BAROMETER. 353 



(). The range of the barometer is greater during 

 winter than summer in the middle latitudes. 



The reason of which is, that the advancement 

 of the sun towards the pole, brings the tempera- 

 ture of the higher latitudes to nearly an equality 

 over forty or fifty parallels, and almost to the same 

 state as within the tropics, as observed by Dr. 

 Dalton : whereas during winter, the warm and 

 rarefied air of the tropical regions is often en- 

 countered by widely extended currents of cold 

 dense air from the polar latitudes ; the conse- 

 quence of which is, that the barometer will rise 

 or fall at the same time, over a great extent of 



two of which were fair ; but that when it stood above its mean 

 monthly height, there was but little rain, and generally fair 

 weather. 



He also found, that the heaviest rains fell when the barometer 

 was about 29'47, and not when at a minimum, as- might have 

 been expected, d priori. His explanation of this fact is simple 

 and satisfactory. He observes, that " when the barometer is 

 above the mean high extreme for the season of the year, the air 

 must, relatively speaking, be extremely dry or cold, or both. If 

 extremely dry, it is in a state for absorbing vapour ; and if ex- 

 tremely cold, no further degree of cold can be expected, and 

 therefore in neither case can there be any considerable precipita- 

 tion. On the contrary, when the barometer is very low for the 

 season, the air must relatively be extremely warm, or extremely 

 moist, or both ; if extremely warm, it is in a similar state to dry 

 air for imbibing vapour ; and if extremely moist, there must be a 

 degree of cold introduced to precipitate the vapour, which cold, 

 at the same time, raises the barometer. From which it follows, 

 that no very heavy and continued rains can be expected to 

 happen whilst the barometer remains about the lowest extreme.'* 

 (Meteorological Essays, p. 143.) 



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