APPENDIX. 269 



a vacuum, for it is a property of gases that each will occupy a given space 

 in almost the same manner as it would if the others were not present. But 

 as the vapor expands into the surrounding cooler air its temperature is low- 

 ered, and, unless the air be very dry, a portion of the vapor is condensed, 

 forming a visible fog or mist. Now, all the heat which was consumed in 

 evaporating the water again becomes sensible upon its condensation, and 

 tends to raise the temperature of the surrounding air. The heat thus set 

 free will be in great part confined to the particles of water composing the 

 fog, which are too dense to rise, and thus they will tend to prevent the 

 escape of the heat, and at the same time they are so small that they float in 

 the air as fog, with hardly a perceptible tendency to fall. 



The tendency is, therefore, to trap the heat produced by the fire and dis- 

 tribute it thi'oughout the space near the surface which needs protection. 

 The excessive heat about the fire, which, with dry fuel, produces the waste- 

 ful upward draft, is, in this instance, utilized in evajjorating the water in 

 the fuel. The vapor then, by the operation of its gaseous property, dis- 

 tributes itself quickly throughout the surrounding cooler space, where, in 

 condensing, it sets free its latent heat, warming the region, and, by the 

 density of the fine particles of water thus warmed, the heat is retained near 

 the surface. 



After a considerable study of various methods of protecting orchards 

 against frost which have thus far been made public, the writer has become 

 convinced that those which depend for their success upon this principle are 

 generally the most efficient. In very dry climates, however, where the 

 dew-point at times of danger is 10° or more below the temperature of the 

 air this method is inefficient since the vapor does not condense in sufficient 

 quantities to protect, and. owing to the vapor being lighter than air. it 

 escapes upward and the heat nece.ssary for its formation is lost. 



Many methods which involve this principle have been suggested and 

 tried, tw^o of which have already been mentioned, namely: 



Fires of damp sfrair and stable mavnre — Have the fuel, in small piles, dis- 

 tributed throughout the orchard in advance ; the more numerous the piles 

 the better. With the same amount of fuel the best protection is obtained 

 from small and frequent fires, since, with small fires, the upward draft is 

 reduced to a minimum, and the more frequent the fires the more uniform 

 will be the distribution of heat. 



Sacks of manure — A decidedly preferable method is to pack damp stable 

 manure in common grain or burlap sacks, by which it can be conveniently 

 handled. They should be distributed through the orchards in rows about 

 one hundred feet apart, and about fifty feet between sacks in each row. 

 When it is found necessary to protect, a small amount of coal oil is poured 

 upon each sack and ignited. It is usually unnecessai'y to fire more than 

 every second or fourth sack, the remainder being left for later occasions. 

 These sacks will burn with a smoldering fire for several hours. 



The amount of heat which is set free by burning one sack of manure 

 weighing about fifty pounds, and condensing the vapor near the surface, 

 would be sufficient to raise the temperature 20° in a space of seventy-five 

 feet square and twenty-five feet deep. If one-fourth of this heat remained 



