100 



THE ATMOSPHERE. 



dividual local characters to the climate in re- 

 spect of warmth, dampness, frequency of winds 

 and storms, and transparency of atmosphere. 

 These circumstances have from time immemo- 

 rial exerted a powerful influence upon the na- 

 ture of the productions of the soil, and on the 

 manners, forms of government, and likings and 

 dislikings of neighbouring races for one anoth- 

 er. The character of the geographical individ- 

 uality reaches its maximum, as it were, where 

 the diversities in the configuration of the sur- 

 face, both in the vertical and the horizontal di- 

 rection, in the relief and the partitioning of con- 

 tinents, are the greatest possible. With such 

 relations of the soil are contrasted the steppes 

 of Northern Asia, the grassy plains (Prairies, 

 Savannas, Llanos, and Pampas) of the New 

 Continent, the heaths or moors of Europe, and 

 the sandy and rocky deserts of Africa. 



The law of the decrement of temperature ac- 

 cording to the height above the sea under dif- 

 ferent parallels of latitude, is one of the most 

 important particulars in connection with the 

 knowledge of meteorological processes, with 

 the geographical distribution of plants, the theo- 

 ry of terrestrial refraction, and the various hy- 

 potheses which bear upon the determination of 

 the height of the atmosphere. In the course 

 of the numerous mountain expeditions I have 

 undertaken, both within and without the trop- 

 ics, the determination of this law has always 

 been one of the principal objects of my obser- 

 vations and experiments(^"). 



Since the true relations of thermal distribu- 

 tion over the surface of the earth, i. e., the in- 

 flections of the isothermal and isotheral lines, 

 and the unequal distances of these from each 

 other in the several systems of eastern and 

 western temperature of Asia, mid-Europe, and 

 North America, have been studied and made 

 more generally known, we must not any long- 

 er inquire, even in a general way, what frac- 

 tional part of the mean annual or summer tem- 

 perature corresponds to a change of one de- 

 gree of. geographical latitude 1 In each system 

 of isothermal lines of like curvature there pre- 

 vails an intimate and necessary connection be- 

 tween three elements : the decrease of tem- 

 perature in the perpendicular direction from 

 below upwards ; the difference of temperature 

 in changing the place of observation by 1° of 

 latitude ; the equality of the mean tempera- 

 ture of a mountain station, and the polar dis- 

 tance of a point laid down on the level of the 

 sea. 



In the East American system, the mean an- 

 nual temperature changes from the coasts of 

 Labrador to Boston for every degree of lati- 

 tude by 0°-88 C. ; from Boston to Charleston 

 by 0°-95 C. ; from Charleston to the tropic of 

 Cancer in Cuba onwards, the change, however, 

 becomes less — there it is only 0°-66 C. With- 

 in the tropics the change is still smaller, the va- 

 riation from Havanna to Cumana, correspond- 

 ing to a degree of latitude, being no more than 

 0° 20 C. 



It is quite different in the system of iso- 

 therms of mid-Europe. Between the parallels 

 of 38° and 71 ° I find the decrease of temperature 

 to coincide very accurately with half a degree 

 (0°-5 C.)for each degree of latitude. But, as 

 in this country, the fall in temperature is 1° C. 



for every 480, or 523 feet of perpendicular rise, 

 it follows that here a rise of from 240 to 262 

 feet above the level of the sea corresponds, in 

 respect of temperature, to one degree of lati- 

 tude. The mean annual temperature of the 

 Convent on Mount St. Bernard, 7,668 feet 

 above the sea-level, in latitude 46° 50', would 

 thus be met with again in the plain, in latitude 

 75° 50'. 



In that part of the chain of the Andes which 

 lies within the tropics, my observations, which 

 have been carried out to an elevation of 18,000 

 feet, indicate a fall of 1° C. for 96 toises, or 576 

 feet ; my friend Boussingault, thirty years later, 

 found 90 toises, or 540 feet, as the mean corre- 

 sponding to the same fall. On comparing the pla- 

 ces which stand among the Cordilleras at equal 

 heights above the sea, whether on the slopes 

 themselves, or on the extensive plateaus which 

 they" form, I found an increase of from l°-6 to 

 2°-3 C. in mean annual temperature of the lat- 

 ter over the former. Without the cooling ef- 

 fects of nocturnal radiation, the difference 

 would be still greater. As the climates are 

 there stratified, as it were, superposed in lay- 

 ers from the Cacao groves of the lowlands up 

 to the line of perpetual snow, and as the tem- 

 perature in the tropical zone varies but very 

 slightly in the course of the whole year, a tol- 

 erably fair idea is formed of the relations in 

 respect of temperature to which the inhabi- 

 tants of the great cities of the Andes are ex- 

 posed, when these relations are compared with 

 the temperature of particular months in the 

 plains of France and Italy. Whilst the tem- 

 perature of the day on the wooded banks of the 

 Orinoco is such that it exceeds, by 4° C, that 

 of the month of August at Palermo, we find 

 when we have ascended the mountains to Po- 

 payan (911 toises), that we are in the tem- 

 perature of the three summer months at Mar- 

 seilles ; in Quito, again (1492 toises), the tem- 

 perature is that of the end of the month of 

 May at Paris, and when we have attained the 

 Paramos or mountain wilds, overgrown with 

 dwarf Alpine plants, still bearing large flowers 

 (1800 toises), we meet with the temperature of 

 the beginning of the month of April at Paris. 



The acute Peter Martyr de Anghiera, one of 

 the friends of Christopher Columbus, was the 

 first who perceived (in the expedition of Rod- 

 rigo Enrique Colmenares, Oct. 1510), that the 

 snow-line always rises higher the nearer the 

 equator is approached. 1 find these words in 

 the beautiful work„ De Rebus Oceanicis(^'"*) : 

 " The River Gaira comes from a mountain (in 

 the Sierra Nevada de Santa Marta), which, 

 from the reports of the companions of Colme- 

 nares, is higher than any mountain yet discov- 

 ered. It must undoubtedly be so, if, in a zone 

 which is at most 10° from the equinoctial line, 

 it retains its covering of snow continually." 

 The inferior limit of the eternal snow in a given 

 latitude is the summer limit of the snow-line ; 

 that is, the maximum height to which the 

 snow-line recedes in the course of the entire 

 year. From this summer limit of the snow- 

 line, three other phenomena must be distin- 

 guished : Annual fluctuations of the snow-line ; 

 occasional or sporadic falls of snow ; and gla- 

 ciers, which appear to be peculiar to the tem- 

 perate and frigid zones, on which Saussure's 



