3 i4 THE LITTLENESS OF KNO WLEDGE. 



venous blood, flows from the circumference towards the 

 centre: this is the solar heat which the earth continues to 

 receive through its crust. 



The first of these sources lies beyond the domain of 

 experiment. It has been the object of numerous hypotheses 

 and diverse speculations, with which we shall not here con- 

 cern ourselves. The second source is alone accessible to 

 our investigations, and yet the network of isothermal lines is 

 scarcely defined. 



Since the year 1817, when Alexander von Humboldt 

 conceived the felicitous idea of representing by lines the 

 same mean temperatures enjoyed, in a given space of time, 

 by the different regions of the globe, researches of this 

 nature have very considerably multiplied. But these re- 

 searches do not forget refer rather to the temperature 

 of the atmosphere than to the heating of the inferior stratum 

 of that gaseous ocean whose bed or foundation is the ter- 

 restrial crust. And it is the penetration of the latter by the 

 sun's calorific rays which we would especially desire to under- 

 stand. Here, then, a sufficient margin is left for our curiosity. 



"If the king, my father, does not rest from his con- 

 quests," cried Alexander of Macedon, while still a child, 

 " he will leave me nothing to do when I shall have reached 

 manhood." To such a complaint, be you sure, dear reader, 

 that the conquests made by science will never give rise. 

 Every step in advance is but a step into the infinite : what 

 we have done only shows us the boundless extent of what 

 we have to do. 



