PART I.] 



Nature of Strata in Surface-soil of Calcutta. 



235 



these varies greatly even within areas of very limited extent. This is well shown in 

 the following diagram, illustrating the nature of the strata encountered in three of the 

 borings made in 1875 in the site of the India Museum : — 



Alluvial. 



Water-level. 



Hard clay mixed 

 with sand. 



Hard clay. 



1 



VIO' 



!► 3' 



Alluvial. 



y 7- '5 



Water- level. 



Very firm hard sand. 



)-6-'5 



Alluvial. 



Water-level. 



Pure sand, moderately 

 firm. 



Quick sand. 



1 



\1 



\v 



M' 



It is clear that water must tend to close the pores in such a soil so far as it penetrates, 

 and when added from above as by rainfall, must therefore tend to interpose an 

 impermeable partition between the air of the atmosphere and that contained in the 

 soil beneath the moistened layer. 



Water deposited on soil like that in Calcutta takes long to penetrate to any 

 distance from the surface — a fact which must be familiar to all who have had opportunities 

 of observing the sections presented by fresh excavations during the early part of the 

 rainy season — and the increase in amount of carbonic acid begins to occur long ere the 

 water has reached the stratum from which the air containing it is derived. This alone 

 is almost conclusive in favour of the increase at this time being due to accumulation, 

 but there are other grounds for regarding the degree of soil-ventilation as the principal 

 factor regulating the fluctuations in the amount of carbonic acid throughout the 

 year. 



The temperature of the soil cannot be regarded as the determinant, as we find 

 the amount of carbonic acid on the one hand varying at different times independent of 

 corresponding variations in soil-temperature, and on the other hand remaining constant 



