1 94 The Soil in Relation to Disease. [part i. 



Qi) Comparison of the amount of Carbonic Acid with the Velocity of the Wirid. — 



(Chart IV.) 



The velocity of the wind does not appear to exert any very distinct influence 

 on the amount of carbonic acid in the soil. It is, however, possible that the extreme 

 and continued elevation in velocity of the wind during April and May may have 

 been influential in producing the sudden depression in the amount of carbonic acid 

 in the upper layer of the soil of the first locality in the latter month. There 

 was no corresponding depression in the upper layer of the other locality, but as the 

 latter was much more sheltered than the first locality, the discrepancy rather goes 

 to support the idea that the wind may have had some effect. The question also 

 arises, whether the marked elevation in amount of carbonic acid in both localities 

 in January may not have been partially dependent on the long continuance of still 

 weather, and consequent diminished ventilation of the soil, which preceded it. 



(3.)— Temperature of the Soil.— (Chart III.) 

 Little need be said regarding this, as the principal phenomena appear very 

 clearly in the chart. So long as the weather remains dry, the fluctuations in 

 temperature in the upper layer of soil follow those of atmospheric temperature very 

 regularly ; but on the occurrence of rain this correspondence ceases. The fluctuations 

 in the temperature in the lower layer are naturally much less marked and sudden, 

 and the line of elevation and depression follows a long, gentle curve. The maxima 

 of temperature in the two layers approach more closely than the minima., a point 

 in which the relations of temperature correspond with those of carbonic acid. During 

 the cold weather the temperature of the lower layer considerably exceeds that of 

 the upper one. These relations are reversed during the hot weather. A period ensues 

 on the onset of the rains, in which the temperatures of both layers are nearly alike 

 — sometimes one, sometimes the other showing a slight excess — and this is followed 

 by a prolonged and continuous fall of the temperature of the upper layer beneath 

 that of the lower until the maximum difference is attained in January and February, 

 coincident with the minimum absolute temperature. 



(4.)-Water-Level.-(Chart II.) 



The only point calling for special notice here is the demonstration afforded by 

 the chart, that the water-level in Calcutta is really essentially dependent on the 

 local rain-fall. In so far as weekly averages are concerned, the influence exerted 

 by the tides is so slight as to be almost inappreciable, and the same holds in regard 

 to drainage into the delta from the melting of the snows on the Himalaya, and 

 other non-local supplies of water, which might have been expected to produce very 

 evident effects in a soil such as that in and around Calcutta. The three years' 

 Table (No. VII) demonstrates the same fact for a longer period. 



