On the Barometer. 137 



rain, at the place of observation, or in its vicinity ; especially 

 if the barometer be at or below changeable. 



The barometer, stationary, with a convex surface of the 

 mercury at the top, is a strong indication oifair iveatheTf 

 at the place of observation, or in its vicinity; especially if 

 the barometer be at or ahove changeable. 



In the first instance, I would account for this circum- 

 stance, by the natural spring or elasticity of the air being 

 suspended or diminished (the density or weight of the at- 

 mosphere remaining the same) by the intervention of vapour 

 collectino; into amass; and in the latter instance, from 

 the dispersion of the interposed vapour, by which the na- 

 tural .-pring or elasticity of the air is restored. 



For the same reason it is, I presume, that during a steady 

 fall of rain, the barometer is commonly stationary, with a 

 concave siirface at the top ; and as soon as the weather be- 

 gins to clear up, the top of the mercury in the barometer 

 assumes a convex surface. 



The quick ascent into the atmosphere, oi smoke, vapour, 

 &c. \nA\c3.\.ts fair-iveatker ; and the slow ascent, and parti- 

 cularly the descent of them, indicates rain. The former, in 

 consequence of an influx of denser ov heavier 3^\v,\nio the 

 lower stratum of the atmosphere, commonly from the north, 

 and east points ; and the latter, in consequence of an influx 

 oi rarer or lighter air, into the lower stratum of the at- 

 mosphere, commonly from the soiith and ivest points. 



Upon the whole there is more wet weather, whilst the 

 barometer is above changeable ; than there is dry weather, 

 whilst the barometer is below changeable. Hence it 

 follows that, caeleris paribus, the former state of the baro- 

 meter is not so strong an indication affair weather, as the 

 latter is o'i lainy weather. 



The barometer rising, especially if it be progressive, 

 whilst the wind is in tlie south' or south-west points, indi- 

 cates a change of the wind to the north or east points ; and 

 conversely, the barometer sinking, whilst the wind is in 

 the opposite points, indicates a change to the south or west 

 points. 



In the spring, and as summer advances, rain brings suc- 

 cessively warmer weather niter \t', and m autumn, and as 

 winter approaches, rai/t brings successively colder wealhei 

 after it. Hence, successive showers gradually change, as 

 it w ere, spring into summer, and auhirnn into winter, 



Ihe reason (jf this seems to be, that during ybir ueather 

 alu/ief tiie true character of each season, with respect to 



temperature, 



