CLIMATOLOGY. 319 



York with Naples ; St. Augustine, in Florida, with Cairo, we 

 find that, under the same degrees of latitude, the differences 

 of the mean annual temperature between Eastern America 

 and Western Europe, proceeding from north to south, are suc- 

 cessively 20-7, 13-9, 6-8, and almost 0. The gradual 

 decrease of the differences in this series extending over 28 

 of latitude is very striking. Further to the south, under the 

 tropics, the isothermal lines are every where parallel to the 

 equator in both hemispheres. We see, from the above exam- 

 ples, that the questions often asked in society, how many de- 

 grees America (without distinguishing between the eastern 

 and western shores) is colder than Europe ? and how much 

 the mean annual temperature of Canada and the United 

 States is lower than that of corresponding latitudes in Eu- 

 rope ? are, when thus generally expressed, devoid of meaning. 

 There is a separate difference for each parallel of latitude, and 

 without a special comparison of the winter and summer tem- 

 peratures of the opposite coasts, it will be impossible to arrive 

 at a correct idea of climatic relations, in their influence on 

 agriculture and other industrial pursuits, or on the individual 

 comfort or discomfort of mankind in general. 



In enumerating the causes which produce disturbances in 

 the form of the isothermal lines, I would distinguish between 

 those which raise and those which lower the temperature 

 To the first class belong the proximity of a western coast in 

 the temperate zone ; the divided configuration of a continent 

 into peninsulas, with deeply-indented bays and inland seas ; 

 the aspect or the position of a portion of the land with refer- 

 ence either to a sea of ice spreading far into the polar circle, 

 or to a mass of continental land of considerable extent, lying 

 in the same meridian, either under the equator, or, at least, 

 within a portion of the tropical zone ; the prevalence of south- 

 erly or westerly winds on the western shore of a continent in 

 the temperate northern zone ; chains of mountains acting as 

 protecting walls against winds coming from colder regions ; 

 the infrequency of swamps, which, in the spring and begin- 

 ning of summer, long remain covered with ice, and the ab- 

 sence of woods in a dry, sandy soil ; finally, the constant se- 

 renity of the sky in the summer months, and the vicinity of 

 an oceanic current, bringing water which is of a higher tem- 

 perature than that of the surrounding sea. 



Among the causes which tend to lower the mean annual 

 temperature I include the following : elevation above the level 

 of the sea, when not forming part of an extended plain ; the 



