18 Dr. Hall on one of the Causes 



Meteorological Observations of Mr. Dalton*. "Very dark and 

 dense clouds," he observes, " pass over without rain when the 

 barometer is high ; whereas when the barometer is low, it some- 

 times rains without any appearance of clouds." 



The views just given are greatly supported by the observation 

 that the variations in the barometer are confined to the lower 

 region of the atmosphere, and diminish rapidly as we approach the 

 upper limit of the region of the clouds. 



I now proceed to make a few observations on the influence of 

 the condensation of atmospheric moisture as a cause of wind. 

 The causes of the north-east wind have been already stated as 

 given by various authors t. But I am not aware that any satis- 

 factory cause has been assigned for the prevalence of winds from 

 the south or from the west. It is plain, however, from the pre- 

 ceding remarks, that such winds must be produced by great and 

 rapid formations of clouds and rain, the air being drawn from all 

 the adjacent regions to that in which this condensation of atmo- 

 spheric vapour takes place, and thus under particular circum- 

 stances a south-west wind or gale will be produced. 



Mr. Playfair has made some observations which appear to con- 

 firm this view of the subject. He observes, " the sudden sinking 

 of the barometer almost always indicates a gale of wind, though 

 a gale that is sometimes at a considerable distance J." And 

 further, " there is in our climate hardly any instance of rain 

 without a change of wind, and very rarely a change of wind 

 without rain in a greater or less quantity §." Mr. Dalton ob- 

 serves, " the barometer generally rises with a wind between the 

 north and east, and is very low in winter when a strong and 

 warm S. or S.W. wind blows J." He adds, the lowest extreme of 

 the barometer for five years, was accompanied with a strong S. 

 or S.W. wind and and heavy rain, and occurred just after the 



* Page 196. 



t See La Place, Syst&me du Monde, t. 2. p. 174 ; Playfair's Outlines of 

 Natural Philosophy, v, I. p. 293. 

 X Outlines, v. I. p. 299. § Ibid. p. 305. 



f| Met. Obs. p. 118. 



