464 POPULAR SCIENCE MONTHLY 



to other lands? The difference between a country formerly well irri- 

 gated and fertile, and a present-day sandy, inhospitable waste may be 

 the result of a former compulsion of the people, by a strong governing 

 power, to till the soil and to irrigate, while now, without that compul- 

 sion, no attempt is made to keep up the work. The incapacity of the 

 present inhabitants, or of their rulers, is often responsible for effects 

 which have been interpreted as due to climatic change. It has been 

 shown that where irrigation is resorted to in parts of the districts about 

 the Mediterranean which have been reported to be drying up, there the 

 former fruitfulness returns. In many cases the reports of increasing 

 dryness really concern only the decrease in the water supply from rivers 

 and springs, and it is well known that a change in the cultivation of the 

 soil, or in the extent of the forests, may bring about marked changes 

 in the flow of springs and rivers without any essential change in the 

 actual amount of rainfall. These conditions are particularly likely to 

 occur in regions where there is no snow covering, and where the rain 

 falls in a few months only. In Tripoli, the Vicomte de Mathuisieulx 

 finds that the Latin texts and monuments seem to establish the fact 

 that, so far as atmospheric conditions and soil are concerned, everything 

 is just as it was in ancient times. The present condition of the country 

 is ascribed to the idleness of the Arabs, who have allowed wells to 

 become choked and vegetation to perish. " In a country so little 

 favored by nature, the first requisite is a diligent and hard-working 

 population. The Eomans took several centuries to make the land pro- 

 ductive by damming rivers and sinking wells in the wady beds." In an 

 arid region, man has a hard task if he is to overcome the climatic diffi- 

 culties of his situation. Irrigation ; the choice of suitable crops adapted 

 to arid conditions; steady, thoughtful work, are absolutely essential. 

 To a large extent, an intelligent man may thus overcome many of the 

 obstacles which nature has put in his way. On the other hand, a region 

 of deficient rainfall, once thickly settled and prosperous, may readily 

 become an apparently hopeless desert, even without the intervention 

 of war and pestilence, if man allows the climate to master him. 



Lastly, a region whose normal rainfall is at best barely sufficient for 

 man's needs, may be abandoned by its inhabitants during a few years 

 of deficient precipitation, and not again occupied even when, a few 

 years later, normal or excessive rainfall occurs. It is a very striking 

 fact that the districts from which comes most of the evidence of 

 changes of climate within historical times are subtropical or subequa- 

 torial, i. e., they are in just those latitudes in which a slightly greater 

 or a slightly less migration of the rain-bringing conditions easily pro- 

 duces a very considerable increase or decrease in the annual rainfall. 



