490 METEOROLOGY. DEW. 



At half-past five in the evening ; the air calm, the sky cloudless, 

 the sun already concealed for some time : thermometer under the 

 snow, 32° F. ; thermometer upon the snow, 30° F. ; thermometer 

 in the air, 40" F. 



Feb. 14. Seven in the mornino^, wind W., a fine rain falling: 

 thermometer under the snow, 32" F. ; thermometer upon the snow. 

 32" F. ; thermometer in the air, 35.7° F. 



When we reflect upon the losses occasioned to formers and mar- 

 ket gardeners by frosts that are entirely due to nocturnal radiation 

 ft seasons of the year when vegetation has already made considera- 

 ble progress, we ask eagerly if there be no possible means of guard- 

 ing against them. I shall here make known a metiiod imagined and 

 successfully followed by South American agriculturists with this 

 view. The natives of the upper country in Peru who inhabit the 

 elevated plains of Cusco are perhaps more than any other people 

 accustomed to see their harvest destroyed by the effects of nocturnal 

 radiation. The Incas appear to have ascertained the conditions 

 under which frost during the night was most to be apprehended. 

 They had observed that it only froze when the night was clear and 

 the air calm : knowing consequently that the presence of clouds 

 prevented frost, they contrived to make as it were artificial clouds 

 to preserve their fields against the cold. When the evening led 

 them to apprehend a frost — that is to say, when the stars shone with 

 brilliancy, and the air was still — the Indians set fire to a heap of wet 

 straw or dung, and by this means raised a cloud of smoke, and so 

 destroyed the transparency of the atmosphere from which they had 

 so much to apprehend. It is easy in fact to conceive that the 

 transparency of the air can readily be dvstroyed by raising a smoke 

 in calm weather ; it would be otherwise were there any wind stir- 

 ring ; but then the precaution itself becomes unnecessary, for with 

 air in motion, with a breeze blowing, there is no reason to apprehend 

 frost from nocturnal radiation. 



The practice t'ollowed by the Indians just described is mentioned 

 by the Inca (iarcillaso de la A cga in his Royal Commentaries of 

 Peru. Garcillaso in the imperial city of Cusca, and in his youth, 

 had fiequi ntly seen the Indians raise a smoke to preserve the fields 

 of maize from the frost.* 



The cooling of bodies occasioned by nocturnal radiation is always 

 accomi)anicd by a dcposile of moisture ujion their surface under the 

 form of minute globules : this is dew. The ingenious experiments 

 ot' Wells having demonstrated that the appearance of dew always 

 follows, never j)recedes the fall in temperature of the bodies on 

 ^vhich it is deposited, the phenomenon cannot b«i attributetl to any 

 thing more than a simple condensation of the watery vapor con- 

 tained in the air, comparable in all respects to that which takes 

 place upon the surface of a vessel containing a fluid that is colder 

 than the air.f The quantity of moisture dissolved in the atmosphere 



* The piod efFfcts of smoke in proventinp nortiiriial congelation are nlso signaliied 

 by Pliny tlic naturalist. 



t Arn.no, .Annuaire des Longitudes, .\\jn*o 1837, p. 160. 



