INTRODUCTION. 41 



lion of the laws of attraction or repulsion of the poles, or the 

 means of eliciting either permanent or transitory electro-mag- 

 netic currents. Physical geography depicts in broad outlines 

 the even or irregular configuration of continents, the relations 

 of superficial area, and the distribution of continental masses in 

 the two hemispheres, a distribution which exercises a power- 

 ful influence on the diversity of climate and the meteorological 

 modifications of the atmosphere ; this science defines the cha- 

 racter of mountain-chains, which, having been elevated at dif- 

 ferent epochs, constitute distinct systems, whether they run in 

 parallel lines, or intersect one another ; determines the mean 

 height of continents above the level of the sea, the position 

 of the centre of gravity of their volume, and the relation of 

 the highest summits of mountain-chains to the mean elevation 

 of their crests, or to their proximity with the sea -shore. It 

 depicts the eruptive rocks as principles of movement, acting 

 upon the sedimentary rocks by traversing, uplifting, and inclin- 

 *'ng them at various angles ; it considers volcanoes either as 

 isolated or ranged in single or in double series, and extend- 

 ing their sphere of action to various distances, either by rais- 

 ing long and narrow lines of rocks, or by means of circles of 

 commotion, which expand or diminish in diameter in the 

 course of ages. This terrestrial portion of the science of the 

 Cosmos describes the strife of the liquid element with the solid 

 land ; it indicates the features possessed in common by all 

 great rivers in the upper and lower portion of their course, 

 and in their mode of bifurcation when their basins are unclosed; 

 and shows us rivers breaking through the highest mountain- 

 chains, or following for a long time a course parallel to them, 

 either at their base, or at a considerable distance, where the 

 elevation of the strata of the mountain system and the direc- 

 tion of their inclination correspond to the configuration of 

 the table-land. It is only the general results of compara- 

 tive orography and hydrography that belong to the science 

 whose true limits I am desirous of determining, and not the 

 special enumeration of the greatest elevations of our globe, of 

 active volcanoes, of rivers, and the number of their tributaries ; 

 these details falling rather within the domain of geography 

 properly so called. We would here only consider phenomena 

 in their mutual connection, and in their relations to different 

 soues of oui- planet, and to its physical constitution generally. 



