496 



PHYSICAL GEOGRAPHY. 



small caravan lost twenty-two persons in this manner during the last three days of its 

 journey across the Desert towards the city. 



In the United States, the prevailing winds remarkably affect the temperature, pro- 

 ducing the cold of the polar regions and the warmth of the torrid zone, according as they 

 blow from the frozen shores of Hudson's Bay or from the hot regions of the Gulf of 

 Mexico. In Venezuela, the temperature, which is from 87 to 90 in March, rises to 104 

 or 105, whenever the wind blows from the parched surface of the llanos or great plains. 

 Siberia, and the northern parts of North America, have their cold greatly increased by 

 the polar winds, which are not intercepted by mountains, in addition to the effects of a 

 northern declivity. But the same wind, in various countries, will produce opposite 

 effects upon the temperature at different seasons of the year. Poeppig mentions a 

 singular instance of this in the southern districts of Chili. The east winds, called los 

 Puelches, blowing in spring, are so cold as rapidly to depress the temperature 15 or 18; 

 but towards the end of summer they raise it nearly as much. The former effect is attri- 

 buted to the deep snow lying in spring upon the Andes chilling the adjacent air, and the 

 latter to the heat which the sandy pampas of Buenos Ayres acquire in the summer months. 

 The east winds of England exhibit this alternating character. Late in spring, having 

 passed over the plains of the Baltic, yet bearing the chill of winter, they are cold ; but in 

 autumn, they are warm enough to raise the temperature, the sandy plains in their passage 

 having been heating through the summer. 



The preceding causes are those by which physical climate is chiefly determined. A 

 few results of observation respecting the mean annual temperature in different localities, 

 and seasonal temperature, may now be stated, chiefly upon the authority of 

 Humboldt. 



The mean temperature of a country is the average of heat and cold throughout the 

 year, and may be obtained by observing at stated periods, during each day, the indications 

 of the thermometer, taking a series of such observations continued through an interval 

 of ten or fifteen years. This is a tedious and protracted process, and the lessons of expe- 

 rience have superseded the necessity for it. De la Hire was the first who observed the 



The Andes. 



