Frost and the Prevention of Damage by It. 35 



when the danger point has been reached, regardless of the tem- 

 perature. 



When small apples or pears commence to freeze minute blisters 

 will begin to form on the skin. By keeping constantly on the watch 

 for the first appearance of these blisters on the fruit in the coldest 

 part of the orchard, the firing can be begun at exactly the right time ; 

 no fruit will be lost and no oil wasted. By carefully cutting the 

 blossoms of deciduous fruit it is generally possible to note with ice 

 crystals first begin to form in them, and thus regulate the beginning 

 of firing. 



When oranges begin to freeze, the section of the skin exposed to the 

 sky takes on a transparent appearance, generally known as the 

 "water-mark," probably caused by the water in the rind freezing 

 and leaving the oil separated. On the following day these oranges 

 can be picked out easily and are called " shiners." By timing the 

 firing with the first appearance of the " water-mark " in the orchard, 

 it is possible to save the fruit and yet prevent waste of oil. Some 

 experience is necessary before the fruit grower is able to use these 

 methods of timing the firing; but the importance of saving oil is 

 well worth giving the matter close attention. 



If the small lard-pail heaters are set 100 to the acre, alternate 

 heaters in every fourth row should be lighted first, followed im- 

 mediately by alternate heaters in every second row if the tempera- 

 ture has been falling rapidly. The effect on the temperature should 

 then be noted and decision made as to whether additional firing is 

 necessary at that time. As soon as a row of heaters begins to burn 

 low, reserve heaters should be lighted, as the amount of heat given 

 off during the last half hour of burning is small. 



If the large capacity down-draft heaters are used, all may be 

 lighted at once if desired and the consumption of oil regulated by 

 manipulating the drafts. 



During a cold night an isolated cloud passing overhead, by cut- 

 ting off radiation and to a certain extent reflecting radiation from 

 the earth, may cause the temperature to rise. As the cloud drifts 

 toward the horizon the temperature falls again. Likewise, sudden 

 temporary rises in temperature are caused by gusts of wind of short 

 duration which mix the upper and the surface air. As a general 

 rule the temperature falls rapidly after the wind or cloud has passed 

 and cases are on record where entire crops were lost through ex- 

 tinguishing the heaters at such a time. If clouds are overspreading 

 the whole sky or a sudden rise in temperature due to wind occurs just 

 before sunrise, the heaters may be extinguished, but if the sky re- 

 mains clear and sunrise is an hour or more away, the temperature 

 should be watched closely during the remainder of the night. 



