moist soils at night is less quick, but on the whole such soils 

 are colder and are called cold soils. Nocturnal radiation 

 results in quicker formation of dew in the interstices of soils 

 where water vapour accumulates in larger proportions than 

 in the air. 



94. Retention of heat. Quick or slow cooling depends 

 partly on specific heat but chiefly on fineness or largeness 

 of particles of the soil, finely divided particles cooling more 

 readily. Soils covered with gravels, cool more slowly than 

 sandy soils. Sandy soils also retain heat longer than clay 

 soils and these longer than humus soils. Water being a bad 

 conductor of heat, wet soils differ little from one another in 

 the absorption and retention of heat. A wet plot may be 

 as much as yC higher in temperature early in the morning 

 or 7C lower in temperature at 3 or 4 P.M. in day time than 

 a neighbouring dry plot. The physical effect of irrigation 

 on soils in equalising temperature and keeping soils from 

 getting too hot cannot be overrated in a climate like that of 

 India. In England coldness of soils is avoided by drainage. 

 Drainage for this purpose alone is not required in this climate. 



95. Evenness of temperature and slow nocturnal radia- 

 tion are very helpful to the growth of plants. Uniformity 

 of temperature occurs in sea-side places the climate of which 

 should be considered favourable to vegetation for this reason 

 only. It should be noted however that cold is helpful for 

 developing the germinating power of seed in the case of 

 many agricultural crops of the temperate climate, and the 

 difference of summer and winter is therefore beneficial. In 

 sea-side places, high winds prove an obstacle to agricultural 

 operations. From January to May the difference between the 

 day and night temperatures is the greatest in the plains of 

 Bengal, while in July and August it is the least. Vegeta- 

 tive processes are hampered therefore from January to May 

 and highly facilitated in July and August. In Calcutta the 

 nocturnal radiation in January amounts to 9-2 F. and in 



