Causes of Diseases in Crops 77 



about also by a sudden change from wind to calmness 

 of air. Winds prevent frost formation because they 

 prevent the accumulation of the colder air at the 

 surface. The trend of temperature is also an im- 

 portant consideration. A temperature of forty 

 degrees at about 6 p. m. with a clear sky may 

 indicate the approach of frost. A fall of tempera- 

 ture oi two degrees an hour in the afternoon would 

 also indicate the approach of frost. If the air pres- 

 sure is increasing rapidly, as indicated by a rapid 

 rise in the barometer, frost may be approaching. A 

 change in pressure usually precedes, by a short in- 

 terval, the change in direction of wind. 



How to Protect Crops from Frost. Crops may 

 be protected from frost in two ways, (i) Arti- 

 ficial covering is an old practice widely used by 

 truckers, and consists in protecting the plants by 

 covering them with newspapers, carpets, sacks, straw, 

 tar paper, or a mulch of soil. This, however, is 

 applicable only to small gardens or to seed beds. 

 On a large scale it is not practical because of the 

 labor involved. (2) Smudging and heating consists 

 in the bioming of any combustible material capable 

 of producing heavy smoke, such as moist straw or 

 coal tar. Through smudging we prevent the escape 

 of the earth's heat. 



A better method consists in heating the air of the 

 field by means of evenly distributed small fires gen- 

 erally supplied by ovens of various designs. The 

 material used is wood, coal, or oil, the choice being 

 ^etarmined by the local price and supply. With 



