468 On the Barometer, 



mer season, and the almost constant moist stale of the 

 lower stratum of the atmosphere during the wiNTt r sea- 

 son ; the air, however, being sometimes sufficiently dense, 

 as in the clear weather which accompanies a freezing atmo- 

 sphere, to retain the water in a state of chemical combina- 

 tion, notwithstanding the diminution of temperature. 



The same circumstance accounts likewise for the dif- 

 ferent states, with respect to moisture and dryness, of the 

 middle seasons, viz. spring and autumn, accordingly 

 as these participate in their nature more or less of either of 

 the former seasons ; observing that, cceteris paribus , there is 

 more rain and misty weather during autumn than spring, 

 in consequence of the greater quantity of water which has 

 been raised into the atmosphere during the summer than 



the WINTER SEASON. 



All the circumstances I have had occasion to mentibn, 

 depending upon the greater or less density, and the higher 

 and lower degrees of temperature of the atmosphere, are 

 exemplified by the two following familiar experiments: 



In the first instance, by means of pumping out of a glass 

 receiver (containing air apparently dry and perfectly trans- 

 parent) a certain portion of the air it contains, when the 

 air being rarefied deposits a certain portion of the water it 

 originally Contained in chemical combination in a cloudy 

 vapour, which, upon re-admission of the air, is re-absorbed; 

 and in the second instance, by abstracting heat from a glass 

 vessel containing atmospherical air, and again restoring the 

 heat. The latter circumstance is likewise instanced, natu- 

 rally, by what commonly happens in the course of a hot 

 summer's day, particularly when the ground has become 

 very moist by previous rain ; the vapour ascending visibly in 

 the morning, disappearing during the middle of 'the day, 

 and descending visibly again in the evening*. 



The variations of temperature in the atmosphere inde- 

 pendent of those which proceed from the direct influence of 

 the sun, arise from the conversion of water into vapour, 

 which produces cold ; and the condensation of vapour into 

 water, which produces heat. Hence it commonly follows, 

 that in proportion as the barometer rises, the thermometer 

 sinks, and vice versa, throughout the year; the direct influ- 

 ence of the sun in clear weather being abstractedf. 



* At Lima, in Peru, it never rains ; the moisture raised in the day time 

 being restored again at night in the state of mist. 



f In summer, during fair wearher, the nights, or rather the mornings be- 

 fore sun rise, are cold, approaching even to frost. 



• Thunder 



