NOTES ON THfi BOIL. 49l 



soil varies inversely as its porosity, it is sufficient to measure 

 the porosity. Sand is very porous, but has very little 

 capillarity. Clay shows strong capillary action, but is very 

 slightly porous. Porosity and capillarity are determined 

 simply by using a cylinder of soil, and noting the rate of the 

 downward or upward flow of water through it. By similar 

 experiments we can compare the water-absorbing capacity of 

 different soils and the rate of evaporation from them. 



(2) Water-content depends absolutely upon rainfall, ex- 

 cept, of course, in habitats with a constant water-supply 

 owing to the presence of springs, streams, ponds, or other 

 bodies of water. The soil becomes saturated after heavy 

 rain, and its water-content gradually decreases through a 

 dry period. Snow is often of importance, since it acts as a 

 cover, preventing evaporation, and enters the soil on 

 melting. 



(3) Humidity is of great importance as the chief stimulus 

 which controls the loss of water by transpiration. It is 

 influenced by temperature, wind, air-pressure, altitude, and 

 exposure, as well as by the water-content of the soil and the 

 presence or absence of a covering of dead or living vegeta- 

 tion. Heat increases the capacity of the air for moisture and 

 therefore lowers the relative humidity. Dry winds and high 

 winds remove the humid air around plants, and keep the 

 humidity low. Air-pressure affects humidity by varying 

 the density of the air, and therefore its power of holding 

 moisture. 



The relatively small daily barometric changes, which con- 

 stitute "weather," are of little importance except in their 

 relation to rainfall, but the permanently low pressure at high 

 altitudes increases evaporation owing to the rarefaction of 

 the air. Exposure to sun and to prevailing winds, as seen on 

 slopes, has a great influence on humidity ; slopes with a 

 southern exposure receive most light, heat, and wind, and are 

 therefore drier than slopes facing northwards. 



A dense covering of vegetation on a slope prevents rain- 

 water from running off before it can be absorbed, besides 

 hindering the washing away of soil ; the cover also increases 

 humidity by reducing the influence of temperature and wind 

 on evaporation, and a cover of living plants keeps the air 

 moist by the evaporation of water from the plants. 



