rain, cloudy or wet weather ; and when 

 fluctuating mid- way, changeable weather. 



It occasionally happens, however, that the atmosphere 

 is cloudy, and even wet, whilst the barometer is as high as 

 fair; and clear and dry, whilst the barometer stands as 

 low as rain. The reason of this, in the first instance, is, 

 that the air, having become replete or over-loaded with 

 water, is incapable (by an alteration of temperature, viz. 

 the air and its contents having become colder) of retaining 

 or suspending it in a state of chemical combination ; and in 

 the latter case, which happens after rain, succeeding a con- 

 tinned dry state of the atmosphere, which having swept 

 down the vapour with it in its descent ; the air, though then 

 in a rarer state, is yet sufficient to retain the proportion of 

 water, now much reduced in quantity, in a state of chemical 

 combination. 



The 



