290 REASONING. 



were able to observe a and e together, and although 

 even d, our only direct mark of e, may be not per- 

 ceptible in those objects, but only inferrible. Or 

 varying the first metaphor, 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 

 transforms 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 &, 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 ; &, 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 grand 

 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 appa- 

 rently 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 



