152 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



V. — The Cevennes mountains form a high barrier on the 

 west side of the Rhone valley, and also face the Mediterranean. 

 The rivers which rise in these mountains flow either towards 

 the Mediterranean or the Atlantic, and are very liable to sudden 

 and very high floods. For example, the Ardeche will rise 

 10 to 15 metres in from 10 to 12 hours. In i8go the flow 

 of this river reached 7500 cubic metres a second, or 3000 times 

 its minimum flow. 



M. Chaudey, basing himself on an erudite publication by 

 Professor Maurice Parde, considers the question whether these 

 sudden cloud-bursts and floods can at all be prevented by the 

 action of man. Can the clouds be made to deliver up their 

 rain in a longer period of time, so that the water may pass off 

 more gradually, and therefore cause less damage? Or can the 

 rain when it reaches the earth be checked and passed off more 

 slowly? The seriousness of the question may be understood 

 when it is stated that these cloud-bursts have been known 

 actually to produce 31 inches of rain in 24 hours. Their 

 occurrence, which is usually in early autumn, is ascribed to 

 the fact that after a period of drought, when the atmosphere 

 becomes charged with heated vapour, the wind blowing from 

 the south-east (that is, from the Mediterranean) drives this 

 vapour against the bare (that is, unwooded) summit and flank 

 of the Cevennes, these mountains having been just previously 

 cooled by north winds. 



Bare areas become more rapidly heated, and also more 

 rapidly cooled, than wooded areas. A wooded area keeps the 

 air above it cooler, and also nearer the point of saturation, than 

 does a bare area, so that it takes less cooling in the former 

 case to precipitate the rain, and accordingly the clouds empty 

 themselves on more numerous occasions, less falling at a time. 

 A wooded area is always moister than a bare one, and as it is 

 on its humidity that the temperature of a soil depends, and as, 

 further, water is a five-times worse conductor of heat than bare 

 ground, a wooded country will have a more equable temperature 

 than an unwooded one. 



Thus a wooded country is likely prima facie to receive 

 its rain in numerous, relatively small showers, instead of 

 in sudden, heavy downpours. And when the water does 

 come down a wooded country will pass it off to the 

 rivers much more gradually than bare country. The 



