174 THE APRIL MOON. 



rapidity of the process. dLemistry points out tte reason of this, vapour 

 having a greater capacity for heat than water, the heat sensible in the 

 water hecomes latent in its vapour, and the sensible temperature falls ; 

 additional heat to keep up the temperature not being quickly enough 

 supplied by the surrounding media. Let us look at the effect of this 

 evening's supply of water to plants : the air is irj, evaporation goes on 

 briskly ; the temperature sinks, the plants are chiUed, there are no 

 sun's rays to communicate fresh warmth, and their growth is sometimes 

 even more unsatisfactory than that of such plants as are growing in the 

 apparently arid soU, and which have been allowed to take their chance. 

 The other source of diminished temperature I noticed was radiation : 

 every warm body tends continually to throw off its heat to all others of 

 lower temperature, near or remote: but radiation in meteorology is 

 more particularly confined to ' the radiation of heat from the surface 

 of the earth and objects on it into a clear sky.' All objects do not 

 radiate heat with equal rapidity : rough surfaces doit more readily than 

 smooth, and dark surfaces than those of a lighter shade of colour. 

 Apply the latter remark to the process of evening watering : almost all 

 soils are darkened in their colour by moisture, and hence soil by this 

 practice is reduced to the best possible condition for getting cooled down 

 during the night." 



This is, in fact, a commentary upon what the French call the Zune 

 JRousse or April moon, which they fancy rusts their crops. M. Ajago 

 has shown that this notion is erroneous, the effect aUuded to being clearly 

 owing to another cause, but one which must necessarily be in active 

 operation on bright moonlight nights. He observed that in the months 

 of April and May the temperature at night is often not more than four 

 or six degrees above the freezing point ; and under these circumstances, 

 when the sky is most unclouded and the moon shining brightest, heat 

 wiU be radiated from the earth sufficient to reduce the temperature at 

 the surface some degrees lower, or below freezing point; henoe the 

 tender leaves and roots of plants are nipped by cold, and that appear- 

 ance given to the former which is intended to be conveyed by the 

 French word rousse. But it maybe doubted whether, under such con- 

 ditions as are above assumed to exist, watering should be practised at aU. 

 The principle is not to water if it can be avoided ; it is in hot. dry 

 weather that the operation is most needed, and at that time the 

 lowering of temperature at night is more beneficial than disadvanta- 

 geous. It is evident indeed that the arguments just quoted are alto- 

 gether one-sided, the real questions to be determined are, 1st, "Whether 

 such a loss of heat is detrimental to plants ? and 2ndly, Whether there 

 may not be some compensating advantages? "We believe that all 

 plants are retained in a more healthy state by lowering their tempera- 

 ture at night, and that no «rror is greater than that of supposing warm 

 nights advantageous to them. In aU countries nature cools down the 



