MKTEOROLOGY. 1 1 



of jur above the earth's surface, as is well known, decreases in tem- 

 perature in a certain ratio to its altitude ; in this atmosphere rises 

 another of aqueous vapour, exhaled from the earth and ocean in quan- 

 tities proportionate to the heat prevailing at the surface, and which 

 also decreases in temperature as it rises in the air : the two atmos- 

 pheres, it should be noticed, continuing quite distinct and disunited. 

 Now if both atmospheres cooled in ascending at the same rate, as 

 much vapour would continually exist, as the atmosphere of air could 

 sustain — in other words, it would always be saturated with moisture ; 

 the effect would be that a thin mist would generally obscure the sky, 

 and the least fall of temperature produce copious depositions of dew. 

 By a beautiful arrangement, however, the air cools more rapidly 

 upwards than the vapour, from which it follows that in the higher 

 regions the vapoiu: mingling with colder air, gives out its latent heat 

 and is condensed into clouds ; the pressure on the vapour below being 

 thus removed, more Water rises by evaporation, and when as much 

 exists in the upper regions as their temperature will sustain in the 

 form of gas, a fall of rain ensues. In the mean time the lower part of 

 the atmosphere is kept clear, or unmistified, and considerably above 

 the point of saturation ; thus instead of obscuring the entire depth of 

 the atmosphere by thick, palpable, and almost continual mists, the 

 vapour on condensing is formed into thin strata or layers of clouds, 

 leaving the great bulk of the atmosphere cleai' and transparent. It may 

 be noticed that after a warm day, a clear sky at night induces a mpid 

 radiation of heat from the earth's surface, and by this means a great 

 fall of its temperature ; on tliese occasions there are copious deposits 

 of dew, and the vapour becomes visible in condensing, as a thick mist 

 creeping along the surface; this does not generally rise very high, the 

 temperature being then always lowest near the earth, the air above 

 being warmer the vapour there remains uncondensed and invisible : 

 a fall, however, of several degrees of the thermometer is necessary to 

 produce these mists and dews, and if an equable decrease of atmos- 

 pheric and aqueous temperature had occurred in ascending, such mists 

 and dews would have been continual and pervading the whole depth 

 of the atmospheie. The general operation of the two atmospheres is 

 thus simply stated. The atmosphere of air, rarified by heat at the 

 equator, ascends and proceeds in upper currents towards each pole, a 

 returning current sets in towaids the equator at the surface filling up 



