EVILS OE MOSS AND LICHEN. 349 



be instantly removed by flowing off and being eva- 

 porated; and although the moss presents much more 

 surface to the air than that of the soil on which it 

 grows, it is so much cooler that the evaporation 

 from it is considerably less. The absence of 

 mosses is among the reasons why sandy and 

 chalky places are so soon parched up. 



But although, within certain limits, the growth 

 of those plants is good, yet, when those limits are 

 exceeded, it becomes an evil. For, though their 

 tendency be to mitigate the severity of both heat 

 and cold, they do in all cases produce cold by 

 making the total evaporation greater than it other- 

 wise would be. They take off the extremes of 

 evaporation during the great heats, but they also 

 occasion evaporation at times when otherwise 

 there would be little or none, and thus they keep 

 a moist atmosphere all the year round, and so 

 where they abound, the climate is less healthy. 

 It also rains much more frequently, because that 

 air being always nearly saturated with moisture, 

 is of course disposed to part with that moisture in 

 the state of rain, much more readily, that is, with 

 much less atmospheric action, than when the de- 

 gree of saturation is less. Consequently they are 

 injurious to cultivated grounds. To the annual 

 crops they, indeed, do small harm, as they attain 

 but little size, and are under the shade of these. 

 But on grass lands they are much more destruc- 

 tive ; and would in time change a good soil and 

 climate into the opposite. The finest grasses, 

 though they thrive well with occasional irriga- 

 tions, decay when they are too moist, as they 

 always are in old pastures that have got mossed ; 

 and if the surface had little drainage, the mosses 



Hh 



