498 



PHYSICAL GEOGRAPHY. 



The preceding table gives only the difference in the mean temperature of different months; 

 but, taking the days into the account, the thermometer exhibits a far wider range. Thus, 

 at Pekin, it has been observed as high as 110; while the freezing of mercury, which 

 takes place at 39 or 40 below zero, is no unusual thing at Quebec. 



The farther we recede from the equator to the poles, the greater is the difference 

 between the mean temperature of summer and winter. 



But, however great the oscillations of the thermometer in the course of a single year, 

 a comparison of years shows that the mean annual temperature in every place is remark- 

 ably uniform, and seldom varies from the standard peculiar to the locality to any extent, 

 even in those years that are marked by excessive seasonal heat or cold. Thus, in the 

 year 1788, when the frost was so severe at London that the Thames was passable on the 

 ice, the mean temperature for that year was within the fraction of a degree of the stan- 

 dard. This was the case also in 1796, when the greatest cold that was ever observed at 

 London occurred; and in 1813-14, when the Thames, Tyne, and other large rivers were 

 completely frozen over, the variation amounted to little more than a degree below the 

 .standard. When also, in 1808, the summer was so hot in London that the thermometer 

 stood at 931, the mean temperature for the year did not rise above the usual average. 

 A large collection of data justifies the remark of Humboldt, that the quantity of heat 

 which any point of the g]obe receives, is much more equal during a long series of years 

 than we should be led to believe from the testimony of our sensations and the variable 

 product of our harvests. It is more a change in the distribution of the heat through the 



