296 Our Climatic f^icissitudes. [June, 



OVSR CLIMATIC VICISSITUDES. 



A BRIEF account of tlie most rioted peculiarities of our climate was 

 given on page 82. It was there stated, that the most plausible theory 

 to account for our sudden changes, was the one which looked to the 

 Piocky Mountains as deflecting northerly, the warm trade-winds of the 

 Atlantic, thus producing our remarkably "warm spells," often followed 

 by sudden " cold snaps," occasioned by the rapid descent of the cool 

 upper western current. 



We may here remark that this current from the west is, owing to its 

 dryness, colder, in effect, than the thermometer would indicate. Dry air 

 eagerly drinks in moisture ; even ice will evaporate when exposed to its 

 influence ; but evaporation cannot go on without taking up heat, as every 

 one knows who has been chilled by the drying of wet garments upon 

 him. Hence, we often see the ground solidly frozen, especially when 

 the western wind is blowing, although the thermometer is several degrees 

 above the freezing point. 



A few preliminary considerations will show the philosophy of this 

 matter of the weather so plainly, that even those who have never given 

 any attention to the subject, may have the key to the whole mystery. 



" Our wondrous atmosphere " is a great water drinker, and, like the 

 laborer, its thirst is in proportion to its warmth. Whatever it drinks 

 is so completely assimilated, that the sharpest optics are unable to detect 

 its presence in the pure pellucid ether, so that vast amounts of the liquid 

 element are floating unseen in the firmament above. Or, to state it 

 more technically, the amount of moisture air is capable of taking up is 

 dependent on its temperature. One hundred and sixty pounds of air, at 

 the freezing point, can dissolve one pound of water and no more ; but if it 

 be twenty-seven degrees warmer, that is, at fifty-nine, it can dissolve 

 twice as much ; and thus on, doubling the amount for every additional 

 twenty-seven degrees of heat. Hence, it is evident, that if one hundred 

 and sixty pounds of air, saturated at fifty-nine, should, by any means, be 

 cooled to the freezing point, it would precipitate one pound of water ; 

 and should a similar portion at eighty-six be cooled to fifty-nine, it 

 would precipitate two pounds, and in a like proportion for other degrees. 

 A deposition of moisture in some form, then, will occur whenever two 

 portions of air of difi'erent temperatures meet in such a way as to reduce 

 them below the point of saturation. If the cooling occur suddenly, 



