246 REASONING. 



the ladder from a to e by a process of ratiocination ; we can 

 conclude that a is a mark of e, and that every object which 

 has the mark a has the property e, although, perhaps, we 

 never were able to observe a and e together, and although 

 even d, our only direct mark of e, may not be perceptible in 

 those objects, but only inferrible. Or, varying the first meta- 

 phor, we may be said to get from a to e underground : the 

 marks b, c, d, which indicate the route, must all be possessed 

 somewhere by the objects concerning which we are inquiring; 

 but they are below the surface : a is the only mark that is 

 visible, and by it we are able to trace in succession all the 

 rest. 



6. We can now understand how an experimental may 

 transform itself into a deductive science by the mere progress 

 of experiment. In an experimental science, the inductions, 

 as we have said, lie detached, as, a a mark of b, c a mark of 

 d, e a mark of/, and so on : now, a new set of instances, and 

 a consequent new induction, may at any time bridge over 

 the interval between two of these unconnected arches ; b, for 

 example, may be ascertained to be a mark of c, which enables 

 us thenceforth to prove deductively that a is a mark of c. 

 Or, as sometimes happens, some comprehensive induction 

 may raise an arch high in the air, which bridges over hosts 

 of them at once : b, d, /, and all the rest, turning out to be 

 marks of some one thing, or of things between which a con- 

 nexion has already been traced. As when Newton discovered 

 that the motions, whether regular or apparently anomalous, 

 of all the bodies of the solar system, (each of which motions 

 had been inferred by a separate logical operation, from 

 separate marks,) were all marks of moving round a common 

 centre, with a centripetal force varying directly as the mass, 

 and inversely as the square of the distance from that centre. 

 This is the greatest example which has yet occurred of the 

 transformation, at one stroke, of a science which was still 

 to a great degree merely experimental, into a deductive 

 science. 



Transformations of the same nature, but on a smaller scale, 



