TRAINS OF REASONING. 295 



some of the elements concerned; while the fact sought 

 presupposes a certain relation between the quantities 

 of some other elements : now, if these last quantities 

 are dependent in some known manner upon the former, 

 or vice versd, we can argue from the numerical relation 

 between the one set of quantities, to determine that 

 which subsists between the other set ; the theorems 

 of the calculus affording the intermediate links. And 

 thus the one of the two physical facts becomes a mark 

 of the other, by being a mark of a mark of a mark 

 of it. 



