PHYSICAL CLIMATE. 495 



to warm the Atlantic States of America by which it sweeps, whereas the mean temperature 

 there is lower than in Western Europe ; but this is owing to the narrowness of the current 

 in that quarter, and to the prevailing winds being from the west and south-west, which carry 

 the hot circumambient air away from the shore. With reference to the lower degree of 

 heat experienced over the whole continent of America, as compared with corresponding 

 latitudes in the eastern portion of the earth, Humboldt advances the following statement 

 in his Tableaux de la Nature : " The comparative narrowness of this continent its elonga- 

 tion towards the icy poles the ocean, whose unbroken surface is swept by the trade winds 

 the currents of extremely cold water which flow from the straits of Magellan to Peru 

 the numerous chains of mountains, abounding in the sources of rivers, and whose summits, 

 covered with snow, rise far above the region of the clouds the great number of immense 

 rivers, that, after innumerable curves, always tend to the most distant shores deserts, 

 but not of sand, and consequently less susceptible of being impregnated with heat 

 impenetrable forests, that spread over the plains of the equator, abounding in rivers, and 

 which, in those parts of the country that are the farthest distant from mountains and from 

 the ocean, give rise to enormous masses of water, which are either attracted by them, or 

 are formed during these act of vegetation : all the causes produce, in the lower parts of 

 America, a climate which, from its coolness and humidity, is singularly contrasted with 

 that of Africa. To these causes alone must we ascribe that abundant vegetation, so 

 vigorous and so rich in juices, and that thick and umbrageous foliage, which constitute 

 the characteristic features of the new continent." 



4. There are various other circumstances which enter into the constitution of climate, 

 such as the soil of a country, whether light and porous, or clayey and compact, whether 

 open to the solar action and to the play of the winds, or covered with forests, and screened 

 by mountains from the free operation of atmospheric currents. Barren sands admit of a 

 much more intense heat than a loamy soil, and pasture lands are not so susceptible of it as the 

 bare ground. The clearing of a country from trees has the effect of raising the mean 

 annual temperature, but at the same time greater extremes of heat and cold are introduced. 

 Dr. Williams found the temperature of open grounds 12 higher in summer and 7 lower 

 in winter, than that of forests. Open grounds are always frozen deeper than the wood- 

 lands, and hence the spring advances later ; but being more heated in summer, the winter 

 commences later. Several countries have, however, suffered severely from the incautious 

 destruction of the forests, having been deprived thereby of a natural protection needed 

 by their geographical position from the intensity of the solar rays, or from the heated air 

 of neighbouring districts, besides the amount of moisture in the atmosphere being 

 lessened by the loss of the vegetation which promotes evaporation. The climate of some 

 parts of Southern Europe ,has been deterioriated from this cause, and the oppressive heats 

 and dreadful droughts of the Cape Verde Islands are due to it. In addition to the direct 

 effect of the sun's rays, contiguous parts of the earth exert a continual influence upon each 

 other, by reciprocating their temperature through the medium of the winds. In this way, 

 the deserts of Arabia and Africa are like immense furnaces warming all the regions on the 

 Mediterranean Sea, in the south of Europe and the west of Asia ; and in a similar manner, 

 the mountains and table-land of Tartary increase the cold of the surrounding countries. 

 At Bagdad and at Bushire, where the south wind arrives heated by the burning sands of 

 Arabia, the thermometer sometimes stands at 125, and on the west coast of Africa, 

 when a similar cause operates, it has been observed at 130. In the August of 1819, 

 when the fierce simoom blew at Bagdad, the thermometers rose to 120 in the shade, 

 and at midnight were at 108 in the open air, and a heavy rain falling, an unprecedented 

 event, the whole region was converted into a vast hot steam-bath. Multitudes of people, 

 both in the country and in the streets, dropped down dead, owing to the intense heat. A 



