GENERAL CONCLUSIONS. 287 



is because, as these sciences developed, we have recog- 

 nized more clearly the links which unite them, and 

 at last we have perceived a kind of general design of 

 the map of universal science. There are facts com- 

 mon to several sciences, like the common fountain 

 head of streams diverging in all directions, which may 

 be compared to that nodal point of the St. Gothard 

 from which there flow waters that feed four different 

 basins. 



Then we can make our selection of facts with more 

 discernment than our predecessors, who regarded 

 these basins as distinct and separated by impassable 

 barriers. 



It is always simple facts that we must select, but 

 among these simple facts we should prefer those that 

 are situated in these kinds of nodal points of which 

 I have just spoken. 



And when sciences have no direct link, they can 

 still be elucidated mutually by analogy. When the 

 laws that regulate gases were being studied, it was 

 realized that the fact in hand was one that would give 

 a great return, and yet this return was still estimated 

 below its true value, since gases are, from a certain 

 point of view, the image of the Milky Way ; and these 

 facts, which seemed to be of interest only to the 

 physicist, will soon open up new horizons to the 

 astronomer, who little expected it. 



Lastly, when the geodcsist finds that he has to turn 

 his glass a few seconds of arc in order to point it upon 

 a signal that he has erected with much difficulty, it is 

 a very small fact, but it is a fact giving a great return, 

 not only because it reveals the existence of a little 

 hump upon the terrestrial geoid, for the little hump 



