THE ATMOSPHERE— CLIMATE. 



97 



and isochimenal lines cut tho parallel circles, 

 the portion of the convexities or concavities 

 of these lines in respect of the pole of the cor- 

 responding hemisphere, are the effects of calo- 

 rific or frigorific causes which show themselves 

 possessed of more or less power under differ- 

 ent geographical longitudes. 



The progress of Climatology has been favour- 

 ed in a remarkable manner by the spread of 

 European civilization from two opposite sea- 

 boards, by its extension from our Western Eu- 

 ropean coast to an Eastern coast on the other 

 side of the great Atlantic vallct- When the 

 British, after the temporary establishments 

 which had proceeded from Iceland and Green- 

 land, had founded the first permanent colonies 

 on the shores of the United States of America, 

 where religious persecution, fanaticism, and 

 love of freedom, soon swelled the ranks of the 

 settlers, the bold adventurers must have been 

 amazed at the severity of the winters which 

 they encountered, from North Carolina and 

 Virginia to the River St. Lawrence, in com- 

 parison with those which prevail under corre- 

 sponding parallels of latitude in Italy, France, 

 and Great Britain. Such climatic observations, 

 however exciting they must have been, still 

 only bore fruits when they could be based on 

 numerical results of mean annual temperatures. 

 If, between the parallels of 68° and 30° N. lati- 

 tude we compare Nain, on the coast of Labra- 

 dor, with Gottenburg, Halifax with Bordeaux, 

 New York with Naples, St. Augustin in Florida 

 with Cairo, we find the differences in mean an- 

 nual temperature between the East of America 

 and the West of Europe, under similar paral- 

 lels of latitude, progressing from north to south, 

 from 11° -5, 7° -7 and 3°-8 to almost Cent. 

 The gradual decrease of difference in the above 

 series, through 28 degrees of latitude, is very 

 remarkable. Still farther to the south, and 

 within the tropics, the isothermal lines in al- 

 most every part of both divisions of the globe 

 run parallel with the equator. From the ex- 

 amples here given, it is obvious that the ques- 

 tions we hear so constantly repeated in our 

 social circles, as to how many degrees Amer- 

 ica — and without any distinction of East or 

 West coast — is colder than Europe? and how 

 many degrees the mean annual temperature in 

 Canada and the United States of America is 

 lower than under corresponding parallels of 

 latitude in Europe 1 when taken as general ex- 

 pressions, are totally without meaning. The 

 difference under each particular parallel is dif- 

 ferent from what it is under every other paral- 

 lel ; and without special comparisons of the 

 winter and summer temperatures of the oppo- 

 site coasts, no right conception can be formed 

 of the several particular climatic relations in 

 so far as they influence agriculture, trade, and 

 the feelings of comfort and convenience, or the 

 contrary. 



In enumerating the causes that may produce 

 disturbances in the form of the isothermal lines, 

 I distinguish the causes tending to exalt, and 

 the causes tending to depress temperature. To 

 the first class belong : the vicinity of a west 

 coast in the temperate zone ; the configuration 

 of a continent cut up into numerous peninsu- 

 las ; deep bays, and far-penetrating arms of the 

 sea ; the right position of a portion of dry land 

 N 



— i. c. its relations cither to an ocean free from 

 ice which extends beyond the polar circle, or 

 to another continent of considerable extent 

 which lies between the same meridional lines 

 under the equator, or, at all events, in part 

 within the tropics ; farther, the prevalence of 

 southerly and westerly winds on the western 

 confines of a continent in the northern tem- 

 perate zone ; mountain chains, which serve as 

 screens against winds from colder countries ; 

 the rarity of swamps, which continue covered 

 with ice through the spring, and even some 

 way into summer ; the absence of forests on a 

 dry sandy soil ; finally, the constant serenity 

 of the heavens in the summer months, and the 

 neighbourhood of a pelagic stream of running 

 water of a higher temperature than that of the 

 surrounding sea. 



To the second class of causes, or those that 

 tend to depress the mean annual temperature 

 by exciting cold, I enumerate : the elevation of 

 a place above the sea level, without any thing 

 like remarkable elevated plains surrounding it ; 

 the vicinity of an eastern coast in high and 

 middle latitudes ; the massive or unbroken out- 

 line of a continent without indentation of its 

 coasts and deep sea bays ; the wide extension 

 of the land towards the poles up to the region 

 of eternal ice (without the intervention of a 

 sea open in winter) ; a geographical position 

 in longitude of such a kind that the equatorial 

 and tropical regions belong to the ocean — in 

 other words, the absence of a heating, radia- 

 ting tropical country between the same merid- 

 ian lines as the country whose climate is to be 

 determined ; mountain chains whose form and 

 direction are such that they prevent the access 

 of warmer winds ; or the neighbourhood of iso- 

 lated summits down whose slopes cold currents 

 of air descend ; extensive forests, which hinder 

 the sun's rays from reaching the ground, whose 

 appendicular organs (the leaves), by their vital 

 activity, throw off large quantities of watery 

 vapour, and vastly increase the amount of ra- 

 diating or cooling superficial surface, and so 

 act in a threefold manner — by shading, by 

 evaporating, and by radiating ; great swamps, 

 which, up to the middle of summer, in the 

 north, form a kind of subterraneous glacier in 

 the flats; a rnisty or overcast summer sky, 

 which diminishes the effect of the sun's rays 

 by intercepting them in their passage to the 

 earth ; finally, a very clear winter's sky, by 

 which radiation is favoured(^*°). 



The simultaneous activity of disturbing, 

 whether heating or cooling causes, determines 

 as a total effect the inflexions of the isothermal 

 lines projected upon the surface of the earth, 

 their course being especially influenced by the 

 relations of extent and configuration bettveen 

 the opaque continental and the. fluid oceanic 

 masses. The perturbating causes engender 

 convex or concave summits of the isothermal 

 curves. But there are disturbing causes of 

 different orders, each of which must first be 

 separately considered ; subsequently, in order 

 to ascertain the whole effect upon the motion 

 (direction or local curving) of the isothermal 

 lines, it must be discovered which of the sev- 

 eral influences in their combinations modify, 

 annul, or strengthen each other, as happens in 

 the case of other small oscillations that meet 



