M. BECQUEREL, ON THE CLIMATIC EFFECTS OF FORESTS, 327 



volumes of different argillaceous or calcareous soils, in fine powder, or 

 with humus, or with arable or garden soils, are the poorest condu<;tors 

 of heat. For this reason, sandy soils in summer, even during the night, 

 preserve a high temperature, and we may conclude from this that when 

 a sandy soil has been cleared, the local temperature should be raised, 

 and the more so as cooling causes do not exist. After sands come, 

 successively, argillaceous soils, arable and garden soils, and finally 

 humus, which comes last. Eepresenting calcareous sand, in its faculty 

 for retaining heat by 100, we have the following relative classification : 

 sand, 95.6; argillaceous soil, 68.4 ; garden-soil, 04.8; humus, 49.0. 



We must further remember, that the faculty of retaining heat, is pro- 

 portioned to the size of the particles. It is from this reason, that a soil 

 covered with silicious pebbles, cools more slowly than silicious sands, 

 and that gravelly soils agree better with the ripening of grapes than 

 chalky and clayey soils, which cool more rapidly. We see, therefore, 

 by this, how important it is, in the examination of the thermal effects 

 resulting from cooling, to have regard to the physical properties of the 

 soil, when once it has become denuded. This is probably the reason 

 why Humboldt drew the conclusions deduced from thermometric obser- 

 vations made at stations in North America, in not taking into account 

 the nature of the soil of the denuded countries, and M. BousfSiiugault 

 came to different results by taking these causes into consideration. 



It is therefore well proved that a soil of silicious sand and gravel, 

 when cleared, ought to raise the mean temperature of the air more than 

 all other soils, while at the same time it removes one cause of humidity, 

 while if the soil is argillaceous, dry or humid, the property of warming 

 the air and of retaining heat, as com|)ared with the forme.", is in the 

 ratio of 68.4 to 100. The thermal effect ought, therefore, to be much 

 less in clearing a dry ground. 



We see from this in what manner we should look upon the influence 

 of clearing upon the temperature of the air. The effects are, moreover, 

 so complex, that we can only determine the result by the aid of daily 

 observations of temperature. It will als» be necessary to collect max- 

 ima and minima temperatures, as these play an important part in the 

 constitutions of climates as regards the nature of the soil. We will 

 take up this subject again in a future memoir. 



The following example is furthermore calculated to give an idea of 

 the influences which vast forests may exert upon the climate of a coun- 

 try. The pret^ence of immense forests in the tropic.il regions of the 

 African continent, under the meridians of Western Europe, doubtless 

 modifies the ascending current of hot air resulting from the warming 

 of a sandy soil, and which descends within the middle portions of 

 Europe. If in the course of ages, the sands of Sahara should become 

 covered with wood, these sands would not become warm in the solar 

 heat as now, and, as a consequence, the south winds which now amelio- 

 rate our climate, having no longer so high a temperature as now, would 

 render the climate more rude. It is sufiicient to prove this, if we ex- 

 amine what passes on the American continent, where the tropical 

 regions are covered by vast forests, immense savannas, or great water- 

 courses. The descending currents of air do not warm and temper the 

 climate of countries situated in the middle latitudes of North America 

 so much as those of warm air coming from the Sahara, and hence the 

 difference between countries located in the same latitudes. It is pre- 

 cisely lor this reason that the American continent in the same latitudes 

 is colder than ours, judging from the cultivation of the two continents, 

 and the direction of the isothermal lines, in regions similarly exposed. 



