PHYSICAL CLIMATE. 



497 



day may always be obtained by three readings of the thermometer, namely, at sunrise, at 

 two o'clock in the afternoon, and at sunset ; and then dividing the sum of the temperatures 

 by three. When the mean temperature of all the days of the month has been found, the 

 sum of the diurnal temperatures divided by the number of days, gives the mean tempera 

 ture of the month. In like manner, the sum of the mean monthly temperatures in the 

 year divided by the number of months, gives the mean temperature of the particular year. 

 By comparing a series of these annual returns, and striking the average, the standard 

 annual mean temperature of a place is deduced. It is a curious and highly useful result 

 of thermometric observation, that at many places in the north temperate zone, where alone 

 meteorological notices have been multiplied, the mean temperature for the months of 

 April and October is identical, or very nearly so, with the mean temperature of the year. 

 The close approximation of the mean temperature of these months to the annual 

 mean is apparent from the comparison in the table; and is most conspicuous with 

 reference to October. 



In almost all northern latitudes, January or February is the coldest month of the year, 

 and July or August the warmest. The greatest cold during the day is usually about an 

 hour before sunrise. The greatest heat in latitudes between 35 and 60 is from two to 

 three o'clock, and from one to two o'clock between the equator and 35. 



The mean temperature of different months, in various places, takes a very wide range 

 above and below the mean annual temperature, and constitutes what Buffon has indicated 

 by the name of " excessive" climates, where the winter and summer are in violent contrast. 

 These are chiefly found in North and Eastern Europe, in Asia and America. 



The climates of the northern United States, Canada, a great part of Russia, and northern 

 China, are among the most " excessive," the winters and summers strongly contrasting 

 in their temperature. Thus, at New York, says Humboldt, we find the summer of Rome 

 and the winter of Copenhagen. At Quebec, grapes sometimes ripen in the open air, 

 whereas the winter is that of Petersburg!!, during which the snow lies five feet deep for 

 several months, and travelling is performed in sledges, frequently on the ice of the 

 St. Lawrence. At Pekin in China, where the mean temperature of the year is that of 

 the coasts of Brittany, the scorching heats of summer are greater than at Cairo, and 

 the winters as rigorous as at Upsal. These violent contrasts render such climates 

 trying to the constitution of a western European, unused to such extremes of temperature, 

 the excessive heat of summer, after the rigour of winter, increasing the irritability of the 

 nervous system. 



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