Example. — The celebrated Cavendish experiment for deter- 

 mining the weight of the Earth by measuring the force of attraction 

 of two large lead weights on two small lead balls rigidly connected 

 with a thin rod and delicately poised. To avoid draughts of air 

 the apparatus was placed in a closed room and worked from 

 without, and light was thrown on the scales from outside, and 

 positions read off by means of a telescope similarly placed. 

 Corrections were also made for the mutual attractions of the 

 different parts in different positions. 



III. The joint method of Agreement and Difference con- 

 sists in the successive application of I. and II. to phenomena. 



Example. — Whenever incandescent hydrogen is present two 

 bright lines (C and F) are always present, and absorption spectra 

 obtained by light passing through incandescent hydrogen, always 

 shows the same two lines, but dark. On the other hand no other 

 incandescent substance produces these two lines. 



B. Qualitative Methods 

 (especially fruitful in results). 



IV. Method of Residues. — Subtract from a phenomenon 

 such part as is known to be the effect of certain antecedents, and 

 the residue of the phenomenon is the effect of the remaining 

 antecedents. 



Example. — The disturbance in the motion of Uranus forms a 

 residual phenomenon, which could be accounted for by the exist- 

 ence of an outer planet ; this led to the discovery of Neptune by 

 Adams and Levevrier. 



V. Method of Concomitant Variations. — Whenever pheno- 

 mena vary conjointly in any manner, there is a casual relation 

 between them. 



Example. — History shows that in proportion as Vesuvius has 

 been active the volcanic region round about (Bay of Baiae, Iscoia, 

 Solfatara, &c.) has been quiescent and subsiding, and vice versn. 

 Lyell accounts for this by supposing that Vesuvius forms a 

 natural vent, and when active diminishes the internal pressure of 

 the molten rocks. 



Such was the complexity of scientific phenomena that these 

 Canons were mostly combined in the search after truth. The most 

 important form of imperfect Induction was Analogy, which was 

 only valuable when the points of resemblance greatly outweighed 

 the points of difference. The problem of the climate of Mars and 

 its habitability was a good instance of this difficulty. The 



