PREVENTION OF SUMMER FROST 185 



atmosphere (see p. 151), while the absence of wind prevents 

 the warmer air of greater altitudes or of surrounding districts 

 from counteracting the rapid cooling which is in progress. 



The conditions necessary for the production of frost being 

 so few and simple, a careful observer should have no difficulty 

 in predicting its occurrence, especially if he is able to ascertain 

 the state of dryness of the atmosphere by means of a 

 hygrometer. In the United States the prediction of frost over 

 wide areas is one of the functions performed by the Weather 

 Bureau. Slight frosts are, however, often confined to well- 

 marked localities ; low-lying land is especially likely to suffer, 

 the fall in temperature being assisted in this case by the 

 descent of cold air from the higher regions around. 



It has been known from ancient times that the production 

 of smoke when frost is apprehended will often be the means 

 of saving a crop from injury. Pliny apparently refers to 

 this practice (Book xviii, chap. 69, 70) ; he says : 



' The moon, however, is productive of no noxious effects 

 at either of these periods, except when the nights are clear, 

 and every movement of the air is lulled ; for so long as 

 clouds prevail, or the wind is blowing, the night dews 

 never fall. And then, besides, there are certain remedies 

 to counteract these noxious influences. When you have 

 reason to fear these influences, make bonfires in the fields 

 and vineyards of cuttings or heaps of chaff", or else of the 

 weeds that have been rooted up ; the smoke will act as a 

 good preservative.' 



Boussingault in treating of this subject (Chimie agricole, 

 ii. 378) quotes also from the Commentaries reales of the 

 Inca, Garcilaso de la Vega, showing that before the conquest 

 of Peru, the Indians at Cuzco on the flanks of the Andes, 



