On the Barometer. 1 39 



instance the clouds come from a denser to a rarer medium, 

 and in the latter case, from a rarer into a de?tser medium. 



The maximum and minimum heights of the barometer, 

 in this and every other country, are limited, by nature, thus : 

 When the atn)Osphere is charged with as much ivater m 

 a state of chemical combiiiation, under the most favourable 

 circumstances for absorbing it, as the atmosphere can 

 sustam, it is then ktaviest, and the barometer is at its 

 maximum ; — and when the air is most free from, or contams 

 the least quantity of, tiater in a state of chemical combina- 

 tion, other circumstances concurring, the atmosphere is 

 then lightest, and the baromettr is at its mimnmm of 

 height. ° Hence, cceleris paribus, the absorption of water, 

 by air, renders the atmosphere heavier, and its parting with 

 it renders it lighter*. 



During many successive years of observation on the ba- 

 rometer In England, it may be found, that its greatest 

 height will not exceed one inch and a quarter above 

 CHANGEABLE ; DOf its kast height, the same distance beloiv 



CHANGEABLE. 



The variations in the barometer are greatest towards the 

 poles; less, intemperate latitudes; and /eas^, between the 

 tropics. 



The barometer was observed, some years ago, at Oxford, 

 as low as 28 inches, within two hundredths, at which tune 

 the weather was perfectly fair and calm; but it appeared 

 afterwards that an earthquake had happened at the time, at 

 a distant part of the world. This circumstance puzzled 

 prdinary observers, and injured the reputation of the 

 weather-glass ; but the intelligence afterwards of the event 

 which had happened restored its character. 



The moisture on the ground and in other situations 

 freezes, as is well known,'vvhen the atmosphere is tuarmer 

 than 32**, the freezing point, in consequence of the cold 

 produced by evaporaiion. Under the most favourable cir- 

 cumstances, viz. a dry air and a brisk wind, this may hap- 

 pen, when the air is eight or ten degrees above xhe freezing 

 point. The rise or fall of one-tenth of an inch, in the 

 barometer, is usually accompanied, or followed, by a very 

 perceptible change in the weather. 



• Anionj; oihcr circumstances, which show the power of air for absorb- 

 ing water or vapour, the follc.win^-is a very curious as well as satisfactory 

 one : if a small cloud, or frapnent of a cloud, in a loose or rare state, insu- 

 lated from others, in a clear part of the atmosphere, especially when the 

 air is warm and calm, be observed, it will be found to melt, or dissolve, as 

 it were, in the aimosphere, gradually, and at length to vanish entirely 



Raia 



