130 



DISCOVERY 



growing leaf is always cold, the reason being that the 

 plant has discovered exactly what amount of moisture 

 it is necessary to evaporate from its pores to keep its 

 temperature down. Humanity does exactly the same 

 thing when it is properly acclimatised. In the hottest 

 climates, the natives' skins are normally as cold as a 

 snake's, whilst the visitor is streaming with useless 

 perspiration, too profuse to evaporate. The plant 

 has to absorb water through its roots from the ground, 

 so that if it cannot penetrate deep enough to get 

 sufiicient moisture, it dies. The leaves of a tree are 

 always cool and evaporating moisture, and it follows 

 that every breeze that blows over them is cooled and 

 moistened, which means that the dew-point has been 

 lowered. If the wind now encounters a hill-slope, on 

 blowing up it the dew-point is still further lowered by 

 the chill caused, and rain is all the more likely to 

 result. The ground also under the tree is shaded 

 from the sun, and is therefore cooler and moister than 

 if it were exposed. In addition, when rain falls, it is 

 easily conducted down the holes made by the roots 

 into the soil, and there is not so much left on the 

 surface to run off or be lost in evaporation. 



In the other case, when there are no trees, the soil 

 gets very hot and dry, and the herbage, having roots 

 that do not penetrate so deeply, gets parched. Since 

 the ground surface is caked, any rain runs off freely, 

 and as the earth has been baked hot by the sun, there is 

 much loss in evaporation. More than this, since the 

 quantity of water running off is so much larger, its 

 scoring and carrying powers are much greater. The 

 result is that a great quantity of stones, earth, humus, 

 and detritus is always swept off slopes that have been 

 deforested, lilling up and choking the river-beds, where- 

 ever the slopes are too flat to enable the current to 

 carry it farther. 



A good example of this is the port of Pisa. For 

 fifteen hundred years at least, the mouth of the Arno 

 was a port which supported a w^ealthy town. Later, 

 the town grew into a state, rivalling even Genoa. It 

 had colonies in Majorca and Minorca, an army and a 

 navy with which it fought its neighbouring states. 



Effect of Cutting the Apennine Forests 



In the fourteenth and fifteenth centuries the 

 Apennines became deforested, some say because it was 

 believed that the devastating plagues of the Middle 

 Ages were caused by trees, or it maj' have been to get 

 more pasture for the growth of the wool which made 

 Florence so rich and famous. At any rate the result 

 was disastrous. The Apennine pastures dried up, the 

 rivers rapidly rose in flood and carried more and more 

 gravel down to the sea. The harbour of Pisa became 

 choked and obliterated, the Pisans ignorantly blaming 

 the Genoese for having done it in one of their raids. 



Now there are miles of unhealthy marshes between 

 Pisa and the sea, and there is no harbour for even a 

 row-boat to shelter in, and this is all traceable to the 

 ground getting hard and caked in the absence of trees. 

 Where there are forests the soil is filled with water 

 which is kept cool, and, therefore, the springs are well 

 supplied , keeping up the flow of the rivers for a longer 

 time and at a higher level. At the same time the 

 surface of the subterranean reservoir is kept higher, 

 and the roots of plants and trees have a better chance 

 of getting their necessary moisture from it. Without 

 the forests, rain rushes off rapidly, causing severe floods, 

 masses of gravel and detritus pour over fertile fields, 

 followed by a rapid fall in the flood level. A shrivelled 

 current is all there is for most of the year in a vast bed 

 of gravel, which is quite useless for agriculture because 

 it is covered with water so frequently. The sub- 

 terranean reservoir falls then to a much lower level, 

 and the surface plants and trees consequently suffer 

 accordingly. 



Saturation Level 



In every country this subterranean reservoir exists 

 at a greater or less depth below the surface. It is the 

 level of saturation which, of course, varies from time to 

 time according to the rainfall. At the sea, it coincides 

 with the mean tide level, but it rises more and more 

 on going inland, and it is the level to which wells must 

 be sunk before water appears in them. It is caused by 

 the rain, which is usually said to run oft to the extent 

 of one-third ; another third sinks in to form this reser- 

 voir, and the remainder is lost in evaporation. When 

 following a river valley, one often notices a line of 

 springs appearing at a certain level ; this is when the 

 valley has been cut down to below the subterranean 

 reservoir, which then forms a wet trough for it to run in. 

 When the reverse is the case, the river loses a great 

 deal of its water by its percolating into the dry soil 

 around and beneath it. In the East this last is very 

 common, so that rivers very often get smaller and 

 smaller the farther they go, till at last they dry up 

 altogether. 



We see now that the denudation of trees has cumula- 

 tive ill effects which tend to reduce the fertility of the 

 country. The reverse is also the case ; a large growth 

 of forests has accumulati\'e good effects tending to 

 greatly increase the humidity of the air, the equability 

 of the temperature, and the fertility of the region. The 

 moisture in the atmosphere, largely supplied by leaves, 

 has a very great, but often unnoticed, effect on a 

 climate. The aqueous vapour is impervious to heat 

 rays, unless they come from a greatly heated source. 

 In fact, it acts in much the same way as glass. The 

 heat rays from the sun pass freely through, but when 

 the same rays are reflected back from the earth, the 



