SPRING FROSTS 



125 



treated as regards pruning just as ordinary hedges would be. 

 Such hard pruning, however, renders the trees composing them 

 useless as fruit producers : often there is practically no fruit 

 throughout the run of the hedge, though profuse fruiting occurs 

 on any branches which may have been left unpruned with the 

 object of overarching a path, etc. ; thus affording a striking 

 illustration of the antagonism between hard pruning and fruit- 

 ing (p. 67). Lists of these hedges, arranged according to their 

 merits from various points of view, are given below. 



The damage done by wind-frosts, however, is not confined 

 to that taking place when the blossoms are actually killed, for 

 indirect damage occurs through the cold interfering with the 

 visits of those insects on which pollination depends. 



As to radiation-frosts, a few words on the general principles 

 governing them may be necessary, for these principles are not 

 very widely known, as may be gathered from the frequency with 

 which are heard such expressions as " dew falling," " mist 

 rising," etc. 



Heat may be received, or lost, by a body in two distinct 

 ways : by conduction, as when heat passes gradually from the 

 hot end of a metal bar to the cooler end, there being a regular 

 gradation of temperature from one end to the other; or by 

 radiation, as when we warm ourselves by standing in front of 

 a fire, the heating in that case being due to the rays of heat 

 which are given out by the burning coal being absorbed by our 

 bodies, though they pass through the' air without raising its 

 temperature appreciably. Such exchange of heat is always 

 occurring between bodies, whatever their temperatures may be, 

 and whether a body becomes cooler or hotter, depends on whether 

 it loses more heat than it receives, or vice versa. 



During the day the earth receives more heat from the sun 

 than it loses, and, consequently, becomes hotter ; but during the 

 night it loses more heat than it gains, for the temperature of 

 space is excessively low, and practically no heat is radiated 



