Cultivation and tilUige 



91 



untouched, carefully cultivate with a penknife every 

 two or three days the top quarter of an inch of the 

 second, and cover the third with a layer of grass. 

 After a week notice again the levels of the liquid and 

 mark with paper; you find that the water has fallen 

 most in the untouched jar, showing that more has been 

 lost from this than from the jars covered with a mulch 

 either of soil or of dry grass. 



A slate or flat stone acts like a mulch ; if you leave 

 one on the soil for a few days in hot weather and then 

 lift it up on a hot day you will see that the soil under- 

 neath is quite moist ; you may also find several slugs or 

 other animals that have gone there for the sake of the 

 coolness and the moisture. Plants and trees also keep 

 ofi^ the sun's heat and so make the soil cold and moist 

 Grass land is in summer and autumn, and even in early 

 winter, cooler near the surface than bare land. At 

 Harpenden we found: — 



Even if the ground is not covered a certain amount 

 of protection is still possible. Trees are often planted 

 round ponds to prevent evaporation of the water. The 

 wind helps to dry the soil very much, and a hedge 



